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The Mechanics & Pathomechanics
of Human Movement

Second Edition

Carol A. Oatis

Welters Kluwer

HtAlh

Lippi ncott
Williams & Wilkins

rhePoint:

Kinesiology

The Mechanics and Pathomechanics
of Human Movement

Second Edition

Carol A. Oatis, PT, PhD

Professor

Department of Physical Therapy
Arcadia University
Gienside, Pennsylvania

With contributors

Acquisitions Editor: Emily J. Lupash
Managing Editor: Andrea M. Klingler
Marketing Manager: Mis si Carmen
Production Editor: Sally Anne Glover
Designer: Doug Smock

Typesetter: International Typesetting and Composition

Second Edition

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Baltimore, MD 21201 Philadelphia, PA 19106

Printed in India.

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987654321

Library of Congress Cataloging-in-Publication Data

Oatis, Carol A.

Kinesiology : the mechanics and pathomechanics of human movement /

Carol A. Oatis, with contributors.—2nd ed.
p. ; cm.

Includes bibliographical references and index.

ISBN-13: 978-0-7817-7422-2
ISBN-10: 0-7817-7422-5

1. Kinesiology. 2. Human mechanics. 3. Movement disorders.

I. Title.

[DNLM: 1. Biomechanics. 2. Kinesiology, Applied.

3. Movement—physiology. 4. Movement Disorders. WE 103 Ollk 2009]

QP303.O38 2009
612.7’6—dc22

2007037068

DISCLAIMER

Care has been taken to confirm the accuracy of the information present and to describe generally accepted practices. However, the
authors, editors, and publisher are not responsible for errors or omissions or for any consequences from application of the information
in this book and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the
publication. Application of this information in a particular situation remains the professional responsibility of the practitioner; the clini¬
cal treatments described and recommended may not be considered absolute and universal recommendations.

The authors, editors, and publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accor¬
dance with the current recommendations and practice at the time of publication. However, in view of ongoing research, changes in govern¬
ment regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package
insert for each drug for any change in indications and dosage and for added warnings and precautions. This is particularly important when
the recommended agent is a new or infrequently employed drug.

Some drugs and medical devices presented in this publication have Food and Drug Administration (FDA) clearance for limited use
in restricted research settings. It is the responsibility of the health care provider to ascertain the FDA status of each drug or device
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This book is dedicated to the memories of two people who have graced my life
and whose friendships have sustained me:

Marian Magee, PT, MS, a scholar-clinician whose respect for patient, student,
and colleague can serve as a model for all practitioners. She demonstrated the
value of interprofessional practice and mutual respect in her everyday
interactions. She generously shared her wisdom, humor,
and friendship with me.

Steven S. Goldberg, JD, PhD, educator, author, negotiator, and colleague.

He demanded much of his students and of himself. He was a generous colleague
and friend who listened carefully, offered wise and thoughtful advice,
and never failed to make me laugh.

FEATURES OF THE SECOND EDITION

■ Clinical Relevance boxes allow us to emphasize the applicability of the information contained in this textbook. These were
one of the most popular aspects of the first edition and are intended to once again help focus information and enhance
understanding. We have added new Clinical Relevance boxes throughout the text to provide additional examples of how a
clinician can use the information in this text to understand a dysfunction or choose an intervention strategy.

■ Muscle Action tables introduce the discussion of muscle actions for each muscle. Actions of each muscle are now intro¬
duced in table format and include the conclusions drawn from the evidence regarding each action. The evidence is dis¬
cussed in detail after the table. This format allows the reader to identify at a glance which reported actions are supported
by evidence, which are refuted, and which remain controversial.

■ Examining the Forces boxes and Muscle Attachment boxes explain and highlight more advanced mathematical con¬
cepts and provide muscle innervation and attachment information, respectively. The Muscle Attachment boxes now also
include brief descriptions of palpation strategies.

■ New and updated artwork, including illustrations and photographs, have been created and revised specifically for this
text.

■ Updated references continue to lend current, evidence-based support to chapter content and direct the student to
further research resources

ANCILLARIES

■ Approximately 150 video clips provide dynamic illustrations of concepts discussed in the textbook and demonstrate
movement disorders that can occur as a result of impairments. The clips also include demonstrations of palpations of bony
landmarks for each anatomical region. A video icon is used throughout the text to identify concepts with related video
material. These added elements will help the reader integrate the relationships among structure, force, movement, and
function and provide examples for students and teachers to analyze and discuss.

■ Laboratory Manuals for both students and instructors continue to offer activities for students to enhance learning and
applications. The instructors’ laboratory manual includes solutions and brief discussions of most activities. The student
manual for Chapter 1 now has 10 additional problems for students to test their analytical skills. The solutions are provided
in the Instructors’ Manual.

■ The Instructors’ Guide is a chapter-by-chapter outline to assist instructors with preparing class lectures. This ancillary
has been updated to include the materials added to the revised chapters.

Contributors

PAUL F. BEATTIE, PHD, PT, OCS

Clinical Associate Professor
Program in Physical Therapy
Department of Exercise Science
School of Public Health
University of South Carolina
Columbia, SC

EMILY L. CHRISTIAN, PHD, PT

Restore Management Co, LLC
Pelham, AL

JULIE E. DONACHY, PHD, PT

Restore Management Co, LLC
Pelham, AL

Z. ANNETTE IGLARSH, PT, PHD, MBA

Chair and Professor
Department of Physical Therapy
University of the Sciences in Philadelphia
Philadelphia, PA

ANDREW R. KARDUNA, PHD

Assistant Professor

Department of Exercise and Movement Science
University of Oregon
Eugene, OR

MARGERY A. LOCKARD, PT, PHD

Clinical Associate Professor
Pathway to Health Professions Program
Drexel University
Philadelphia, PA

JOSEPH M. MANSOUR, PHD

Professor

Department of Mechanical and Aerospace Engineering
Case Western Reserve University
Cleveland, OH

THOMAS P. MAYHEW, PT, PHD

Associate Professor and Chair
Department of Physical Therapy
School of Allied Health Professions
Virginia Commonwealth University
Richmond, VA

STUART M. McGILL, PHD

Professor

Department of Spine Biomechanics
University of Waterloo
Waterloo, Canada

SUSAN R. MERCER, PHD, BPHTY (HON), FNZCP

Senior Lecturer

Department of Anatomy & Developmental Biology
The University of Queensland
Brisbane, Australia

PETER E. PIDCOE, PT, DPT, PHD

Associate Professor
Department of Physical Therapy
School of Allied Health Professions
Virginia Commonwealth University
Richmond, VA

NEAL PRATT, PHD, PT

Emeritus Professor of Rehabilitation Sciences
Drexel University
Philadelphia, PA

L. D. TIMMIE TOPOLESKI, PHD

Professor

Department of Mechanical Engineering
University of Maryland, Baltimore County
Baltimore, MD

Reviewers

ROSCOE C. BOWEN, PHD

Associate Professor

Campbellsville University

Campbellsville, KY

LEE N MARINKO, PT, OCS, FAAOMPT

Clinical Assistant Professor

Boston University

Sargent College of Health and Rehabilitative Sciences
Boston, MA

BETH KIPPING DESCHENES, PT, MS, OCS

Clinical Assistant Professor

Department of Physical Therapy

UT Southwestern Medical Center at Dallas

Dallas, TX

PATRICIA ANN McGINN, PHD, ATC, CSCS, LAT

Assistant Professor of Athletic Training

Nova Southeastern University

Ft. Lauderdale, FL

JEFF LYNN, PHD

Assistant Professor

Slippery Rock University

Slippery Rock, PA

MARCIA MILLER SPOTO, PT, DC, OCS

Associate Professor

Nazareth College of Rochester

Rochester, NY

CORRIE A. MANCINELLI, PT, PHD

Associate Professor

West Virginia University School of Medicine
Morgantown, WV

KEITH SPENNEWYN, MS

Department Head

Globe University

Minneapolis, MN

ROBIN MARCUS, PT, PHD, OCS

Assistant Professor

University of Utah

Salt Lake City, UT

Foreword

This new edition of Kinesiology: The Mechanics and
Pathomechanics of Human Movement is a very timely arrival!
Hardly a day goes by without a newspaper or magazine arti¬
cle extolling the values of exercise as a regular and enduring
part of daily activity Exercise can only become a sustained
part of daily activity if it does not cause injury, but any exer¬
cise regimen creates the potential for injury to the muscu¬
loskeletal system. A challenge of exercise is finding the right
balance between activity that enhances tissue health versus
that which injures tissues. Optimizing the precision of move¬
ment is the key to achieving this balance. A clear understand¬
ing of the precision of movement and its contributing factors
requires a thorough knowledge of kinesiology. In the field of
physical therapy the focus is on movement and movement-
related dysfunctions or impairments; thus kinesiology is the
science that provides physical therapy’s major foundation.

Since the first kinesiological texts were published, the
depth of material has grown immensely. Although knowledge
in the fields of kinesiology, pathokinesiology, and kine-
siopathology has increased substantially since the first kinesi¬
ological texts were published, the changes that may come
from this new knowledge are not always reflected in clinical
practice. All physical therapy students study kinesiology dur¬
ing their education, but the information is often not retained
for application in the clinic, nor is it expanded by additional
study. The emphasis on functional performance, prompted in
part by reimbursement criteria, has detracted from improv¬
ing the depth of knowledge of impairments underlying the
compromises in performance. Similarly, focus on treatment
techniques applied to conditions without attention to the
underlying movement dysfunction or the techniques’ effects
compromises patient care and the status of the profession.
Kinesiology: The Mechanics and Pathomechanics of Human
Movement is a wonderful example of both the breadth and
depth of the expansion of kinesiological knowledge and the
clinical application of that knowledge. How fortunate for
rehabilitation specialists that the information they need is
readily available in this text.

A strong emphasis is currently being placed on evidence-
based practice. It may be a long time before even a small
percentage of our treatment procedures have met level 3 evi¬
dence, and all evidence is only the best available at a given
time. In the fields of physical therapy, occupational therapy,
and athletic training, evidence for the best treatments and the
methods used when addressing a person’s movement will
change, just as it has for the physicians’ treatments of meta¬
bolic, cardiopulmonary, or neurological conditions. The
improvement in the diagnosis and treatment of any body sys¬
tem is based on increased understanding of mechanisms and
pathophysiology. We therefore have to continue to pursue an

understanding of the mechanisms related to any body system
that therapists, trainers, and exercise instructors address dur¬
ing their care, especially the systems involved in movement
and its dysfunctions or impairments.

Kinesiology: The Mechanics and Pathomechanics of
Human Movement is truly unique in its thoroughly
researched approach and provides convincing evidence to
debunk old and inaccurate theories. This scientific approach
to the clinical application of biomechanics means the infor¬
mation contained in this text is of particular importance to
anyone involved in a rehabilitation specialty.

I have had many opportunities to interact with therapists
and trainers around the world, and I am struck by how few
have a thorough understanding of basic kinesiology, such as an
understanding of scapulohumeral rhythm, lumbar range of
motion, and the determinants of gait. Coupled with this is a
deficiency in the ability to observe movement and recognize
subtle deviations and variations in normal patterns. I attribute
this to an emphasis on both passive techniques and the lack of
a strong basic knowledge about exercise program development.

Kinesiology: The Mechanics and Pathomechanics of
Human Movement is an invaluable resource for people seek¬
ing to correct this deficiency. Physical therapists must clearly
demonstrate themselves to be movement experts and diagnos¬
ticians of movement dysfunctions. This book is the key to
acquiring the knowledge that will enable students and practi¬
tioners to achieve the required level of expertise. The essentials
of kinesiology are all present in this text. The basics of tissue
biomechanics are well explained by experts in the field. The
specifics of muscle action and the biomechanical basis of
those actions, kinetics, and kinematics for each region of the
body are analyzed and well described. This text is suited for
readers who are interested in acquiring either an introductory
and basic knowledge as well as those who want to increase
their understanding of the more detailed and biomechanically
focused knowledge of kinesiology. In selecting the authors for
each chapter, Dr. Oatis has chosen well; each expert has pro¬
vided an excellent and relevant presentation of normal and
abnormal kinesiology. This text is a must-have textbook for
every student and reference for every practitioner of physical
therapy, as well any other rehabilitation specialist or indi¬
vidual desiring knowledge of the biomechanical aspects of the
human movement system.

Shirley Sahrmann, PT, PhD, FAPTA
Professor of Physical Therapy
Departments of Physical Therapy, Neurology,
Cell Biology, and Physiology
Washington University School of Medicine—St. Louis

St. Louis, Missouri

VII

Preface from the First Edition

A clinician in rehabilitation treats patients with many and var¬
ied disorders, and usually goals of intervention include
improving the individuals ability to move [1]. Physical thera¬
pists prevent, identify, assess, and correct or alleviate move¬
ment dysfunction [3]. Similarly, occupational therapists work
to restore or optimize “purposeful actions.” Optimizing
movement and purposeful actions and treating movement
disorders require a firm foundation in kinesiology, the sci¬
entific study of movement of the human body or its parts.

To evaluate and treat movement disorders effectively, the
clinician must address two central questions: What is
required to perform the movement, and what effects does the
movement produce on the individual? This textbook will help
the reader develop knowledge and enhance skills that permit
him or her to answer these questions.

Two general factors govern the movement of a structure:
the composition of the structure and the forces applied to it. A
central principle in kinesiology is that the form or shape of a
biological structure is directly influenced by its function. In
fact, the relationship among movement, structure, and force is
multidirectional. It is a complex interdependent relationship
in which structure influences a body’s movement, its move¬
ment affects the forces applied to the structure, and the forces,
in turn, influence the structure (see Figure). For example, the
unique structure of the tibiofemoral joint produces complex
three-dimensional motion of the knee, leading to intricate
loading patterns (forces) on the tibia and femur that may con¬
tribute to structural changes and osteoarthritis later in life.
Similarly, the type of movement or function and its intensity
influence the forces sustained by a region, which in turn alter
the structure. For instance, as muscles hypertrophy with exer¬
cise and activity, they stimulate bone growth at their attach¬
ment sites; physically active individuals tend to have more
robust skeletons than inactive people.

Function is interdependent among structure, force, and
movement, so that structure affects both the forces on a struc¬
ture and the motion of that structure. Similarly, forces on a
structure influence its structure and movement. Finally,
movement affects both the structure and the forces sustained
by the structure.

An abnormal structure produces abnormal movement as
well as abnormal forces on a structure, contributing to further
alterations in structure. Excessive anteversion of the hip, for
example, leads to torsional deformities at the knee, which
may contribute to abnormal loading patterns at the hip as well
as at the knee or foot, ultimately leading to pain and dysfunc¬
tion. The clinician needs to understand these interrelation¬
ships to design and direct the interventions used to restore or
optimize human movement.

An understanding of the relationship among structure,
force, and movement requires a detailed image of the struc¬
ture of a region as well as a grasp of the basic laws of motion
and the basic material properties of the tissues comprising the
musculoskeletal system. The purposes of this textbook are to:

• Provide a detailed analysis of the structures of the muscu¬
loskeletal system within individual functional regions.

• Discuss how the structures affect function within each

region.

• Analyze the forces sustained at the region during function.

This textbook will help the clinician recognize the rela¬
tionships between form and function, and abnormal structure
and dysfunction. This foundation should lead to improved
evaluation and intervention approaches to movement dys¬
function.

This book uses terminology that is standard within health
care to describe elements of disablement based on a classifi¬
cation of function developed by the World Health
Organization (WHO) and others. In this classification scheme,
a disease process, or pathology, alters a tissue, which then
changes a structures function, producing an impairment.
The impairment may cause an individual to have difficulty
executing a task or activity, producing an activity limitation
or dysfunction. When the dysfunction alters the individual’s
ability to participate in life functions, the individual has par¬
ticipation restriction or a disability [2,4].

Although improving activity and participation are usually
the primary objectives in rehabilitation, the WHO model of
disease provides a vision of how clinicians can improve function
not only by intervening directly at the level of the dysfunction,
but also by addressing the underlying impairments. By under¬
standing the detailed structure and precise movement of an
anatomical region, the clinician has tools to identify impair¬
ments and their influence on function and devise interventions

viii

PREFACE FROM THE FIRST EDITION

that focus on the mechanism producing the dysfunction. This
textbook allows the reader to examine normal structure and
function and then consider the impairments that result from
alterations in structure at anatomical regions, thus providing
insight into the dysfunctions that may follow. For example, by
understanding the normal glenohumeral rhythm of the shoul¬
der the clinician can appreciate the consequences of an unsta¬
ble scapula during arm trunk elevation and develop strategies
to improve function.

The needs of individual readers vary, and I have designed
this book to allow readers to use it in ways that best meet their
needs. Part I of this textbook introduces the reader to the
principles of biomechanics and material properties and then
examines the material properties of the major component tis¬
sues of the musculoskeletal system: bone, muscle, cartilage,
and dense connective tissue. These chapters lay out the bio¬
mechanical foundation for examining human movement.
Parts II through IV explore movement by anatomical region,
investigating the detailed structure of the bones, joints, and
muscles in that region and examining how their structures
influence its movement. The ability of the region to sustain
the forces generated during movements and function also is
explored in Parts II through IV. Finally, Part V considers more
global, or whole-body, movements, specifically posture and
locomotion.

Detailed discussions of forces at joints are presented in
separate chapters so that readers may access that information
as they need it. Although many readers will be interested in
delving into the mathematical analyses used to determine
forces on joint structures, others will find little need for such
detail. The actual calculations are set apart in boxes that
accompany the chapters. Conclusions based on the calcula¬
tions are contained within the chapters’ text so that readers
can read the chapter and glean the essential information and
return to the specific analyses as desired.

Conclusions regarding structure, function, and dysfunc¬
tion in this text are based on the best available evidence, and

each chapter is extensively referenced using both current and
classic resources. I believe that the clinician is best equipped
to evaluate current practice and to debunk long-held beliefs
by having access to the classic resources that have established
a concept and to the most current evidence that confirms or
refutes standard impressions. Throughout this book, common
clinical beliefs that are unsupported—or actually refuted—by
strong evidence are explicitly identified so that the clinician
hones the skill of healthy skepticism and develops the practice
of demanding the evidence to support a concept. The book
also notes where the evidence is meager or inconclusive or
the conclusion is the opinion of the author. A strong,
evidence-based background in kinesiology also helps develop
clinician scholars who can contribute to our understanding of
movement and movement dysfunction through the systematic,
thoughtful observation and reporting of clinical phenomena.
Despite the comment made long ago by a fellow graduate stu¬
dent that there was “nothing left to learn in gross anatomy,”
there is much to be learned yet in functional anatomy and
kinesiology.

Today one can discover the errors of yesterday.

And tomorrow obtain a new light on what seemed
certain today.

—Prayer of Maimonides

References

1. Guide to Physical Therapy Practice, 2nd ed. Phys Ther 2001; 81:
6-746.

2. Nagi SZ: An epidemiology of disability among adults in the
United States. Milbank Mem Fund Q Health Soc 1976; 54:
439-467.

3. Sahrmann SA: Moving precisely? Or taking the path of least
resistance? Twenty-ninth Mary McMillan Lecture. Phys Ther
1998; 78: 1208-1218.

4. www3.who.int/icf/icftemplate.cfm?myurlintroduction.html%
20&mytitle=Introduction

Preface to the Second Edition

The purposes of Kinesiology: The Mechanics and Pathome-
chanics of Human Movement were articulated in the
Preface to the first edition and are unchanged in this new
edition. They are to:

• Provide a detailed analysis of the structures of the muscu¬
loskeletal system within individual functional regions

• Discuss how the structures affect function within each
region

• Analyze the forces sustained at the region during function

If the purposes of this textbook remain the same what is the
point of a second edition? Is there a compelling reason to
undertake a second edition? As I considered these questions,
I recalled the conclusion of the Preface to the first edition, the
Prayer of Maimonides. That prayer provides the impetus for
a second edition:

Today one can discover the errors of yesterday ,

And tomorrow obtain a new light on what seemed certain
today.

—Prayer of Maimonides

A new edition provides the opportunity to correct “the errors
of yesterday” and offer suggestions on where to look for “new
light” tomorrow. The primary goal of this second edition of
Kinesiology: The Mechanics and Pathomechanics of Human
Movement is to ensure that it reflects the most current under¬
standing of kinesiology and biomechanics science. Chapter
contributors reviewed the literature and updated chapters
wherever necessary. We have also explicitly identified new
knowledge and emerging areas of study or controversy. These
additions will aid the reader in the quest for principles that
underlie and drive best practice in the fields of rehabilitation
and exercise.

The second purpose of the revision has been to build on
the strong clinical links available from the first edition. We
provide additional examples of the interrelationships among
structure, force, and movement and their effects on function
(as described in the Preface from the First Edition). A strong
and clear understanding of the interdependency of these fac¬
tors allows practitioners to recognize abnormal movement and
systematically search for and identify the underlying pathome¬
chanics. By recognizing underlying mechanisms, practitioners
will be able to intervene at the level of the mechanism to nor¬
malize or remediate dysfunction. One means to demonstrate
these relationships and enhance the applicability of the infor¬
mation contained in this textbook is through the use of the
Clinical Relevance boxes. We have added more of these boxes

throughout the text to provide more direct examples
of how structure, function, and forces affect movement,
demonstrating ways a clinician can use the information in
this text to understand a dysfunction or choose an interven¬
tion strategy.

Updating the content to reflect new information and
current research and practice also has helped us build on
those clinical links. Although little has changed in the bio¬
mechanical principles outlined in Chapter 1, Dr. Karduna
has clarified certain aspects of analysis. He has also provided
additional “practice problems” for students to access on the
associated website. Drs. Topoleski and Mansour have reor¬
ganized their chapters on basic material properties
(Chapters 2) and on the properties of bone and cartilage
(Chapters 3 and 5) and added clinical examples to help
readers see the connections between engineering princi¬
ples and the clinical issues important to practitioners. Dr.
Lockard has included emerging evidence regarding tissue
response to activity gleaned from new research technolo¬
gies (Chapter 6). Drs. Pidcoe and McGill reorganized their
chapters to help the readers utilize the information and
understand the evidence (Chapters 27, 33, and 34). Dr.
McGill also updated evidence and addressed some con¬
temporary issues. Drs. Beattie and Christian reviewed the
literature to ensure that their chapters reflected an under¬
standing based on the most current scientific evidence
(Chapters 32, 35, and 36).

The final purpose of producing a second edition was to
provide dynamic illustrations of the principles and con¬
cepts presented in this text. We all know that a “picture is
worth a thousand words,” but kinesiology is the study of
movement, and video provides benefits not found in still
images. Recognizing that movement is the central theme
of kinesiology and biomechanics, we have produced a
DVD with approximately 150 video clips to provide action
videos of concepts discussed in the textbook and demon¬
strate movement disorders that can occur as the result of
impairments. These will help the reader integrate the rela¬
tionships among structure, force, movement, and function
and provide examples for students and teachers to analyze
and discuss.

In the second edition we have also slightly modified the
format of the chapters that address muscles of specific
regions. The format change will help the reader quickly rec¬
ognize the strength of the evidence supporting the identified
muscle actions. Actions of each muscle are now presented in
table format, which includes the conclusions drawn from the
evidence regarding each action.

PREFACE TO THE SECOND EDITION

These changes have been made because I firmly believe
that people with musculoskeletal disorders or those who
want to optimize their already normal function require
the wisdom and guidance of individuals who have a clear,
evidence-based understanding of musculoskeletal structure
and function, a firm grasp of biomechanical principles, and

the ability to observe and document movement. This sec¬
ond edition is meant to help further advance the ability of
exercise and rehabilitation specialists to serve this role.

— Carol A. Oatis

Acknowledgments

Completion of this second edition required the work and
commitment of several individuals. Revising chapters is often
less “fun” than writing the original piece. I want to thank the
contributing authors for undertaking the project willingly and
enthusiastically. Their efforts to identify changes in knowl¬
edge or perspective help ensure that this textbook remains at
the forefront of kinesiologic science. I am also grateful to the
contributors to the functional region chapters who also
reviewed and revised their chapters as necessary.

An extensive team at Lippincott Williams & Wilkins has
provided invaluable developmental, managerial, and techni¬
cal support throughout the project. Peter Sabatini,
Acquisitions Editor, helped me articulate my goals for the
project and provided me the freedom and support to under¬
take new approaches. Andrea Klingler, Managing Editor, has
been patient, persistent, and enthusiastic—frequently at the
same time! She has held me to deadlines while simultaneously
acknowledging the exciting challenges we were facing. She
also brought together an exceptional team of talented indi¬
viduals to produce the accompanying DVD. This team
included Freddie Patane, Art Director (video); Ben Kitchens,
Director of Photography (video); and his wonderful crew:
Andrew Wheeler, Gaffer, David Mattson, Grip, and Kevin
Gallagher, Grip. These people made the production of the
DVD not only exciting and successful but wonderfully fun.
They provided extraordinary artistic insight and technical
skill, but always remained focused on the learning objectives
for each clip, wanting to ensure that each clip met the needs
of the student and teacher. Brett McNaughton, the Art
Director of Photography/Illustration, coordinated the models
for video and photography and coordinated the photography
shoot. His wisdom and experience helped make the whole
process of producing videos and photography smooth and
successful. I also want to thank all of the people who were
filmed or photographed, including students and people with
disabilities, who willingly participated so others could learn.

I am indebted to three people who provided clinical
insight, technical and organizational assistance, and moral
support during the production of the DVD. Amy Miller,
DPT, assisted with setting up the EMGs and monitored

those activities. Her understanding of EMG and kinesiology
was invaluable for the production of these clips. Additionally
her enthusiasm for the entire project was a constant support.
Marianne Adler, PT, worked with me to write the scripts for
the video clips. Her understanding of the subject matter and
her logical thinking yielded clear, concise scripts to describe
the action and articulate the principles to be learned.
Michele Stake, MS, DPT, coordinated the overall video and
photography program, from finding and scheduling patients
to helping to direct each shoot and ensuring that the video or
photograph told the story we intended. Without these
womens commitment to the project and their friendship, I
could not have completed the job.

I had the wonderful good fortune of working again with
Kim Battista, the talented artist who created the artwork in
the original textbook. She contributed new art with the same
skill and artistry as in the first edition. Similarly, Gene Smith,
the photographer for the first edition, returned to work with
me again and has provided new photographs that, like the ones
in the first text, “tell the story.” These two artists together have
created images that bring kinesiology and biomechanics alive.
Jennifer Clements, Art Director, oversaw the entire art pro¬
gram and coordinated the production of new and revised art.
She was wonderfully patient and receptive to the little
“tweaks” we requested to optimize the art program.

I am grateful to Jon McCaffrey, DPT, who provided essen¬
tial help in tracking down references as well as proofreading
and offering helpful editorial suggestions, and to Luis Lopez,
SPT, who played a pivotal role in final manuscript production.
Again I wish to thank the Department of Physical Therapy
and Arcadia University for their support during this process.
I am particularly grateful for the support provided by
Margaret M. Fenerty, Esq., who listened to my fears, tolerated
my stress, and encouraged my efforts.

Finally, I wish to thank all the students and colleagues who
have used the first edition and provided insightful feedback
and valuable suggestions that have informed this new edition.
They helped identify errors, offered new ideas, and graciously
told me what worked. I look forward to hearing new ideas and
suggestions for this second edition.

XII

Contents

Contributors.v

Reviewers .vi

Foreword .vii

Preface from the First Edition.viii

Preface to the Second Edition.x

Acknowledgments .xii

PART I: BIOMECHANICAL PRINCIPLES_ 1

1 Introduction to Biomechanical Analysis .3

2 Mechanical Properties of Materials .21

3 Biomechanics of Bone.36

4 Biomechanics of Skeletal Muscle.45

5 Biomechanics of Cartilage.69

6 Biomechanics of Tendons and Ligaments.84

7 Biomechanics of Joints .103

PART II: KINESIOLOGY OF THE UPPER EXTREMITY _ 117

Unit 1: Shoulder Unit: The Shoulder Complex.118

8 Structure and Function of the Bones and Joints of the Shoulder Complex.120

9 Mechanics and Pathomechanics of Muscle Activity at the Shoulder Complex .150

10 Analysis of the Forces on the Shoulder Complex during Activity .188

Unit 2: Elbow Unit.197

11 Structure and Function of the Bones and Noncontractile Elements of the Elbow.198

12 Mechanics and Pathomechanics of Muscle Activity at the Elbow.219

13 Analysis of the Forces at the Elbow during Activity .243

Unit 3: Wrist and Hand Unit.253

14 Structure and Function of the Bones and Joints of the Wrist and Hand.255

15 Mechanics and Pathomechanics of the Muscles of the Forearm .294

16 Analysis of the Forces at the Wrist during Activity.331

17 Mechanics and Pathomechanics of the Special Connective Tissues in the Hand.339

18 Mechanics and Pathomechanics of the Intrinsic Muscles of the Hand .351

19 Mechanics and Pathomechanics of Pinch and Grasp.370

PART III: KINESIOLOGY OF THE HEAD AND SPINE _ 389

Unit 4: Musculoskeletal Functions within the Head.390

20 Mechanics and Pathomechanics of the Muscles of the Face and Eyes.391

21 Mechanics and Pathomechanics of Vocalization .412

22 Mechanics and Pathomechanics of Swallowing.423

23 Structure and Function of the Articular Structures of the TMJ.438

24 Mechanics and Pathomechanics of the Muscles of the TMJ.452

25 Analysis of the Forces on the TMJ during Activity .466

xiii

XIV

CONTENTS

Unit 5: Spine Unit.472

26 Structure and Function of the Bones and Joints of the Cervical Spine . 473

27 Mechanics and Pathomechanics of the Cervical Musculature. 492

28 Analysis of the Forces on the Cervical Spine during Activity. 511

29 Structure and Function of the Bones and Joints of the Thoracic Spine. 520

30 Mechanics and Pathomechanics of the Muscles of the Thoracic Spine. 538

31 Loads Sustained by the Thoracic Spine . 556

32 Structure and Function of the Bones and Joints of the Lumbar Spine . 563

33 Mechanics and Pathomechanics of Muscles Acting on the Lumbar Spine . 587

34 Analysis of the Forces on the Lumbar Spine during Activity. 601

35 Structure and Function of the Bones and Joints of the Pelvis. 620

36 Mechanics and Pathomechanics of Muscle Activity in the Pelvis . 654

37 Analysis of the Forces on the Pelvis during Activity . 676

PART IV: KINESIOLOGY OF THE LOWER EXTREMITY _ 685

Unit 6: Hip Unit.686

38 Structure and Function of the Bones and Noncontractile Elements of the Hip. 687

39 Mechanics and Pathomechanics of Muscle Activity at the Hip. 705

40 Analysis of the Forces on the Hip during Activity. 727

Unit 7: Knee Unit.737

41 Structure and Function of the Bones and Noncontractile Elements of the Knee. 738

42 Mechanics and Pathomechanics of Muscle Activity at the Knee. 767

43 Analysis of the Forces on the Knee during Activity . 791

Unit 8: Ankle and Foot Unit.806

44 Structure and Function of the Bones and Noncontractile Elements of the Ankle and Foot Complex . 807

45 Mechanics and Pathomechanics of Muscle Activity at the Ankle and Foot. 838

46 Analysis of the Forces on the Ankle and Foot during Activity. 865

PART V: POSTURE AND GAIT _ 873

47 Characteristics of Normal Posture and Common Postural Abnormalities . 875

48 Characteristics of Normal Gait and Factors Influencing It. 892

Index

919

PART

Biomechanical Principles

Chapter 1: Introduction to Biomechanical Analysis

Chapter 2: Mechanical Properties of Materials

Chapter 3: Biomechanics of Bone

Chapter 4: Biomechanics of Skeletal Muscle

Chapter 5: Biomechanics of Cartilage

Chapter 6: Biomechanics of Tendons and Ligaments

Chapter 7: Biomechanics of Joints

PARTI

T his part introduces the reader to the basic principles used throughout this book to understand the structure
and function of the musculoskeletal system. Biomechanics is the study of biological systems by the applica¬
tion of the laws of physics. The purposes of this part are to review the principles and tools of mechanical
analysis and to describe the mechanical behavior of the tissues and structural units that compose the musculoskeletal
system. The specific aims of this part are to

■ Review the principles that form the foundation of biomechanical analysis of rigid bodies
■ Review the mathematical approaches used to perform biomechanical analysis of rigid bodies
■ Examine the concepts used to evaluate the material properties of deformable bodies

■ Describe the material properties of the primary biological tissues constituting the musculoskeletal system: bone,
muscle, cartilage, and dense connective tissue
■ Review the components and behavior of joint complexes

By having an understanding of the principles of analysis in biomechanics and the biomechanical properties of the pri¬
mary tissues of the musculoskeletal system, the reader will be prepared to apply these principles to each region of the
body to understand the mechanics of normal movement at each region and to appreciate the effects of impairments
on the pathomechanics of movement.

CHAPTER

Introduction to Biomechanical
Analysis

ANDREW R. KARDUNA, PH.D.

CHAPTER CONTENTS

MATHEMATICAL OVERVIEW.4

Units of Measurement.4

Trigonometry .4

Vector Analysis .5

Coordinate Systems .7

FORCES AND MOMENTS.7

Forces.8

Moments.8

Muscle Forces .10

STATICS.11

Newton's Laws .11

Solving Problems.11

Simple Musculoskeletal Problems.12

Advanced Musculoskeletal Problems .14

KINEMATICS.17

Rotational and Translational Motion .17

Displacement Velocity, and Acceleration.17

KINETICS.18

Inertial Forces .18

Work, Energy, and Power.19

Friction.20

SUMMARY.20

JHk Ithough the human body is an incredibly complex biological system composed of trillions of cells, it is subject
to the same fundamental laws of mechanics that govern simple metal or plastic structures. The study of the
Jmwk response of biological systems to mechanical forces is referred to as biomechanics. Although it wasn't rec¬
ognized as a formal discipline until the 20th century, biomechanics has been studied by the likes of Leonardo da Vinci,
Galileo Galilei, and Aristotle. The application of biomechanics to the musculoskeletal system has led to a better under¬
standing of both joint function and dysfunction, resulting in design improvements in devices such as joint arthroplasty
systems and orthotic devices. Additionally, basic musculoskeletal biomechanics concepts are important for clinicians
such as orthopaedic surgeons and physical and occupational therapists.

Part I I BIOMECHANICAL PRINCIPLES

Biomechanics is often referred to as the link between structure and function. While a therapist typically evaluates a
patient from a kinesiologic perspective, it is often not practical or necessary to perform a complete biomechanical
analysis. However, a comprehensive knowledge of both biomechanics and anatomy is needed to understand how the
musculoskeletal system functions. Biomechanics can also be useful in a critical evaluation of current or newly proposed
patient evaluations and treatments. Finally, a fundamental understanding of biomechanics is necessary to understand
some of the terminology associated with kinesiology (e.g., torque, moment, moment arms).

The purposes of this chapter are to

■ Review some of the basic mathematical principles used in biomechanics

■ Describe forces and moments

■ Discuss principles of static analysis

■ Present the basic concepts in kinematics and kinetics

The analysis is restricted to the study of rigid bodies. Deformable bodies are discussed in Chapters 2-6. The material
in this chapter is an important reference for the force analysis chapters throughout the text.

MATHEMATICAL OVERVIEW

This section is intended as a review of some of the basic math¬
ematical concepts used in biomechanics. Although it can be
skipped if the reader is familiar with this material, it would be
helpful to at least review this section.

Units of Measurement

The importance of including units with measurements cannot
be emphasized enough. Measurements must be accompanied
by a unit for them to have any physical meaning. Sometimes,
there are situations when certain units are assumed. If a cli¬
nician asks for a patients height and the reply is “5-6,” it can
reasonably be assumed that the patient is 5 feet, 6 inches tall.
However, that interpretation would be inaccurate if the
patient was in Europe, where the metric system is used.
There are also situations where the lack of a unit makes a
number completely useless. If a patient was told to perform a
series of exercises for two, the patient would have no idea if
that meant two days, weeks, months, or even years.

The units used in biomechanics can be divided into two
categories. First, there are the four fundamental units of
length, mass, time, and temperature, which are defined on
the basis of universally accepted standards. Every other unit
is considered a derived unit and can be defined in terms of
these fundamental units. For example, velocity is equal to
length divided by time and force is equal to mass multiplied
by length divided by time squared. A list of the units needed
for biomechanics is found in Table 1.1.

Trigonometry

Since angles are so important in the analysis of the muscu¬
loskeletal system, trigonometry is a very useful biomechanics
tool. The accepted unit for measuring angles in the clinic is
the degree. There are 360° in a circle. If only a portion of a
circle is considered, then the angle formed is some fraction of
360°. For example, a quarter of a circle subtends an angle of
90°. Although in general, the unit degree is adopted for this
text, angles also can be described in terms of radians. Since
there are 2ic radians in a circle, there are 57.3° per radian.
When using a calculator, it is important to determine if it is set

TABLE 1.1: Units Used in Biomechanics

Quantity

Metric

British

Conversion

Length

meter (m)

foot (ft)

1 ft = 0.3048 m

Mass

kilogram (kg)

slug

1 slug = 14.59 kg

Time

second (s)

1 s = 1 s

Temperature

Celsius (°C)

Fahrenheit (°F)

°F = (9/5) X °C + 32°

Force

newton (N = kg x m/s 2 )

pound (lb = slug x ft/s 2 )

1 |b = 4.448 N

Pressure

pascal (Pa = N/m 2 )

pounds per square inch (psi = lb/in 2 )

1 psi = 6895 Pa

Energy

joule (J = N x m)

foot pounds (ft-lb)

1 ft-lb = 1.356 J

Power

watt (W = J/s)

horsepower (hp)

1 hp = 7457 W

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

Figure 1.1: Basic trigonometric relationships. These are some of
the basic trigonometric relationships that are useful for biome¬
chanics. A. A right triangle. B. A general triangle.

to use degrees or radians. Additionally, some computer pro¬
grams, such as Microsoft Excel, use radians to perform
trigonometric calculations.

Trigonometric functions are very useful in biomechanics
for resolving forces into their components by relating angles to
distances in a right triangle (a triangle containing a 90° angle).
The most basic of these relationships (sine, cosine, and tan¬
gent) are illustrated in Figure 1.1 A. A simple mnemonic to
help remember these equations is sohcahtoa—sine is the
opposite side divided by the hypotenuse, cosine is the adja¬
cent side divided by the hypotenuse, and tangent is the
opposite side divided by the adjacent side. Although most cal¬
culators can be used to evaluate these functions, some impor¬
tant values worth remembering are

sin (0°) = 0, sin (90°) = 1 (Equation 1.1)

cos (0°) = 1, cos (90°) = 0 (Equation 1.2)

tan (45°) = 1 (Equation 1.3)

Additionally, the Pythagorean theorem states that for a right
triangle, the sum of the squares of the sides forming the right
angle equals the square of the hypotenuse (Fig. 1.1A).
Although less commonly used, there are also equations that
relate angles and side lengths for triangles that do not contain
a right angle (Fig. 1.1 B).

Vector Analysis

Biomechanical parameters can be represented as either
scalar or vector quantities. A scalar is simply represented by
its magnitude. Mass, time, and length are examples of scalar
quantities. A vector is generally described as having both
magnitude and orientation. Additionally, a complete
description of a vector also includes its direction (or sense)
and point of application. Forces and moments are examples
of vector quantities. Consider the situation of a 160-lb man

sitting in a chair for 10 seconds. The force that his weight is
exerting on the chair is represented by a vector with magni¬
tude (160 lb), orientation (vertical), direction (downward),
and point of application (the chair seat). However, the time
spent in the chair is a scalar quantity and can be represented
by its magnitude (10 seconds).

To avoid confusion, throughout this text, bolded notation is
used to distinguish vectors (A) from scalars (B). Alternative
notations for vectors found in the literature (and in class¬
rooms, where it is difficult to bold letters) include putting a
line under the letter (A), a line over the letter A, or an arrow
over the letter A. The magnitude of a given vector (A) is
represented by the same letter, but not bolded (A).

By far, the most common use of vectors in biomechanics is
to represent forces, such as muscle, joint reaction and resist¬
ance forces. These vectors can be represented graphically with
the use of a line with an arrow at one end (Fig. 1.2A). The
length of the line represents its magnitude, the angular posi¬
tion of the line represents its orientation, the location of the
arrowhead represents its direction, and the location of the line
in space represents its point of application. Alternatively, this
same vector can be represented mathematically with the use
of either polar coordinates or component resolution.
Polar coordinates represent the magnitude and orientation of
the vector directly. In polar coordinates, the same vector
would be 5 N at 37° from horizontal (Fig. 1.2 B). With compo¬
nents, the vector is resolved into its relative contributions from
both axes. In this example, vector A is resolved into its
components: A x = 4 N and A y = 3 N (Fig. 1.2C). It is often
useful to break down vectors into components that are aligned
with anatomical directions. For instance, the x and y axes may
correspond to superior and anterior directions, respectively.

application

A = 5 N
0 = 37°

C. Components

Ax = 4 N
Ay = 3 N

Figure 1.2: Vectors. A. In general, a vector has a magnitude,
orientation, point of application, and direction. Sometimes the
point of application is not specifically indicated in the figure. B.

A polar coordinate representation. C. A component representation.

Part I I BIOMECHANICAL PRINCIPLES

Although graphical representations of vectors are useful for
visualization purposes, analytical representations are more
convenient when adding and multiplying vectors.

Note that the directional information (up and to the right)
of the vector is also embedded in this information. A vector
with the same magnitude and orientation as the vector repre¬
sented in Figure 1.2C, but with the opposite direction (down
and to the left) is represented by A x = — 4 N and A y = — 3 N,
or 5 N at 217°. The description of the point-of-application
information is discussed later in this chapter.

VECTOR ADDITION

When studying musculoskeletal biomechanics, it is common to
have more than one force to consider. Therefore, it is important
to understand how to work with more than one vector. When
adding or subtracting two vectors, there are some important
properties to consider. Vector addition is commutative:

A + B = B + A (Equation 1.4)

A — B = A+(—B) (Equation 1.5)

Vector addition is associative:

A + (B + C) = (A + B) + C (Equation 1.6)

Unlike scalars, which can just be added together, both the
magnitude and orientation of a vector must be taken into
account. The detailed procedure for adding two vectors
(A + B = C) is shown in Box 1.1 for the graphical, polar coor¬
dinate, and component representation of vectors. The graphi¬
cal representation uses the “tip to tail” method. The first step
is to draw the first vector, A. Then the second vector, B, is
drawn so that its tail sits on the tip of the first vector. The vec¬
tor representing the sum of these two vectors (C) is obtained
by connecting the tail of vector A and the tip of vector B. Since
vector addition is commutative, the same solution would have
been obtained if vector B were the first vector. When using
polar coordinates, the vectors are drawn as in the graphical
method, and then the law of cosines is used to determine the
magnitude of C and the law of sines is used to determine the
direction of C (see Fig 1.1 for definitions of these laws).

For the component resolution method, each vector is bro¬
ken down into its respective x and y components. The compo¬
nents represent the magnitude of the vector in that direction.
The x and y components are summed:

C x — A x + B x (Equation 1.7)

C Y = A y + B y (Equation 1.8)

The vector C can either be left in terms of its components, C x
and C Y , or be converted into a magnitude, C, using the
Pythagorean theorem, and orientation, 0, using trigonometry.
This method is the most efficient of the three presented and
is used throughout the text.

VECTOR MULTIPLICATION

Multiplication of a vector by a scalar is relatively straightfor¬
ward. Essentially, each component of the vector is individually
multiplied by the scalar, resulting in another vector. For

example, if the vector in Figure 1.2 is multiplied by 5, the
result is A x = 5 X 4 N = 20 N and A y = 5 X 3 N = 15 N.
Another form of vector multiplication is the cross product,
in which two vectors are multiplied together, resulting in

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

Figure 1.3: Vector cross product. C is shown as the cross product
of A and B. Note that A and B could be any two vectors in the
indicated plane and C would still have the same orientation.

another vector (C = A X B). The orientation of C is such that
it is mutually perpendicular to A and B. The magnitude of C
is calculated as C = A X B X sin (0), where 0 represents the
angle between A and B, and X denotes scalar multiplication.
These relationships are illustrated in Figure 1.3. The cross
product is used for calculating joint torques later in this
chapter.

Coordinate Systems

A three-dimensional analysis is necessary for a complete rep¬
resentation of human motion. Such analyses require a coordi¬
nate system, which is typically composed of anatomically
aligned axes: medial/lateral (ML), anterior/posterior (AP),
and superior/inferior (SI). It is often convenient to consider
only a two-dimensional, or planar, analysis, in which only two
of the three axes are considered. In the human body, there
are three perpendicular anatomical planes, which are
referred to as the cardinal planes. The sagittal plane is
formed by the SI and AP axes, the frontal (or coronal)
plane is formed by the SI and ML axes, and the transverse
plane is formed by the AP and ML axes (Fig. 1.4).

The motion of any bone can be referenced with respect to
either a local or global coordinate system. For example, the
motion of the tibia can be described by how it moves with
respect to the femur (local coordinate system) or how it moves
with respect to the room (global coordinate system). Local
coordinate systems are useful for understanding joint function
and assessing range of motion, while global coordinate systems
are useful when functional activities are considered.

Most of this text focuses on two-dimensional analyses, for
several reasons. First, it is difficult to display three-
dimensional information on the two-dimensional pages of a
book. Additionally, the mathematical analysis for a three-
dimensional problem is very complex. Perhaps the most
important reason is that the fundamental biomechanical prin¬
ciples in a two-dimensional analysis are the same as those in a
three-dimensional analysis. It is therefore possible to use a
simplified two-dimensional representation of a three-
dimensional problem to help explain a concept with minimal
mathematical complexity (or at least less complexity).

Superior

Figure 1.4: Cardinal planes. The cardinal planes, sagittal,
frontal, and transverse, are useful reference frames in a three-
dimensional representation of the body. In two-dimensional
analyses, the sagittal plane is the common reference frame.

FORCES AND MOMENTS

The musculoskeletal system is responsible for generating
forces that move the human body in space as well as prevent
unwanted motion. Understanding the mechanics and patho-
mechanics of human motion requires an ability to study the

Part I I BIOMECHANICAL PRINCIPLES

forces and moments applied to, and generated by, the body or
a particular body segment.

Forces

The reader may have a conceptual idea about what a force is but
find it difficult to come up with a formal definition. For the pur¬
poses of this text, a force is defined as a “push or pull” that
results from physical contact between two objects. The only
exception to this rule that is considered in this text is the force
due to gravity, in which there is no direct physical contact
between two objects. Some of the more common force genera¬
tors with respect to the musculoskeletal system include muscles/
tendons, ligaments, friction, ground reaction, and weight.

A distinction must be made between the mass and the
weight of a body. The mass of an object is defined as the
amount of matter composing that object. The weight of an
object is the force acting on that object due to gravity and is
the product of its mass and the acceleration due to gravity
(g = 9.8 m/s 2 ). So while an objects mass is the same on Earth
as it is on the moon, its weight on the moon is less, since the
acceleration due to gravity is lower on the moon. This dis¬
tinction is important in biomechanics, not to help plan a trip
to the moon, but for ensuring that a unit of mass is not treated
as a unit of force.

As mentioned previously, force is a vector quantity with
magnitude, orientation, direction, and a point of application.
Figure 1.5 depicts several forces acting on the leg in the frontal
plane during stance. The forces from the abductor and adduc¬
tor muscles act through their tendinous insertions, while the
hip joint reaction force acts through its respective joint center
of rotation. In general, the point of application of a force (e.g.,
tendon insertion) is located with respect to a fixed point on a
body, usually the joint center of rotation. This information is
used to calculate the moment due to that force.

Moments

In kinesiology, a moment (M) is typically caused by a force
(F) acting at a distance (r) from the center of rotation of a seg¬
ment. A moment tends to cause a rotation and is defined by
the cross product function: M = r X F. Therefore, a moment
is represented by a vector that passes through the point of
interest (e.g., the center of rotation) and is perpendicular to
both the force and distance vectors (Fig. 1.6). For a two-
dimensional analysis, both the force and distance vectors are
in the plane of the paper, so the moment vector is always
directed perpendicular to the page, with a line of action
through the point of interest. Since it has only this one orien¬
tation and line of action, a moment is often treated as a scalar
quantity in a two-dimensional analysis, with only magnitude
and direction. Torque is another term that is synonymous
with a scalar moment. From the definition of a cross product,
the magnitude of a moment (or torque) is calculated as M =
r X F X sin (0). Its direction is referred to as the direction in
which it would tend to cause an object to rotate (Fig. 1.7A).

Figure 1.5: Vectors in anatomy. Example of how vectors can be
combined with anatomical detail to represent the action of
forces. Some of the forces acting on the leg are shown here.

Although there are several different distances that can be
used to connect a vector and a point, the same moment
is calculated no matter which distance is selected (Fig.
1.7 B). The distance that is perpendicular to the force vector
is referred to as the moment arm (MA) of that force (r 2 in
Fig. 1.72$). Since the sine of 90° is equal to 1, the use of a
moment arm simplifies the calculation of moment to

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

Figure 1.6: Three-dimensional moment analysis. The moment
acting on the elbow from the force of the biceps is shown as
a vector aligned with the axis of rotation. F, force vector; r,
distance from force vector to joint COR; M, moment vector.

M = MA X F. The moment arm can also be calculated from
any distance as MA = r X sin (0). Additionally, although there
are four separate angles between the force and distance vec¬
tors, all four angles result in the same moment calculation
(Fig. 1.7 C).

The examples in Figures 1.6 and 1.7 have both force and
moment components. However, consider the situation in
Figure 1.8A. Although the two applied forces create a
moment, they have the same magnitude and orientation but
opposite directions. Therefore, their vector sum is zero. This
is an example of a force couple. A pure force couple results
in rotational motion only, since there are no unbalanced
forces. In the musculoskeletal system, all of these conditions
are seldom met, so pure force couples are rare. In general,
muscles are responsible for producing both forces and
moments, thus resulting in both translational and rotational
motion. However, there are examples in the human body in
which two or more muscles work in concert to produce a
moment, such as the upper trapezius and serratus anterior
(Fig. 1.8 B). Although these muscles do not have identical
magnitudes or orientations, this situation is frequently
referred to as a force couple.

Figure 1.7: Continued

Part I I BIOMECHANICAL PRINCIPLES

Figure 1.7: Two-dimensional moment analysis. A. Plantar flexion
moment created by force at the Achilles tendon. B. Note that no
matter which distance vector is chosen, the value for the moment
is the same. C. Also, no matter which angle is chosen, the value
for the sine of the angle is the same, so the moment is the same.

EXAMINING THE FORCES BOX 1.2

MOMENT ARMS OF THE DELTOID (MA d )
AND THE SUPRASPINATUS (MA S )

Muscle Forces

A. Idealized

F 1

COR

Fl = -F 2 4 F 2

Figure 1.8: Force couples. Distinction between an idealized force
couple (A) and a more realistic one (B). Even though the scapular
example given is not a true force couple, it is typically referred
to as one. COR, center of rotation.

As mentioned previously, there are three important param¬
eters to consider with respect to the force of a muscle: ori¬
entation, magnitude, and point of application. With some
care, it is possible to measure orientation and line of action
from cadavers or imaging techniques such as magnetic res¬
onance imaging (MRI) and computed tomography (CT)
[1,3]. This information is helpful in determining the func¬
tion and efficiency of a muscle in producing a moment. As
an example, two muscles that span the glenohumeral joint,
the supraspinatus and middle deltoid, are shown in Box 1.2.
From the information provided for muscle orientation and
point of application in this position, the moment arm of the
deltoid is approximately equal to that of the supraspinatus,
even though the deltoid insertion on the humerus is much
farther away from the center of rotation than the
supraspinatus insertion.

Clinical Relevance

MUSCLE FORCES: In addition to generating moments that
are responsible for angular motion (rotation), muscles also
produce forces that can cause linear motion (translation).

(< continued )

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

(Continued)

This force can be either a stabilizing or a destabilizing force.
For example\ since the supraspinatus orientation shown in
Box 1.2 is primarily directed medially , it tends to pull the
humeral head into the glenoid fossa. This compressive force
helps stabilize the glenohumeral joint. However ; since the
deltoid orientation is directed superiorly , it tends to produce
a destabilizing force that may result in superior translation
of the humeral head.

These analyses are useful, since they can be performed even
if the magnitude of a muscle s force is unknown. However, to
understand a muscle s function completely, its force magni¬
tude must be known. Although forces can be measured with
invasive force transducers [13], instrumented arthroplasty
systems [6], or simulations in cadaver models [9], there are
currently no noninvasive experimental methods that can be
used to measure the in vivo force of intact muscles.
Consequently, basic concepts borrowed from freshman
physics can be used to predict muscle forces. Although they
often involve many simplifying assumptions, such methods
can be very useful in understanding joint mechanics and are
presented in the next section.

STATICS

Statics is the study of the forces acting on a body at rest or
moving with a constant velocity. Although the human body is
almost always accelerating, a static analysis offers a simple
method of addressing musculoskeletal problems. This analy¬
sis may either solve the problem or provide a basis for a more
sophisticated dynamic analysis.

Newton s Laws

Since the musculoskeletal system is simply a series of objects
in contact with each other, some of the basic physics princi¬
ples developed by Sir Isaac Newton (1642-1727) are useful.
Newtons laws are as follows:

First law: An object remains at rest (or continues moving
at a constant velocity) unless acted upon by an unbal¬
anced external force.

Second law: If there is an unbalanced force acting on an
object, it produces an acceleration in the direction of
the force, directly proportional to the force (f = ma).

Third law: For every action (force) there is a reaction
(opposing force) of equal magnitude but in the oppo¬
site direction.

From Newtons first law, it is clear that if a body is at rest,
there can be no unbalanced external forces acting on it. In
this situation, termed static equilibrium, all of the external
forces acting on a body must add (in a vector sense) to zero.
An extension of this law to objects larger than a particle is that

the sum of the external moments acting on that body must
also be equal to zero for the body to be at rest. Therefore, for
a three-dimensional analysis, there are a total of six equations
that must be satisfied for static equilibrium:

2F X = 0 2F y = 0 2F Z = 0

2M X = 0 2M y = 0 2M Z = 0 (Equation 1.9)

For a two-dimensional analysis (in the x-y plane), there are
only two in-plane force components and one perpendicular
moment (torque) component:

2F X = 0 2F y = 0 2M Z = 0 (Equation 1.10)

Under many conditions, it is reasonable to assume that all
body parts are in a state of static equilibrium and these three
equations can be used to calculate some of the forces acting
on the musculoskeletal system. When a body is not in static
equilibrium, Newtons second law states that any unbalanced
forces and moments are proportional to the acceleration of
the body. That situation is considered later in this chapter.

Solving Problems

A general approach used to solve for forces during static equi¬
librium is as follows:

Step 1 Isolate the body of interest.

Step 2 Sketch this body and all external forces (referred
to as a free body diagram).

Step 3 Sum the forces and moments equal to zero.

Step 4 Solve for the unknown forces.

As a simple example, consider the two 1-kg balls hanging
from strings shown in Box 1.3. What is the force acting on the
top string? Although this is a very simple problem that can be
solved by inspection, a formal analysis is presented. Step 1 is
to sketch the entire system and then place a dotted box

EXAMINING THE FORCES BOX 1.3

A FREE BODY DIAGRAM

£F y = 0
T - F - F = 0
T = 2F = 2 (1 ON)
T = 20N

Free body diagram

Part I I BIOMECHANICAL PRINCIPLES

around the body of interest. Consider a box that encompasses
both balls and part of the string above the top one, as shown
in Box 1.3.

Proceeding to step 2, a free body diagram is sketched. As
indicated by Newtons first law, only external forces are con¬
sidered for these analyses. For this example, everything inside
the dotted box is considered part of the body of interest.
External forces are caused by the contact of two objects, one
inside the box and one outside the box. In this example, there
are three external forces: tension in the top string and the
weight of each of the balls.

Why is the tension on the top string considered an exter¬
nal force, but not the force on the bottom string? The reason
is that the tension on the top string is an external force (part
of the string is in the box and part is outside the box), and the
force on the bottom string is an internal force (the entire
string is located inside the box). This is a very important dis¬
tinction because it allows for isolation of the forces on specific
muscles or joints in the musculoskeletal system.

Why is the weight of each ball considered an external
force? Although gravity is not caused by contact between two
objects, it is caused by the interaction of two objects and is
treated in the same manner as a contact force. One of the
objects is inside the box (the ball) and the other is outside the
box (the Earth). In general, as long as an object is located
within the box, the force of gravity acting on it should be con¬
sidered an external force.

Why is the weight of the string not considered an external
force? To find an exact answer to the problem, it should be
considered. However, since its weight is far less than that of
the balls, it is considered negligible. In biomechanical analy¬
ses, assumptions are often made to ignore certain forces, such
as the weight of someone’s watch during lifting.

Once all the forces are in place, step 3 is to sum all the
forces and moments equal to zero. There are no forces in
the x direction, and since all of the forces pass through the
same point, there are no moments to consider. That leaves
only one equation: sum of the forces in the y direction equal
to zero. The fourth and final step is to solve for the unknown
force. The mass of the balls is converted to force by multiply¬
ing by the acceleration of gravity. The complete analysis is
shown in Box 1.3.

Simple Musculoskeletal Problems

Although most problems can be addressed with the above
approach, there are special situations in which a problem is
simplified. These may be useful both for solving problems
analytically and for quick assessment of clinical problems
from a biomechanical perspective.

LINEAR FORCES

The simplest type of system, linear forces, consists of forces
with the same orientation and line of action. The only things
that can be varied are the force magnitudes and directions.
An example is provided in Box 1.3. Notice that the only

equation needed is summing the forces along the y axis equal
to zero. When dealing with linear forces, it is best to align
either the x or y axis with the orientation of the forces.

PARALLEL FORCES

A slightly more complicated system is one in which all the
forces have the same orientation but not the same line of
action. In other words, the force vectors all run parallel to
each other. In this situation, there are still only forces along
one axis, but there are moments to consider as well.

LEVERS

A lever is an example of a parallel force system that is very
common in the musculoskeletal system. Although not all
levers contain parallel forces, that specific case is focused on
here. A basic understanding of this concept allows for a rudi¬
mentary analysis of a biomechanical problem with very little
mathematics.

A lever consists of a rigid body with two externally applied
forces and a point of rotation. In general, for a musculoskele¬
tal joint, one of the forces is produced by a muscle, one force
is provided by contact with the environment (or by gravity),
and the point of rotation is the center of rotation of the
joint. The two forces can either be on the same side
or different sides of the center of rotation (COR).

If the forces are on different sides of the COR, the system
is considered a first class lever. If the forces are on the same
side of the COR and the external force is closer to the COR
than the muscle force, it is a second class lever. If the forces
are on the same side of the COR and the muscle force is
closer to the COR than the external force, it is a third class
lever. There are several cases of first class levers; however,
most joints in the human body behave as third class levers.
Second class levers are almost never observed within the
body. Examples of all three levers are given in Figure 1.9.

If moments are summed about the COR for any lever, the
resistive force is equal to the muscle force times the ratio of
the muscle and resistive moment arms:

F R = F M x (MA m /MA r ) (Equation 1.11)

The ratio of the muscle and resistive moment arms
(MA m /MA r ) is referred to as the mechanical advan¬
tage of the lever. Based on this equation and the def¬
inition of levers, the mechanical advantage is greater than one
for a second class lever, less than one for a third class lever,
and has no restriction for a first class lever. A consequence of
this is that since most joints behave as third class levers, mus¬
cle forces are greater than the force of the resistive load they
are opposing. Although this may appear to represent an inef¬
ficient design, muscles sacrifice their mechanical advantage
to produce large motions and high-velocity motions. This
equation is also valid in cases where the two forces are not
parallel, as long as their moment arms are known. The effects
of a muscles moment arm on joint motion is discussed in
Chapter 4.

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

Figure 1.9: Classification of lever systems. Examples of the three different classes of levers, where F is the exerted force, R is the reaction
force, and COR is the center of rotation. Most musculoskeletal joints behave as third class levers. A. First class lever. B. Second class
lever. C. Third class lever.

Center of Gravity and Stability

Another example of a parallel force system is the use of the
center of gravity to determine stability The center of grav¬
ity of an object is the point at which all of the weight of that
body can be thought to be concentrated, and it depends on a
body’s shape and mass distribution. The center of gravity of
the human body in the anatomical position is approximately
at the level of the second sacral vertebra [8]. This location
changes as the shape of the body is altered. When a person
bends forward, his or her center of gravity shifts anteriorly
and inferiorly. The location of the center of gravity is also
affected by body mass distribution changes. For
example, if a person were to develop more leg mus¬
cle mass, the center of mass would shift inferiorly
The location of a person s center of gravity is important in
athletics and other fast motions because it simplifies the use of
Newtons second law. More important from a clinical point of
view is the effect of the center of gravity on stability. For

motions in which the acceleration is negligible, it can be shown
with Newtons first law that the center of gravity must be con¬
tained within a person s base of support to maintain stability.

Consider the situation of a person concerned about falling
forward. Assume for the moment that there is a ground reac¬
tion force at his toes and heel. When he is standing upright,
his center of gravity is posterior to his toes, so there is a coun¬
terclockwise moment at his toes (Fig. 1.10A). This is a stable
position, since the moment can be balanced by the ground
reaction force at his heel. If he bends forward at his hips to
touch the ground and leans too far forward, his center of grav¬
ity moves anterior to his toes and the weight of his upper body
produces a clockwise moment at his toes (Fig. 1.10 B). Since
there is no further anterior support, this moment is unbal¬
anced and the man will fall forward. However, if in addition
to hip flexion he plantarflexes at his ankles while keeping his
knee straight, he is in a stable position with his center of grav¬
ity posterior to his toes (Fig. 1.10C).

Part I I BIOMECHANICAL PRINCIPLES

Figure 1.10: Center of gravity. For the man in the figure to main¬
tain his balance, his center of gravity must be maintained within
his base of support. This is not a problem in normal standing
(A). When he bends over at the waist, however, his center of
gravity may shift anterior to the base of support, creating an
unstable situation (B). The man needs to plantarflex at the
ankles to maintain his balance (C).

Advanced Musculoskeletal Problems

One of the most common uses of static equilibrium applied to
the musculoskeletal system is to solve for unknown muscle
forces. This is a very useful tool because as mentioned above,
there are currently no noninvasive experimental methods that
can be used to measure in vivo muscle forces. There are
typically 3 types of forces to consider in a musculoskeletal
problem: (a) the joint reaction force between the two articu¬
lar surfaces, (b) muscle forces and (c) forces due to the body’s
interaction with the outside world. So how many unknown
parameters are associated with these forces? To answer this,
the location of all of the forces with their points of application
must be identified. For the joint reaction force nothing else is
known, so there are two unknown parameters: magnitude and
orientation. The orientation of a muscle force can be meas¬
ured, so there is one unknown parameter, magnitude. Finally,
any force interaction with the outside world can theoretically
be measured, possibly with a handheld dynamometer, force
plate, or by knowing the weight of the segment, so there are
no unknown parameters (Table 1.2) [5,8].

Consequently, there are two unknown parameters for the
joint reaction force and one unknown parameter for each mus¬
cle. However, there are only three equations available from a
two-dimensional analysis of Newtons first law. Therefore, if
there is more than one muscle force to consider, there are more
unknown parameters than available equations. This situation is
referred to as statically indeterminate, and there are an infi¬
nite number of possible solutions. To avoid this problem, only
one muscle force can be considered. Although this is an over¬
simplification of most musculoskeletal situations, solutions
based on a single muscle can provide a general perspective of

TABLE 1.2: Body Segment Parameters [12]

Mass (% of Total

Body Weight)

Location of the

Center of Mass
(% of Limb Segment

Length from

Proximal End)

Radius of Gyration

(% of Limb Segment Length

from Proximal End)

Head and neck

8.1

100.0 a

11.6

Trunk

49.7

50.0

Upper extremity

5.0

53.0

64.5

Arm

2.8

43.6

54.2

Forearm and hand

2.2

68.2

82.7

Forearm

1.6

43.0

52.6

Hand

0.6

50.6

58.7

Lower extremity

16.1

44.7

56.0

Thigh

10.0

43.3

54.0

Leg and foot

6.1

60.6

73.5

Leg

4.7

43.3

52.8

Foot

1.5

50.0

69.0

a Measured from C7-T1 to ear.
NA, not available.

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

the requirements of a task. Options for solving the statically
indeterminate problem are briefly discussed later.

FORCE ANALYSIS WITH A SINGLE MUSCLE

There are additional assumptions that are typically made to
solve for a single muscle force:

• Two-dimensional analysis

• No deformation of any tissues

• No friction in the system

• The single muscle force that has been selected can be con¬
centrated in a single line of action

• No acceleration

The glenohumeral joint shown in Box 1.4 is used as an exam¬
ple to help demonstrate the general strategy for approaching
these problems. Since only one muscle force can be consid¬
ered, the supraspinatus is chosen for analysis. The same gen¬
eral approach introduced earlier in this chapter for address¬
ing a system in static equilibrium is used.

Step one is to isolate the body of interest, which for this
problem is the humerus. In step two, a free body diagram is
drawn, with all of the external forces clearly labeled: the
weight of the arm (F G ), the supraspinatus force (F s ), and the
glenohumeral joint reaction force (Fj) in Box 1.4. Note that
external objects like the scapula are often included in the free
body diagram to make the diagram complete. However, the
scapula is external to the analysis and is only included for con¬
venience (which is why it is drawn in gray). It is important to
keep track of which objects are internal and which ones are
external to the isolated body.

The next step is to sum the forces and moments to zero
to solve for the unknown values. Since the joint reaction
force acts through the COR, a good strategy is to start by
summing the moments to zero at that point. This effective¬
ly eliminates the joint reaction force from this equation
because its moment arm is equal to zero. The forces along
the x and y axes are summed to zero to find those compo¬
nents of the joint reaction force. The fourth and final step is
to solve for the unknown parameters in these three equa¬
tions. The details of these calculations are given in Box 1.4.
In this example, the magnitude of the joint reaction force is
180 N directed laterally and 24 N directed superiorly.
Those components represent the force of the scapula acting
on the humerus. Newtons third law can then be used to find
the force of the humerus acting on the scapula: 180 N medi¬
al and 24 N inferior, with a total magnitude of 182 N.

Note that the muscle force is much larger than the weight
of the arm. This is expected, considering the small moment
arm of the muscle compared with the moment arm of the
force due to gravity. While this puts muscles at a mechanical
disadvantage for force production, it enables them to amplify
their motion. For example, a 1-cm contraction of the
supraspinatus results in a 7.5-cm motion at the hand. This is
discussed in more detail in Chapter 4.

The problem can be solved again by considering the mid¬
dle deltoid instead of the supraspinatus. For those conditions,

Box 1.5 shows that the deltoid muscle force is 200 N and the
force of the humerus acting on the scapula is 115 N directed
medially and 140 N directed superiorly, with a total magni¬
tude of 181 N. Notice that although the force required of
each muscle and the total magnitude of the joint reaction
force is similar for both cases, the deltoid generates a much
higher superior force and the supraspinatus generates a much
higher medial force.

Although the magnitude of the muscle and joint reaction
forces in this example is similar, this might not be the case

Part I I BIOMECHANICAL PRINCIPLES

EXAMINING THE FORCES BOX 1.5

STATIC EQUILIBRIUM EQUATIONS
CONSIDERING ONLY THE DELTOID MUSCLE

IM = 0 (at COR)

(F d )(MA d ) - (F G )(R G )sin(0) = 0
F d = (24 N)(30 cm)sin (30°) = 200 N
1.8 cm

IF x = 0

F d cos(P) + F jx = 0

F JX = -200 cos (55°)= -115 N

XF y = 0

F D sin((3) + F g + Fj Y = 0

Fj Y = - F g - F d sin(p) = - (-24) -200 sin (55°) = -140 N
Fj =4^JX+ ^ = ^(-115 Nf + (-140 Nf = 181 N

for other problems. Consequently, an alternative approach
is to simply document the joint internal moment (generated
by the joint muscles and ligamants) that is necessary to bal¬
ance the joint external moment (generated by the external
forces, gravity in this example). Therefore, in both cases
(supraspinatus and deltoid), the external moment is equal
to a 360 N cm (from F G • R G • sin[0]) adduction moment.
Consequently, the internal moment is a 360 N cm abduction
moment.

Clinical Relevance

SUPRASPINATUS AND DELTOID MUSCLE FORCES:

A clinical application of these results is that under normal
conditions the supraspinatus serves to maintain joint stability
with its medially directed force. However ; if its integrity is
compromised ' as occurs with rotator cuff diseaseand the
deltoid plays a larger role, then there is a lower medial sta¬
bilizing force and a higher superior force that may cause
impingement of the rotator cuff in the subacromial region.

The analysis presented above serves as a model for analyzing
muscle and joint reaction forces in subsequent chapters.
Although some aspects of the problem will clearly vary from
joint to joint, the basic underlying method is the same.

FORCE ANALYSIS WITH MULTIPLE MUSCLES

Although most problems addressed in this text focus on solv¬
ing for muscle forces when only one muscle is taken into con¬
sideration, it would be advantageous to solve problems in
which there is more than one muscle active. However such
systems are statically indeterminate. Additional information is
needed regarding the relative contribution of each muscle to
develop an appropriate solution.

One method for analyzing indeterminate systems is the
optimization method. Since an indeterminate system
allows an infinite number of solutions, the optimization
approach helps select the “best” solution. An optimization
model minimizes some cost function to produce a single solu¬
tion. This function may be the total force in all of the muscles
or possibly the total stress (force/area) in all of the muscles.
While it might make sense that the central nervous system
attempts to minimize the work it has to do to perform a
function, competing demands of a joint must also be met. For
example, in the glenohumeral example above, it might be
most efficient from a force production standpoint to assume
that the deltoid acts alone. However, from a stability stand¬
point, the contribution of the rotator cuff is essential.

Another method for analyzing indeterminate systems is
the reductionist model in which a set of rules is applied for
the relative distribution of muscle forces based on elec¬
tromyographic (EMG) signals. One approach involves devel¬
oping these rules on the basis of the investigators subjective
knowledge of EMG activity, anatomy, and physiological con¬
straints [4]. Another approach is to have subjects perform iso¬
metric contractions at different force levels while measuring
EMG signals and to develop an empirical relationship
between EMG and force level [2,7]. Perhaps the most com¬
mon approach is based on the assumption that muscle force
is proportional to its cross-sectional area and EMG level. This
method has been attempted for many joints, such as the
shoulder [10], knee, and hip. One of the key assumptions in
all these approaches is that there is a known relationship
between EMG levels and force production.

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

KINEMATICS

Until now, the focus has been on studying the static forces
acting on the musculoskeletal system. The next section deals
with kinematics, which is defined as the study of motion
without regard to the forces that cause that motion. As with
the static force analysis, this section is restricted to two-
dimensional, or planar, motion.

Rotational and Translational Motion

Pure linear, or translatory, motion of an entire object occurs
when all points on that object move the same distance (Fig.
1.11 A). However, with the possible exception of passive manip¬
ulation of joints, pure translatory motion does not often occur
at musculoskeletal articulations. Instead, rotational motion is
more common, in which there is one point on a bone that
remains stationary (the COR), and all other points trace arcs of
a circle around this point (Fig. 1.1 LB). For three-dimensional
motion, the COR would be replaced by an axis of rotation,
and there could also be translation along this axis.

Consider the general motion of a bone moving from an ini¬
tial to a final position. The rotational component of this motion
can be measured by tracking the change in orientation of a line
on the bone. Although there are an infinite number of lines to
choose from, it turns out that no matter which line is selected,
the amount of rotation is always the same. Similarly, the trans¬
lational component of this motion can be measured by track¬
ing the change in position of a point on the bone. In this case,
however, the amount of translatory motion is not the same
for all points. In fact, the displacement of a point increases
linearly as its distance from the COR increases (Fig. 1.1 IB).
Therefore, from a practical standpoint, if there is any rotation
of a bone, a description of joint translation or displacement
must refer to a specific point on the bone.

Consider the superior/inferior translation motion of the
humerus in Figure 1.12A, which is rotated 90°. Point 1 repre¬
sents the geometric center of the humeral head and does not
translate from position 1 to 2. However, point 2 on the articu¬
lar surface of the humeral head translates inferiorly. The
motion in Figure 1.12B is similar, except now point 1 translates

Figure 1.11: Translations and rotations. In biomechanics, motion
is typically described in terms of translations and rotations. A. In
translatory motion, all points on the object move the same
distance. B. In rotational motion, all points on the object revolve
around the center of rotation (CO/?), which is fixed in space.

Figure 1.12: Translations and rotations within a joint. For both of
these examples, it is fairly straightforward to describe the rota¬
tional motion of the humerus—it rotates 90°. However, transla¬
tional motion is more complicated, and it is important to refer to
a specific point. Consider the superior/inferior (SI) translation of
two points: point 1 is located at the center of the humeral head
and point 2 sits closer to the articular surface. A. The center of
the rotation of the motion is at point 1, so there is no translation
at point 1, but point 2 moves inferiorly. B. Point 1 moves superi¬
orly, and point 2 moves inferiorly.

superiorly, while point 2 still translates inferiorly. This exam¬
ple demonstrates how important the point of reference is
when describing joint translations.

Displacement, Velocity, and Acceleration

Both linear and angular displacements are measures of dis¬
tance. Position is defined as the location of a point or object
in space. Displacement is defined as the distance traveled
between two locations. For example, consider the knee joint
during gait. If its angular position is 10° of flexion at heel
strike and 70° of flexion at toe off, the angular displacement
from heel strike to toe off is 60° of flexion.

Change in linear and angular position (displacement) over
time is defined as linear and angular velocity, respectively.

Part I I BIOMECHANICAL PRINCIPLES

TABLE 1.3: Kinematic Relationships

Velocity

Acceleration

Position

Instantaneous

Average

Instantaneous

Average

Linear

_ dv

_ V 2 - v x

V dt

V —

*2 - h

3 3

h ~ h

Angular

&|§?

0 2 - 01

&!!“

0) 2 - (Ox

0) —

h ~ h

h ~ ti

q Time

Figure 1.13: Acceleration, velocity, and displacement. Schematic
representation of the motion of an object traveling at constant
acceleration. The velocity increases linearly with time, while the
position increases nonlinearly.

Finding the instantaneous velocity at any given point in time,
requires the use of calculus. Instantaneous velocity is defined
as the differential of position with respect to time. Average
velocity may be calculated by simply considering two separate
locations of an object and taking the change in its position and
dividing by the change in time ( Table 1.3). As the time inter¬
val becomes smaller and approaches zero, the average veloc¬
ity approaches the instantaneous velocity.

Similarly, changes in linear and angular velocity over time
are defined as linear and angular acceleration. Instantaneous
acceleration is defined as the differential of velocity with
respect to time. Average acceleration may be calculated by
simply considering two separate locations of an object and tak¬
ing the change in its velocity and dividing by the change in
time (Table 1.3). An example of the effect of constant acceler¬
ation on velocity and position is shown in Figure 1.13.

KINETICS

Until now, forces and motion have been discussed as separate
topics. Kinetics is the study of motion under the action of
forces. This is a very complex topic that is only introduced
here to give the reader some working definitions. The only
chapter in this text that deals with these terms in any detail is
Chapter 48 on gait analysis.

Inertial Forces

Kinematics and kinetics are bound by Newtons second law,
which states that the external force (f) on an object is propor¬
tional to the product of that objects mass (m) and linear
acceleration (a):

f = ma (Equation 1.12)

For conditions of static equilibrium, there are no external
forces because there is no acceleration, and the sum of the
external forces can be set equal to zero. However, when an
object is accelerating, the so-called inertial forces (due to
acceleration) must be considered, and the sum of the forces
is no longer equal to zero.

Consider a simple example of a linear force system in
which someone is trying to pick up a 20-kg box. If this is per¬
formed very slowly so that the acceleration is negligible, static
equilibrium conditions can be applied (sum of forces equal
zero), and the force required is 200 N ( Box 1.6). However, if
this same box is lifted with an acceleration of 5 m/s 2 , then the

Chapter 1 I INTRODUCTION TO BIOMECHANICAL ANALYSIS

sum of the forces is not equal to zero, and the force required
is 300 N (Box 1.6).

There is an analogous relationship for rotational motion, in
which the external moment (M) on an object is proportional
to that objects moment of inertia (I) and angular accelera¬
tion (a):

M = la (Equation 1.13)

Just as mass is a measure of a resistance to linear acceleration,
moment of inertia is a measure of resistance to angular accel¬
eration. It is affected both by the magnitude of all the point
masses that make up a body and the distance that each mass
is from the center of rotation (r) as follows:

I = 2mr 2 (Equation 1.14)

So the further away the mass of an point mass is from the cen¬
ter of rotation, the larger its contribution to the moment of
inertia. For example, when a figure skater is spinning and tucks
in her arms, she is moving more of her mass closer to the cen¬
ter of rotation, thus reducing her moment of inertia. The con¬
sequence of this action is that her angular velocity increases.

Although equation 1.14 can theoretically be used to calcu¬
late the moment of inertia of a segment, a more practical
approach is to treat the segment as one object with a single
mass (m) and radius of gyration (k):

I = mk 2 (Equation 1.15)

The radius of gyration of an object represents the distance
at which all the mass of the object would be concentrated in
order to have the same moment of inertia as the object
itself. The radius of gyration for various human body
segments is given in Table 1.2. An example of how to calcu¬
late a segment s moment of inertia about its proximal end is
presented in Box 1.7.

Work, Energy, and Power

Another combination of kinematics and kinetics comes in the
form of work, which is defined as the force required to move
an object a certain distance (work = force X distance). The
standard unit of work in the metric system is a joule (J; new¬
ton X meter). For example, if the 20-kg box in Box 1.6 is
lifted 1 m under static equilibrium conditions, the work done
is equal to 200 joules (200 N X 1 m). By analogy with
the analysis in Box 1.6, under dynamic conditions, the work
done is equal to 300 J (300 N X 1 m).

Power is defined as the rate that work is being done
(power = work/time). The standard unit of power is a watt
(W; watt = newton X meter/second). Continuing with the
above example, if the box were lifted over a period of 2 sec¬
onds, the average power would be 100 W under static condi¬
tions and 150 W under dynamic conditions. In practical
terms, the static lift is generating the same amount of power
needed to light a 100 W light bulb for 2 seconds.

The energy of a system refers to its capacity to perform
work. Energy has the same unit as work (J) and can be divided
into potential and kinetic energy. While potential energy
refers to stored energy, kinetic energy is the energy of motion.

Part I I BIOMECHANICAL PRINCIPLES

no motion velocity

F, < |X S N

F f = p k N

In In

Figure 1.14: Friction. A. Under static conditions (no motion), the
magnitude of the frictional force ( F f ) exerted on the box is the
same as the applied force (F) and cannot be larger than the coef¬
ficient of static friction (|jl s ) multiplied by the normal force (A/). If
the applied force exceeds the maximum static frictional force,
the box will move and shift to dynamic conditions. B. Under
dynamic conditions, the friction force is equal to the coefficient
of dynamic friction (|ji k ) multiplied by the normal force.

Friction

Frictional forces can prevent the motion of an object when
it is at rest and resist the movement of an object when it is
in motion. This discussion focuses specifically on Coulomb
friction, or friction between two dry surfaces [11]. Consider a
box with a weight (W) resting on the ground (Fig. 1.14). If a
force (F) applied along the x axis is equal to the frictional force
(F f ), the box is in static equilibrium. However, if the applied
force is greater than the frictional force, the box accelerates to
the right because of an unbalanced external force.

The frictional force matches the applied force until it
reaches a critical value, F = pyN, where N is the reaction
force of the floor pushing up on the box and p, s is the coeffi¬
cient of static friction. In this example, N is equal to the mag¬
nitude of the force due to the weight of the box. Once this
critical value is reached, there is still a frictional force, but it
is now defined by: F = |x k N, where p, k is the coefficient of
dynamic friction.

The values for the coefficient of friction depend on several
parameters, such as the composition and roughness of the
two surfaces in contact. In general, the dynamic coefficient of
friction is lower than the static coefficient of friction. As a con¬
sequence, it would take less force the keep the box in Figure
1.14 moving than it would take to start it moving.

SUMMARY

This chapter starts with a review of some important mathe¬
matical principles associated with kinesiology and proceeds to
cover statics, kinematics, and kinetics from a biomechanics
perspective. This information is used throughout the text for
analysis of such activities as lifting, crutch use, and single-limb
stance. The reader may find it useful to refer to this chapter
when these problems are addressed.

References

1. An KN, Takahashi K, Harrigan TP, Chao EY: Determination of
muscle orientations and moment arms. J Biomech Eng 1984;
106: 280-282.

2. Arwert HJ. de Groot J, Van Woensel WWLM, Rozing PM:
Electromyography of shoulder muscles in relation to force
direction. J Shoulder Elbow Surg 1997; 6: 360-370.

3. Chao EY, Lynch JD, Vanderploeg MJ: Simulation and anima¬
tion of musculoskeletal joint system. J Biomech Eng 1993; 115:
562-568.

4. Dempster WT: Space requirements of the seated operator. In:
Human Mechanics; Four Monographs Abridged AMRL-TDR-
63-123. Krogman WM, Johnston FE, eds. Wright-Patterson
Air Force Base, OH: Behavioral Sciences Laboratory, 6570th
Aerospace Medical Research Laboratories, Aerospace Medical
Division, Air Force Systems Command, 1963; 215-340.

5. Fuller JJ, Winters JM: Assessment of 3-D joint contact load
preditions during postural/stretching exercises in aged females.
Ann Biomed Eng 1993; 21: 277-288.

6. Krebs DE, Robbins CE, Lavine L, Mann RW: Hip biomechan¬
ics during gait. J Orthop Sports Phys Ther 1998; 28: 51-59.

7. Laursne B, Jensen BR, Nemeth G, Sjpgaard G: A model
predicting individual shoulder muscle forces based on relation¬
ship between electromyographic and 3D external forces in static
position. J Biomech 1998; 31: 731-739.

8. LeVeau BF: Williams and Lissners Biomechanics of Human
Motion, 3rd ed. Philadelphia: WB Saunders, 1992.

9. McMahon PJ, Debski RE, Thompson WO, et al.: Shoulder
muscle forces and tendon excursions during glenohumeral
abduction in the scapular plane. J Shoulder Elbow Surg 1995;
4: 199-208.

10. Poppen NK, Walker PS: Forces at the glenohumeral joint in
abduction. Clin Orthop 1978; 135: 165-170.

11. Stevens KK: Statics and Strength of Materials. Englewood
Cliffs, NJ: Prentice-Hall, 1987.

12. Winter DA: Biomechanics and Motor Control of Human
Movement, 3rd ed. Hoboken, NJ: John Wiley & Sons, 2005.

13. Xu WS, Butler DL, Stouffer DC, et al: Theoretical analysis of
an implantable force transducer for tendon and ligament struc¬
tures. J Biomech Eng 1992; 114: 170-177.

Musculoskeletal Biomechanics Textbooks

Bell F: Principles of Mechanics and Biomechanics. Cheltenham,
UK: Stanley Thornes Ltd, 1998.

Enoka R: Neuromechanics of Human Movement, 3rd ed.

Champaign, IL: Human Kinetics, 2002.

Hall S: Basic Biomechanics, 5th ed. Boston: WCB/McGraw-Hill,
2006.

Hamill J, Knutzen K. Biomechanical Basis of Human Movement,
2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2003.

Low J, Reed A: Basic Biomechanics Explained. Oxford, UK:
Butterworth-Heinemann 1996.

Lucas G, Cooke F, Friis, E: A Primer of Biomechanics. New York:
Springer, 1999.

McGinnis P: Biomechanics of Sports and Exercise. Champaign, IL:
Human Kinetics, 2005.

Nigg B, Herzog W: Biomechanics of the Musculo-skeletal System,
3rd ed. New York: John Wiley and Sons, 2007.

Nordin M, Frankel V: Basic Biomechanics of the Musculoskeletal
System, 3rd ed. Philadelphia: Lippincott Williams & Wilkins,
2001 .

Ozkaya N, Nordin M: Fundamentals of Biomechanics: Equilibrium,
Motion and Deformation, 2nd ed. New York: Springer, 1999.

CHAPTER

Mechanical Properties
of Materials

L.D. TIMMIE TOPOLESKl, PH.D.

CHAPTER CONTENTS

BASIC MATERIAL PROPERTIES.21

STRESS AND STRAIN.22

THE TENSION TEST.25

The Basics (Young's Modulus, Poisson's Ratio) .25

Load to Failure .27

MATERIAL FRACTURE .30

Fracture Toughness.30

Fatigue.31

LOADING RATE.31

BENDING AND TORSION .32

Bending .32

Torsion.33

SUMMARY .34

T here are over 1,000 types of properties (reactions to external stimuli) that describe a material's behavior. Since
all human tissue is composed of one material or another, the same material properties that describe materials
like steel, concrete, and rubber can be applied to the behavior of human tissue. Readers may have an intuitive
sense that different tissues behave differently; for example, skin and muscle seem to stretch or deform more easily
than bone. It is also easier to cut skin or muscle than to cut bone. Defining and measuring properties of different mate¬
rials quantifies the differences between materials (how much easier is it to cut muscle than bone?), and predicts how
a material will behave under a known environment (how much force is required to break a bone during, say, a skiing
accident?).

The purposes of this chapter are to

■ Familiarize readers with basic definitions of mechanics and materials terms
■ Describe some of the most useful and general properties of materials

■ Provide an appreciation of the clinical relevance of mechanical properties of biological tissues

BASIC MATERIAL PROPERTIES

Most people, regardless of their background, can name some
material properties. An initial thought might be weight.
Weight, however, is defined by the acceleration of a mass in

Earth s gravity So mass may be a better candidate for a mate¬
rial property But then someone may ask, which has more
mass, a kilogram of gold or a kilogram of paper? Well, a kilo¬
gram is a kilogram, so the mass of a kilogram of gold is the
same as the mass of a kilogram of paper. But it certainly takes

Part I I BIOMECHANICAL PRINCIPLES

less gold than paper to make up a kilogram. So, the mass of
an object depends on how much of the material there is.
Properties that depend on the amount of a material are called
extensive properties. Volume and internal energy are
other examples of extensive properties.

It is easier to compare the behavior of materials without
worrying about whether they are of the same mass or volume.
So properties are often normalized by dividing by the mass or
volume. For example, the mass/unit volume is the density of
a material. The density of gold is the same for an ounce, a
kilogram, or a stone of gold (a stone is an old British unit of
weight, equal to 14 pounds). Density is an example of an
intensive property, a property that does not depend on the
amount of a material. Many of the useful material properties
in biomechanics are intensive properties.

One of the most important properties of a material is the
materials strength. People are especially likely to wonder
about the strength of a material when crossing over a bridge,
for example. Perhaps every person who has ever lived has won¬
dered at some point “is this object strong enough?” meaning,
will the object in use break during the intended use? Some
materials seem to break readily and at inconvenient times. For
example, many people will remember when the tines from the
plastic fork broke at the picnic, when the tire blew out on the
highway, or when shoelaces snapped while being tied.
Engineers have devoted much time and effort to ensure that
materials do not break, especially when people s lives depend
on the integrity of materials (e.g., materials used in a bridge).

Many factors other than strength influence how and when
a material will break. A simple experiment with a paper clip
should convince the reader that material failure may occur
under conditions that are not initially obvious (Box 2.1).

Before considering the details of how strength is meas¬
ured, consider whether strength is an intensive or an exten¬
sive property. Can a thin strand of steel wire hold the same

EXAMINING THE FORCES BOX 2.1

THE PAPER CLIP EXPERIMENT

Most readers have experienced a remarkable phe¬
nomenon. "Uncoil" a common paper clip, and grab
both ends, say with a pair of pliers. You are unlikely
to break the paper clip, even if you pull with all of
your strength; yet if you bend the paper clip back
and forth, you will eventually break a piece of steel
with your bare hands! How is this possible? The
answer is that whether a material breaks depends
on the conditions applied to the material; pulling
the two ends of a paper clip creates different condi¬
tions from bending the paper clip back and forth.
The first relates to the tensile strength of the metal,
and the second to a form of fatigue strength; both
are discussed in detail soon.

EXAMINING THE FORCES BOX 2.2

EXPERIMENT TO TEST THE BREAKING
POINT OF WIRES OF DIFFERENT
THICKNESS

Perform a simple experiment to answer the ques¬
tion. For this experiment, we need wires of differ¬
ent diameters made from the same material. Most
hardware stores have some kind of wire, for exam¬
ple copper or lead, available in different diameters.
Lead solder is a good material to use, since it usually
breaks under moderate loads. Hang the wires from
a convenient ledge, and place the same weight (or
mass) on each of the wires. Next, add more weight
to each wire, keeping the weight on each wire
equal. Which wire breaks first? The thinnest, of
course, and you probably predicted it. The experi¬
ment demonstrates a very important fact: strength,
defined as the maximum weight a wire can hold
before breaking, is an extensive property.

weight as a thick beam? No, because strength is an extensive
property. Another simple experiment demonstrates that the
thickness of a wire, for example, influences when that wire
breaks (Box 2.2).

STRESS AND STRAIN

The results of the experiment in Box 2.2 present a problem.
It would be much more convenient if a material, whether it is
a type of steel or polymer, artificial or biological, had a single
value for strength; that is, if strength were an intensive prop¬
erty. To arrive at a definition of strength that is an intensive
property, some additional background is necessary, specifically
the definition and understanding of two concepts, stress and
strain. Long before the words were used to describe how stu¬
dents feel before final exams, stress and strain were used as
measures of a materials behavior. Most importantly, stress
and strain have distinct meanings; they are not synonyms and
cannot be used interchangeably. Stress is defined by units of
force/area, the same units used for pressure (e.g., pounds per
square inch, newtons per square meter). Stress is a measure
that is independent of the amount of a material. This simple
concept is incredibly useful. A 10-lb weight hanging on a
wire with a cross-sectional area of 0.1 in 2 produces a stress
inside the wire of 10 lb/0.1 in 2 (force/area), or 100 psi (pounds
per square inch; 1 psi is equal to about 6,900 N/m 2 ) (Fig. 2.1).
A thinner wire, for example one with a cross-sectional area of
0.05 in 2 , requires less applied force to sustain the same 100-psi
stress. The full calculation of stress in the thinner wire is
presented in Box 2.3. Note that stress is a measure of load (or
energy) that is in an object. Stress is similar to an internal
pressure in a solid material or a normalized load (or weight)

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

0.1 in 2

Figure 2.1: Stress in a simple bar is defined as the load (in this
case the weight of the monkey) divided by the cross-sectional
area of the bar (with units of psi, pounds per square inch).

on a material. While it is not a material property, stress has the
sense of being intensive, because it does not depend on the
amount of material.

Stress has a complementary measure, called strain. Just as
stress is like a force normalized by the cross-section (or
amount of material) of a wire, strain is like a normalized
stretch or displacement of a material (Box 2.4).

To define stress or strain more formally, imagine a cylinder
of any material under an applied load (or force). The length
and the diameter of the cylinder have been measured. The
cross-sectional area is easily calculated from the diameter.
The direction in which the cylinder is loaded is important;

EXAMINING THE FORCES BOX 2.3

CALCULATION OF STRESS IN A WIRE

How much weight must be added to a wire with a
cross-section of 0.05 in 2 to produce a stress of 100 psi?

Set up an equation:

100 lb _ X
in 2 0.05 in 2

where X is the necessary weight, in pounds. As
noted in Chapter 1, it is always important to keep
track of units when performing any mechanics cal¬
culations. Readers who do not think units are
important should not have any trouble giving the
author a $20 bill, and accepting 20 cents in return!
After all, 20 is 20, if the units are disregarded.
Solving the equation, X is equal to 5 lb. Therefore,
to achieve 100 psi of stress in a 0.1-in 2 wire, use 10
lb of weight, but in a 0.05-in 2 wire use only 5 lb of
weight.

pushing along the axis of the cylinder is called compression,
or in other words, the cylinder is under a compressive
load (Fig. 2.2A). Pulling the cylinder is called tension,
or the cylinder is under a tensile load (Fig. 2.2 B).

Knowing the force applied to the cylinder, the stress in the
cylinder is calculated as

CT = A

(Equation 2.1)

where a is a generally accepted symbol for stress, F is the
applied load, and A is the cross-sectional area. For any force

EXAMINING THE FORCES BOX 2.4

EXPERIMENT TO ASSESS THE STRAIN IN
RUBBER BANDS OF DIFFERENT LENGTHS

Take rubber bands of the same material and thick¬
ness but of different lengths, and place on the same
convenient ledge as in Box 2.2. Hang the same size
weight from each. Which stretches the farthest?

The longest rubber band. Think of it this way: sup¬
pose that the rubber bands, pulled by the weight in
the experiment, stretch by 25%. A 1-in rubber band
stretches to 1.25 in, adding 0.25 in, but the 10-in
rubber band stretches to 12.5 in, adding 2.5 in. The
absolute stretch, or gain in length, depends on
the original length of the material. But, you say, the
original premise was that each rubber band stretched
25% of its original length, so on a percentage basis,
the relative stretch is the same.

I BIOMECHANICAL PRINCIPLES

Part I

► <

Figure 2.2: A. Compression. As the
elephants push on the bone, the
bone is subjected to compression.

B. Tension. As the elephants pull on
the bone, the bone is subjected to
tension.

F, if the stress is calculated on the basis of the original cross-
sectional area (call it A 0 ), then the stress is called the engi¬
neering stress. Why is this important? It turns out that
when a material is in tension, the diameter decreases (this is
easy to see in a rubber cylinder), and if the material is in
compression, the diameter increases (the cylinder bulges).
The more you pull (or push), the more the diameter
decreases (or increases). For many metals and other materi¬
als, it is difficult to detect the squeeze or bulge, and a dif¬
ference of a few square micrometers in area doesn’t change
the value of stress much. Thus, it may not be necessary to
measure the cylinders diameter for every force. For other
materials, such as rubber, the bulge can be a significant per¬
centage of change, and so it is important to know the exact
cross-sectional area (called the instantaneous area). Using
the instantaneous or actual area in the stress calculation
(equation 2.1) gives the true stress.

Strain also comes in “engineering” and “true” definitions.
The engineering strain (e) is defined as the change in
length of a specimen divided by the original length:

L 0

(Equation 2.2)

where L is the length at the time of measurement, L 0 is the
original length, and AL is then the calculated change in
length. The true strain (e) is defined by

8 = In— (Equation 2.3)

L 0

Notice that strain appears to be dimensionless. However, strain
is usually presented in terms of inches/inch, or millimeters/
millimeter. The units cancel each other out, but they give a
sense of scale and often impart information on the size of the
specimens tested.

Many materials (e.g., the structural metals) strain only
a little before they break. Therefore, their useful life
takes place at small strains. The definitions of strains (in
equations 2.2 and 2.3) are really approximations, and they
hold only for small strains. The definition of small strain
may vary from person to person, but it is probably safe to
say that anything less than 1% is a small strain, and the
definitions above are applicable. Equation 2.3 for true
strain is determined by integrating differential length
increments over the entire specimen. For small strains,
less than 1%, engineering strain is a reasonable approxi¬
mation to true strain. Certain simplifying assumptions are
used in calculating strain for small strains. Between 1 and
10%, the definitions may be useful, depending on the
material system and the accuracy of the information that
is needed. If the strains are larger than 10%, as they may
be for some soft tissues, then those assumptions are no
longer valid. More complex treatments are needed to
define “large” or “finite strains.” In biomechanics, the
definitions for finite strains are important, because many
tissues undergo finite strains; however, a discussion of
finite strain is beyond the scope of this book. With stress,
it doesn’t matter so much. Both stress and strain have
precise mathematical definitions and actually have sever¬
al different definitions, which are based largely on the
point of view the definer takes. The in-depth treatments

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

of stress and strain appear in courses in continuum
mechanics and finite deformations.

There is one additional topic to be discussed related to
small strains. Tension is defined by forces that tend to pull
something apart. That is like pulling hands apart before clap¬
ping. Compression is the force applied when the hands come
together for the clap. To continue the hand/stress analogy,
consider a person rubbing his or her hands together, for
example, when the hands are cold. The back-and-forth
motion can occur in materials, and it is known as shear strain
(usually designated by the Greek y). Shear strain has a coun¬
terpart, shear stress (designated by the Greek t) (Fig. 2.3).
The shear stress, t is defined as the shear force, F § , divided by
the area over which it acts, A:

t = (Equation 2.4)

Shear stresses are very important, because many failure
theories for ductile materials (defined below) show that
failure occurs when the maximum shear stress is
reached. If an experimenter pulled apart a cylinder of cop¬
per, a ductile material, it would fracture at a 45° angle to
the direction of the applied forces. Why? Because that is
the direction of the maximum shear stress.

The concepts of stress and strain are not necessarily
easy to grasp. Many readers may feel comfortable with
the description of stress as an “internal pressure.” Strain
may best be thought of as a percentage change in length.
Once stress and strain are no longer a mystery, however,
defining most of the mechanical properties is relatively
straightforward.

Figure 2.3: Shear. As the elephants slide relative to each other,
the boards they are standing on are subjected to shear.

THE TENSION TEST

The Basics (Young's Modulus,

Poisson's Ratio)

The definitions of stress and strain are the first steps toward
understanding and defining some of the most useful proper¬
ties of a material, such as the nebulous concept of strength
discussed earlier. It is easier to make sense of some of the
most important material properties in the context of a simple
tensile test on a cylindrical specimen of material. Everything
that applies to the simple tension test also applies to a simple
compression test; however, the values of the properties in
tension and compression may not be simple negatives of
each other!

Imagine that the director of new materials development at
a huge, multinational biotechnology corporation has just
received samples of a new material developed by one of the
engineers. The engineer believes that the new material is an
excellent bone replacement candidate. The new material
would be most useful if it had properties similar to those of
natural bone. The first task that the materials engineers must
tackle is determining the properties of the new material and
comparing them to the properties of bone (assume that appro¬
priately sized cylinders are available to perform the test).

To test the properties of the new material, the test speci¬
mens are carefully placed in the grips of a tension-testing
machine, and the test begins (with the stress and strain both
initially zero). More and more tensile force is gradually
applied to the specimen. Most tension-testing machines
provide force and displacement data, rather than stress and
strain. However, this is no problem; stress and strain are easily
calculated from equations 2.1 through 2.3, since the specimen
diameter and length are known. A stress-strain diagram
(or graph) is a plot of the stress for each strain. At first, the
engineer may tend to be a little cautious, perhaps because
there is only a limited supply of the sample material. So
the test is programmed to produce only 20-30 lb of force.
After calculating the stress and strain for the initial
test, a stress-strain diagram is generated, which may
look something like Figure 2.4.

Notice that in the figure, when the specimen is unloaded,
the stress-strain curve goes back down the same line that it
went up and stops right where it started. That means the
strain is zero at the end of the test, just as at the beginning.
When a material returns back to its original shape after it is
loaded and unloaded, the deformations, or strains, that
occurred during loading are said to be elastic. This is a dif¬
ferent definition than most people are used to. For example,
the term elastic may produce images of elastic bands
in clothing; elastic is sometimes taken to mean “stretchy.” In
the language of mechanics of materials, elastic has a precise
definition. Elastic means that the material returns to its orig¬
inal shape after loading. What happens if the material does
not return to its original shape? The experiment must con¬
tinue for the answer to be revealed.

Part I I BIOMECHANICAL PRINCIPLES

Figure 2.4: Stress-strain diagram. The applied force (FJ on the
test specimen is the independent variable. The displacement is
measured after the force is applied, and thus displacement is the
dependent variable. Traditionally, however, stress-strain diagrams
are created with the stress acting as the dependent (or "y") vari¬
able, and the strain as the independent (or "x") variable. A cylin¬
drical test specimen (cross-sectional area = 1 in 2 ) is loaded with
an applied force of 20 lb, and thus the uniform stress in the spec¬
imen is 20 psi. The stress-strain diagram is linear from zero stress
to the maximum stress at 20 psi (point B).

Because the specimen appears to have suffered no damage,
the engineers become a little braver and increase the load on
the specimen, perhaps to about 100 lb. The new, extended
plot on the stress-strain diagram still looks like a straight line
(Fig. 2.5). Indeed, many materials exhibit this linear behavior
(e.g., steel, concrete, glass). It is extremely important, however,
to know that most soft tissue (e.g., muscle, skin, ligament,
tendon, cartilage) does not show this linear behavior; the
materials exhibit nonlinear behavior. Nonlinear materials are
more difficult to understand and require a fundamental
knowledge of how linear materials behave.

The slope of the straight line in a stress-strain diagram is
one of the most important and fundamental properties of any
material; the slope is called the Young’s modulus, or modu¬
lus of elasticity (engineers often use the letter E to repre¬
sent the Youngs modulus) (Fig. 2.6). The Youngs modulus is
similar to a spring constant, except it relates to stress and
strain, not force and displacement. The Youngs modulus indi¬
cates either (a) how much a material stretches or strains when
it is subjected to a certain stress or (b) how much stress builds
up in a material when it is stretched or strained by a certain
amount. The Youngs modulus also is sometimes called the
materials stiffness. A material with a high Youngs modulus
undergoes less strain under a given load than a material with
a lower Youngs modulus (Fig. 2.7), and hence it is a stiffer
material. The Youngs modulus is an intensive property, since

Figure 2.5: Stress-strain diagram for larger forces. The test
specimen is loaded with a force (F 2 ) of 100 lb. The corresponding
engineering stress is 100 psi. The stress-strain diagram shows
that the stress-strain relationship remains linear from point B
(20 psi) to point C (100 psi). Note that the corresponding strains
at point B (0.0005 g) and point C (0.0025 g) are also recorded.

Figure 2.6: Young's modulus. The stress-strain relationship between
points B and C is linear, and the slope of the line is the Young's
modulus, or modulus of elasticity. In the case of the test specimen,
the calculations show that the Young's modulus, E, is 40,000 psi.

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

Figure 2.7: Young's moduli for two different materials. On this
stress—strain diagram are illustrated two different materials, one
with a slope greater than the other. The material with the
greater slope (or Young's modulus) is "stiff" relative to the mate¬
rial with the lower slope (or Young's modulus).

it is based on stress and strain and does not depend on the size
of the specimen.

The engineers have carefully observed the specimen dur¬
ing the testing and notice that the diameter of the specimen
decreases as the specimen is pulled! Just as strain is measured
in the direction that the specimen is pulled (along the axis of
the cylinder), strain can also be measured in the direction
across the cylinder. The true strain in the direction perpendi¬
cular to the loading direction can also be calculated as

8 = 2 In

(Equation 2.5)

where D is the original diameter and D is the current diam¬
eter. The ratio of the axial strain to the lateral strain is called
Poisson’s ratio and denoted by the Greek letter nu, v :

—lateral strain
axial strain

(Equation 2.6)

The negative sign appears because while the material is
stretched in the axial direction (a positive strain), the material
is shrinking in the lateral direction (a negative strain). The
negative sign ensures that the resultant Poissons ratio will
always be positive.

The Youngs modulus and the Poissons ratio allow a basic
comparison of material behavior of the new material and
human bone. If the values are the same, the engineers can be
confident that the new material behaves somewhat like human
bone. There are several different ways to measure the Youngs
modulus and Poisson s ratio; the tension test is only one poten¬
tial and simple way. Youngs modulus and Poissons ratio are

material properties because their values are the same
regardless of the type of test or the specimen geometry. They
are also intensive properties, since they are the same regard¬
less of the amount of the material. There may be only a subtle
distinction between a material property and an intensive prop¬
erty. A material property must be the same regardless of the
way the material is tested; it is conceivable that there could be
two different test methods that use the same amount of mate¬
rial. Of course, there may be no difference at all. Therefore, if
the cylinders have different diameters but are of the same
material and the Youngs modulus is determined for each type
of specimen, the value should be the same each time!

The Youngs modulus and the Poissons ratio also allow cal¬
culation of stresses and strains in directions other than that of
the load, through the relationship between stress and strain
called Hooke’s law (Box 2.5).

Load to Failure

Once the Youngs modulus of the new material is known with
some confidence, the next testing objective is to discover when
the specimen breaks. As the specimen is loaded more and
more, one of two things commonly happens. In the first case,
the stress and strain continue to increase in a straight line, and
then suddenly . . . WHAM! the specimen breaks. This is quite
an exciting event when it happens, because (although it is not
calculated here) lots of elastic energy has been stored in the
material, and the remaining fragments spring rapidly back to
their original shape. The largest stress that the material with¬
stands before it breaks is called the ultimate tensile stress, or
ultimate tensile strength, or just the ultimate strength. The
ultimate strength is an intensive definition of strength based on
the maximum stress a material can withstand (Fig. 2.8).

The second possible material response to the increasing
stress is that as the test progresses, the stress-strain line devi¬
ates from the nice straight line it had been following since the
beginning of the test. Usually, the slope of the line decreases
(Fig. 2.9). If the test is reversed after the “bend” in the
stress-strain plot becomes evident, the specimen does not
unload along the same line it described when the load was
increasing. The plot exhibits hysteresis (Fig. 2.10). When the
stress reaches zero, the strain does not return to zero, as it did
in the elastic deformation case. The permanent deformation
is known as inelastic, or plastic, deformation. A material
deforms plastically when it does not return to its original
shape when the load is removed. It is quite easy to demon¬
strate plastic deformation (e.g., by bending a paperclip).

When a material begins to deform plastically, the material
has yielded. The point on the stress-strain graph where the
bend and plastic deformation begin is called the yield point,
or elastic limit. The stress at which the material begins to
deform plastically is called the yield strength. It is important
to reiterate that once a material has exceeded its yield
strength, it is permanently deformed, and it will not recover
to its original shape. This introduces an interesting twist to the
concept of “failure.” It is easy to say that a broken piece of

Part I I BIOMECHANICAL PRINCIPLES

The general form of Hooke's law is three equations,
one for each direction (x,yz):

£x = ^Ox - v(o-y + cr z ))

£y = —(cr y - v((J x + CT Z ))

e z = -(a z - v((j x + a y ))

where e x , e, and e z are the strains in the x, y and z
directions; cr x , a , and a z are the stresses in the x, y and
z directions; and E is the Young's modulus. There are
analogous relationships between shear stress (t) and
shear strain ( 7 ), also part of Hooke's law:

and v is Poisson's ratio.

7xy

7yz

7xz

*xy

~G~

Tyz

where 7 - is the shear strain on an i-facing surface in the
j direction, t.. is the shear stress on an i-facing surface in
the j direction (figure), and G is called the shear modu¬
lus (analogous to the Young's modulus). The shear mod¬
ulus, Young's modulus, and Poisson's ratio are related by

G =

2(1 + v)

Shear stresses. Shear stresses are designated by two sub¬
scripts (e.g., t ), where the first subscript indicates the
face on which the shear stress is acting, and the second
subscript indicates the direction in which it is acting.
Officially, in mechanics, the normal stresses also have two
subscripts. The equations use a frequently adopted short¬
hand: the normal stress in the y direction, which is desig¬
nated a , is more correctly written as a , which indicates
that the stress is acting on an “y" face in a y direction.

Figure 2.8: Brittle failure. One scenario for the failure of the test
specimen illustrated in Figures 2.4 and 2.5. is that with a continued
increase in load on the specimen, the specimen breaks at point D.
Note that the stress-strain relationship remains linear. Point D
represents the ultimate tensile strength of the material.

Figure 2.9: Yield. Another scenario for failure of the test speci¬
mens illustrated in Figures 2.4 and 2.5 is that the material's
behavior is linear until point C'. At point C', the yield point, the
stress-strain relationship is no longer linear, although the
material does not break.

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

plastic strain

Figure 2.10: Permanent deformation. After the stress in the
specimen has passed point C', say to point O, the load is
removed from the specimen. When there is no load on the speci¬
men, there is still a measurable strain; the specimen has not
returned to its original shape and is permanently deformed. The
onset of inelastic or plastic strain at point C identifies C as the
yield point (or yield stress) of the material.

metal has “failed” when it has exceeded its ultimate tensile
strength and is in two pieces. However, a material need not
be in two pieces to fail. Engineers speak of a failure when the
part or material no longer functions as it was designed. So, for
example, if a bridge were made of a material that yielded a lit¬
tle every time a person walks over it, say an inch or two, even¬
tually that bridge may yield all the way into the water! The
bridge is no longer functioning as designed (i.e., to keep feet
dry as people walk over the water), and therefore the bridge
fails because of excessive yielding. If sharks are in the water,
then the point is even clearer.

When the tensile test is continued, and the specimen is
pulled beyond its yield point, eventually the specimen will
break. The breaking point, again, is sometimes called the

Fracture

Figure 2.11: Ductile failure. After the specimen yields, with a
continued increase in the applied load, the specimen eventually
fractures (point D). Because the specimen yields before fracture,
this is an example of a ductile fracture. For this ductile material,
point D represents the ultimate tensile strength.

ultimate strength , even though the material has yielded
before reaching its breaking point (Fig. 2.11). Box 2.6 provides
comparisons of some material properties, including ultimate
strength of several materials.

Figures 2.8 and 2.11 illustrate two ways that a material can
behave before it breaks. The first is that the deformation before
fracture is all elastic (Fig. 2.8). If there is no plastic deformation
before the material breaks, then that materials behavior is
brittle. Notice that only the materials behavior is described as
brittle; the material itself is not brittle. Some will not worry
about the distinction, but it is important. If the material
deforms plastically before it breaks, that is, it yields first (Fig.
2.11), then the materials behavior is ductile. Everyday materi¬
als with brittle behavior include glass, concrete, and ceramic
coffee mugs. If a materials behavior is not brittle, then it is duc¬
tile; the only question is one of degree. One interesting point is
that for almost all materials, temperature plays a big role in

EXAMINING THE FORCES BOX 2.6

PROPERTIES OF DIFFERENT MATERIALS

A given material property varies over a considerable
range. This should be no surprise, however. The reader
has likely handled many different materials, even in
the past week. We notice, for instance, that items
made from plastics (e.g., a CD jewel case or plastic
drinking cup) are light and somewhat flexible, while
items made from metal (e.g., a hammer) are heavier
and tend to be less flexible. The table lists some

common materials and their Young's moduli and yield
strengths. Students should make use of the given data
to perform a valuable exercise: On a single sheet of
graph paper (or using a computer program) plot the
stress-strain behavior of the materials in the table up
to their yield strengths. Note the differences in yield
strengths. The graphic representation may help to
build an appreciation for the differences in material
behaviors.

( Continued)

Part I I BIOMECHANICAL PRINCIPLES

Material

EXAMINING THE FORCES BOX 2.6 (CONTINUED)

Young's Modulus
(GPa)

Yield Strength
(MPa)

Ultimate Tensile Strength
(MPa)

2014-T4 Al alloy 3

324

469

Cortical bone b

17-20 (compressive)

182

195

304 L stainless steel 3

193

206

517

Ti6AI4V 3

114

965

1103

Gold c

207

220

Tungsten 3

407

(1,516) e

1,516

PMMA (bone cement) 0 '

2.0

(40) e

a From Budinski KG, Budinski MK: Engineering Materials, Properties and Selection, 6th ed. Upper Saddle River, NJ: Prentice Hall, 1999.

Trom Cowin SC: The mechanical properties of cortical bone tissue. In: Bone Mechanics. Cowin SC, ed. Boca Raton, FL: CRC Press, 1989. Note that the measured
properties of bone vary widely, depending on age of bone, method of testing, etc., and the numbers given are "reasonable" example values.
c From Properties of Some Metals and Alloys, 3rd ed, a publication of the International Nickel Company, 1968.

Values are approximate and are based on the summary of different experimental measures presented in Lewis G: Properties of acrylic bone cement: state of the
art review. J Biomed Mater Res Appl Biomater 1997; 38: 155-182.
e Brittle (no yield before fracture).

whether the behavior is brittle or ductile. Many readers have
witnessed the demonstration in which a bouncy ball is placed
into liquid nitrogen, after which it no longer bounces. Instead
it shatters when it is dropped! Materials undergo a
ductile-brittle transition at a specific temperature. Thus, a
material that shows ductile behavior in the laboratory may be
brittle in outer space. A material that appears to be brittle in the
laboratory may be ductile at body temperature.

MATERIAL FRACTURE

Fracture Toughness

An in-depth discussion on the fracture of materials is beyond
the scope of this book, but this section introduces some of the
more important concepts. For several reasons, it is difficult to
quantify the fracture properties of a material from the tension-
test data. In the middle of the 20th century, around the
1940s-1950s, people noticed, for example, that large ships
split apart while sitting in the harbor, even though they were
not loaded past their yield point. Seeing a big ship split in the
harbor probably made quite an impression, and people who
questioned why this occurred created a branch of mechanics
called fracture mechanics. From fracture mechanics came
the concept of fracture toughness. Fracture toughness is
analogous to the ultimate tensile strength of a material but
really is a measure of how fracture resistant a material is if a
small crack or flaw already exists. Fracture toughness can be
determined by experiments similar to the tensile test, which
are discussed only briefly here. It is interesting that the units
of ultimate tensile stress or yield stress are the same as those
for stress, for example, pounds per square inch (psi) or mega¬
pascals (MPa; where one pascal = one newton/meter 2 ). The
units for fracture toughness are in psi (inches) 172 or MPa
(m) 172 . The square root of length appears because the fracture
toughness depends on the length of the crack in the material.

It turns out that the longer the starting crack, the less force is
necessary to break the material.

Another way to quantify the fracture strength of a material
is by using the impact test. The most common is called the
Charpy impact test (Fig. 2.12). In the Charpy test, a heavy
pendulum is released with the specimen in its path. The pen¬
dulum swings through, and presumably breaks, the specimen.
The difference in the starting height and the final height
of the pendulum represents an energy loss. That energy loss
is the energy that is needed to break the specimen. The frac¬
ture strength from a Charpy test is given in energy units, such

Potential energy = mgh

Energy loss from fracture
= mg (initial height-final height)

Figure 2.12: The Charpy impact test. In the Charpy impact test, a
mass on a pendulum is raised to some initial height (and thus
has an initial potential energy, mass x acceleration due to gravity x
height). The pendulum is released and swings through an arc that
brings it into contact with the test specimen, causing it to fracture.
The final height of the specimen (and thus the final potential energy)
can be measured. The difference in potential energy is equal to the
energy absorbed by the specimen as it fractures.

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

as newton-meters or joules. A Charpy impact test, for exam¬
ple, can provide useful information on whether a bone breaks
under a certain impact.

Fatigue

Another common word found in the engineering failure dic¬
tionary is “fatigue.” A person who is tired is said to be
“fatigued.” Fatigue fracture is a type of material fracture that
potentially can be very dangerous, because it can sneak up
unexpectedly. Consider the example of a paper clip discussed
earlier. Although the paper clip cannot be pulled apart, it even¬
tually breaks after repeated bending. This is a form of fatigue.
Fatigue fracture occurs when a material is loaded
and unloaded, loaded and unloaded, repeatedly, and
the maximum loads are below the ultimate tensile
strength or yield strength. When a paper clip is bent a lot, it
stays bent, and thus it has yielded. Fatigue can occur at loads
above or below yield. Fatigue failure is harder to quantify than
any of the behaviors discussed so far. It is difficult to identify an
intensive material property for fatigue failure. One property
that may be of interest is called the fatigue, or endurance,
limit. For a given test setup, the endurance limit is defined as
the stress below which the material will never fail in fatigue.
For example, a paper clip can withstand countless small back
and forth bends. If the bending is increased, however, the
paper clip breaks. Somewhere between “a lot” and “a little” is
the endurance limit. Life is complicated more, because some
materials appear to have no endurance limit, and some materi¬
als appear to have an endurance limit under certain conditions.
The fatigue behavior of a material, including the endurance
limit, depends on many variables such as temperature, which
determines whether the materials behavior is brittle or ductile.

Another factor that controls the failure of a material that
is loaded so that the maximum stress is lower than the yield
or fracture stress is the initial (or nominal) maximum stress.
The nominal maximum stress is the maximum stress at
the start of a fatigue test, before any damage occurs to a
specimen. In general, the higher the nominal stress, the
sooner the material fails. An S-N curve (S, nominal maxi¬
mum stress; N, number of loading cycles to failure; bending
the paperclip in the previous example once, back and forth,
is considered one cycle) is one graphic tool that researchers
use to understand how the initial load affects the life of a
material in fatigue (Fig. 2.13).

Clinical Relevance

STRESS FRACTURES: Biological tissues exhibit fatigue
failure. Repeated loadings that generate stresses less than the
ultimate tensile stress may lead to fatigue failures; when this
occurs in bone\ the injury is referred to as a stress fracture. The
term stress fracture is really a misnomer ; since the reader has ,
by now , learned that all fractures are caused by stress , in the
language of engineering mechanics. Thus , the clinical stress
fracture may be better termed subcritical fatigue damage.

(N)

(repetition)

Figure 2.13: Fatigue life. The fatigue life of a material is strongly
influenced by the loads (and thus the stresses) applied to the
material. The higher the stress, the shorter the fatigue life, or
the fewer cycles needed for failure.

LOADING RATE

Many materials behave differently if they are loaded at differ¬
ent rates. Stress rates may be given in psi/second or mega¬
pascals/second. Strain rates may be given as either a pure fre¬
quency, per second, or in units such as inch/inch/second or
millimeter/millimeter/second. In general, the faster a material
is loaded, the more brittle it behaves. Anyone who has played
with modeling clay, Silly Putty® or even bread dough has
experienced this. When the clay is pulled apart rapidly, it
seems to snap apart. When it is pulled apart slowly, it sags, and
extends for quite a while before breaking. Another illustration
is putting a hand in water. When the palm of the hand is
slapped rapidly on the water, it stings as if hitting concrete. Put
a hand in slowly, and the water moves readily out of the way.
Most structural materials, like steel and concrete, behave the
same way regardless of the strain rate. Polymers and many soft
(biological) tissues, on the other hand, are sensitive to strain
rate. In this case, sensitive means that the materials behave dif¬
ferently under different loading conditions. It is especially
important to realize this in the context of injuries, for example,
bone fracture or ligament injuries. Chapters 3 and 6 discuss
this issue for bone and ligaments, respectively.

Clinical Relevance

LOADING RATES: Fast or slow strain rates can lead to differ¬
ent types of injury. For example\ some ligaments fail under very
high loading rates , while the bone where the ligament attaches
fails at lower loading rates. Low loading rates appear to pro¬
duce avulsion fractures at the bone-ligament interface\ while
high loading rates produce failures in the central portions of
ligaments. The type of tissue damaged or the kind of injury
sustained is useful in explaining the mechanism of injury. Bio¬
mechanics experts are often called as "expert witnesses" in trials
to establish the mechanism of injury in personal injury cases.

Part I I BIOMECHANICAL PRINCIPLES

The reason that strain or loading rate can lead to different
behaviors is directly related to the complex structure of the
materials. Many materials, especially soft tissue, have a fluid-
like component to their behavior, very much like the dampers
used on doors to keep them from banging into walls. The flu-
idlike component of behavior leads to time-dependent behav¬
iors. Differences in behaviors for different strain rates is an
example of time dependence, for it takes much less time to
achieve a strain of 20% (or 0.2 mm/mm) at a rate of 0.05/s
than at a rate of 0.001/s. A more detailed discussion of the
relationships between a specific tissue s composition and its
mechanical properties follows in Chapters 3-6.

Fluids have a property that all are familiar with, informally
known as “gooeyness,” formally known as viscosity. Fluids
with a high viscosity (e.g., honey) flow slowly. Fluids with a
lower viscosity (e.g., water) flow quickly. Ice at the bottom of
a glacier, under the tremendous weight of all the ice on top,
acts like an extremely viscous fluid, perhaps moving only a
few inches per year. Therefore, solids that have time-
dependent mechanical behaviors, because of a fluidlike com¬
ponent, are viscoelastic. The simplest models of viscoelastic
behaviors in materials contain a single elastic element, such
as a spring, and a viscous element, represented by a dashpot
(the damper on the door), either in series (lined up one after
another, the “Maxwell model”) or in parallel (lined up next
to each other, the “Kelvin-Voigt model”) (Fig. 2.14). The

A. The Maxwell Model

Force

Elastic

element

Viscous

element

Force

B. The Kelvin-Voigt Model

Elastic

element

Figure 2.14: Viscoelastic models. Two simple models of a vis¬
coelastic material behavior. A. In the Maxwell model the viscous
element, or dashpot, is in series with the spring. The displace¬
ment of the dashpot does not recover like that of the spring
does. When the load is removed, the spring returns to its original
position, but the dashpot does not, and thus there is permanent
or plastic deformation of the model. B. In the Kelvin-Voigt
model, the spring and dashpot are in parallel. When the load is
removed, the spring recovers, and pulls the dashpot back with it;
there is no plastic deformation. In each case, the deformation of
the dashpot depends on the loading rate.

viscoelastic nature of materials leads to two important and
related behaviors, creep and stress relaxation.

Creep is the continued deformation of a material over time
as the material is subjected to a constant load. The simplest
illustration of creep is a cylindrical material loaded with a
fixed weight applied in the axial direction for a long time.
Initially, the weight creates a stress in the material, and the
material deforms according to the elastic strain. Because the
material in this case is viscoelastic, the material continues to
deform and stretch over time, usually very slowly. In creep
experiments, a fixed load is maintained on the specimen, and
the strain is measured as a function of time. The slow, steady
closing of a door with a damper is an example of creep. The
door (and possibly the door spring) applies a constant force
on the damper. The damper slowly deforms and allows the
door to close.

The complementary experiment to the creep experiment
is the stress relaxation test. Stress relaxation is the reduction
of stress within a material over time as the material is sub¬
jected to a constant deformation. For the stress relaxation
test, instead of applying a fixed load to the specimen, a fixed
displacement, or strain, is maintained, and the resisting force
(from which stress is calculated) is measured as a function of
time. Stress generally decreases with time and hence the label
“relaxation.” Both creep and stress relaxation are important
behaviors in biological soft tissue.

Clinical Relevance

SPLINTING TO STRETCH A JOINT CONTRACTURE:

Many splinting techniques directly apply the concepts of
creep and stress relaxation to increase joint range of
motion. Some splints are spring-loaded and apply a con¬
stant stretching force to a joint , allowing a gradual increase
in joint excursion (strain). Other static splints hold a joint in
a fixed position (constant strain) for a prolonged period such
as overnight or for a few days, producing a gradual relax¬
ation of the soft tissues that need to be stretched. After
removal of the splint , more excursion is available. These
valuable clinical techniques demonstrate the applicability
of materials science in clinical practice.

BENDING AND TORSION

Bending

Many bones act as structural beams in the body. Thus, they
are subjected to various forces, like beams, and may be ana¬
lyzed as beams. Beams that are used in bridges or buildings
that are oriented vertically often are subjected to compressive
loads. The beams that are oriented horizontally, however, are
often loaded at or near the midspan and are subjected to
bending. Most readers have, for example, broken a stick by
bending it.

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

EXAMINING THE FORCES BOX 2.7

THE BEAM BENDING EQUATION

For a beam of single material (concrete, steel, or
bone, for example), the stresses generated by pure
bending of a beam may be calculated from

a “ ~T

where a is the stress calculated at position y, M is the
applied bending moment, and I is the moment inertia
(a function of the beam's cross-sectional area).

What is most amazing about the formula for stress in a beam
subjected to bending is that material properties are absent!
(Box 2.7) This means that for a beam of concrete, steel, or
bone, if the cross section has the same geometry and the
applied loads are the same, the stresses are exactly the same.
Thus, the difference in behavior of beams is largely controlled
by the Youngs modulus (to determine how much the beam

will deform) and by the failure strength of the material.
Another interesting result of beam analysis is that there is an
axis along which there is no stress, called the neutral axis.
Stresses on one side of the neutral axis are compressive and
on the opposite side, tensile. For beams with symmetric
cross-sectional geometries, the neutral axis is through the
center of the beam. Box 2.8 calculates the stresses in a beam
subjected to bending and uses the calculation to determine
the stress in the ulna when lifting a 5-lb load.

Torsion

This section considers torsion of a cylinder. The treatment of
torsion for cross sections that are not cylindrical is beyond the
scope of this text. Torsion of cylinders, however, is directly
applicable to the torsion in long bones, where the cross sec¬
tion can be approximated as a cylinder. In torsion, a torque,
or twisting, is applied to the beam instead of a bending
moment (Box 2.9). Torsion generates shear stresses in a beam,
and the equation to calculate the shear stress is directly analo¬
gous to the bending equation.

EXAMINING THE FORCES BOX 2.8

WILL THE ULNA BREAK WHEN
BY LIFTING A LOAD?

'BENT"

To determine whether a beam fails under a bending
moment, consider only the maximum value of y the
distance from the neutral axis (Fig. A). For a beam with
a rectangular cross section, the maximum value is h/2
or one half the beam's height. For a beam with a circu¬
lar cross section, the maximum value is r, the radius of
the beam. Using the formulas shown in the figure, the
maximum stress in a beam with a rectangular cross sec¬
tion is 6M/(b X h 2 ), and in a beam with a circular cross
section is 4M/(tt x r 3 ).

Results calculated elsewhere in this textbook (Chapter 13)
can be used to estimate the stresses in one of the fore¬
arm bones (e.g., in the midshaft of the ulna). Assume
that the ulna can be modeled as a beam with a circular
cross section (albeit hollow) (Fig. B), with an outer radius
of r o and an inner radius of r r Chapter 13 (Box 13.2),
reports an internal moment of 13.4 Nm; that is, the
moment that exists within the bone material to resist
external forces (lifting a 5-lb load) and keep the bone in
equilibrium, is 13.4 Nm. Box 2.7 demonstrates calculation
of the stresses in a beam under a bending moment. For a
beam with a circular cross section, like the model of the
ulna, the neutral axis is the geometric center of the
beam. Therefore, the maximum stress occurs at y = r Q ,
the distance farthest from the neutral axis. The bending
moment, M, is 13.4 Nm. The only information that is miss¬
ing is the moment of inertia of the ulna, l ulna . To calculate

I for a hollow circle, first calculate I for the bone as if it
were a solid cylinder, and then subtract I for the inner
hollow circle. Thus,

1T ( r o) 4

Pouter circle

ii,

inner circle

^(n ) 4

•ulna

= I

outer circle

- i

Tr(r 0 ) ^(nr

inner circle

= y(0 4 - (n) 4 ]

Now it is easy to calculate l ulna for any values of r o and
q. For the sake of this example, assume that the outer
radius is 25 mm (0.025 m), and the inner radius is 20
mm (0.020 m). Hence, l ulna = 180 x 10 -9 m 4 . Apply the
equation in Box 2.7 to find

13.4 Nm X (0.025 m)

180 X 10“ 9 m 4

= 185 X 10 6 N/m 2 , or 1.85 Mpa.

Recall that in a beam subjected to bending, one side of
the beam is in compression and the other is in tension.
Comparison of the calculated stress value with the
strength values listed in the table in Box 2.6 clearly
shows that the stress in the ulna is well below the yield
or ultimate tensile stress of bone. This is, of course, a
good thing and what is expected, since people rarely
break their ulnas when lifting a 5-lb bag of sugar!

(Continued)

Part I I BIOMECHANICAL PRINCIPLES

EXAMINING THE FORCES BOX 2.8 (CONTINUED)

Beam bending. A bending moment, M, applied to a long,
thin beam, creates a stress distribution shown in equation
2.6. The moment of inertia (I) is given for beams with a rec¬
tangular and a circular cross section.

Cross-section
B of ulna

Idealized, cylindrical
cross-section of ulna

EXAMINING THE FORCES BOX 2.9

SHEAR STRESSES IN A BEAM SUBJECTED
TO TORSION

The shear stress in a beam that is subjected to tor¬
sion or "twisting" is calculated from:

T = 7

where the shear stress, t, is calculated at radius r, T
is the applied torque, or twist, and J is the polar
moment of inertia.

Shear stress, T = —-
J

j =

A torque T twisting a cylinder creates a shear stress
distribution shown in the equation. The polar moment
of inerta (J) is for the cylindrical cross-section.

Clinical Relevance

BENDING VERSUS TORSION FRACTURES: Many bone
fractures occur because of either excessive bending or tor¬
sion on the bone. For examplea torsional fracture may
occur during a skiing accident if a ski is caught , say , on a
treeand the bindings do not release. The momentum of the
skier causes the body to spin , thus applying torsion to the
bone. Depending on the speed at which the torsion is
applied , this may result in a complex "spiral" fracture or an
even more serious shattering of the bone. If a bone breaks
under bending loads , however ; the fracture is likely to be a
simpler ; linear-type fracture. Another common skiing injury ,
a boot-top fracture , occurs with a bending load as the skier
tumbles forward over a relatively motionless ski and the
tibia "bends" over the stationary boot.

SUMMARY

This chapter discusses the basic definitions of intensive (e.g.,
density) and extensive (e.g., volume) material properties.
Using an example of a simple tension test, some of the most
important material properties are defined in the language of
mechanics of materials: tensile or compressive modulus
(modulus of elasticity, or Youngs modulus), Poissons ratio,
ultimate tensile strength, yield point and yield strength,
endurance limit, and fracture toughness. The simple tension
test also allows the introduction of the concepts of load and
deformation, stress and strain, materials testing, and brittle
versus ductile behavior. Additional concepts that are impor¬
tant to understanding the behavior of biological materials
include fatigue failure, beam bending, torsion, and loading

Chapter 2 I MECHANICAL PROPERTIES OF MATERIALS

rate (viscoelasticity). The properties specific to biological
materials such as bone, tendons, and ligaments are discussed
in detail in the appropriate chapters.

Additional Reading

Beer FP, Johnston ER: Mechanics of Materials, 2nd ed. New York:
McGraw-Hill, 1992.

Gere JM, Timoshenko SP: Mechanics of Materials, 4th ed. Boston:
PWS Publishing, 1997.

Panjabi MM, White AA III. Biomechanics in the Musculoskeletal
System. New York: Churchill Livingstone, 2001.

Popov EP: Mechanics of Materials, 2nd ed. Englewood Cliffs, NJ:
Prentice Hall, 1976.

Ruoff AL: Materials Science. Englewood Cliffs, NJ: Prentice Hall,
1973.

CHAPTER

P* 3

Biomechanics of Bone

L.D. TIMMIE TOPOLESKI, PH.D.

CHAPTER CONTENTS

BRIEF REVIEW OF BONE BIOLOGY STRUCTURE, AND CHEMICAL COMPOSITION .37

MECHANICAL PROPERTIES OF BONE .38

Material Properties versus Geometry .39

Anisotropy.39

Elastic Constants of Bone.40

Strength.41

Fracture Toughness.41

Strain Rate.41

CHANGES IN MECHANICAL PROPERTIES WITH AGE AND ACTIVITY .42

FRACTURE HEALING.42

SUMMARY .43

II living organisms have a strategy for supporting and protecting their bodies' parts. Marine invertebrates like
the jellyfish rely on the balance of the internal and external water pressure for support. A jellyfish out of
m m water is really pretty much a blob of jelly (as readers who are beach walkers have witnessed). Insects, lob¬
sters, and other small skittering creatures may make use of an external or exoskeleton (a hard outer layer composed of
chitin in many cases) to give support and keep their insides in. Biomechanically, an exoskeleton does not work for larger
animals; the animals and their organs get too heavy. Larger land-dwelling animals are equipped with the familiar
endoskeleton (or internal skeleton) for support. The skeleton in the human body provides support for the body, acts
as a rigid system of levers that transfer forces from muscles, and provides protection for vital organs (e.g., the skull for
the brain and the ribcage for the heart and thoracic organs). There are also some curious little bones in the ears of
some animals (including humans) that are needed to transmit sound to enable hearing.

Chapter 2 introduced the reader to the language and principles needed to understand the general mechanical behavior
of materials. This chapter discusses the structural and support aspects of bone, providing information on the mechani¬
cal properties of bone as a specific material. The purposes of this chapter are to

■ Briefly review basic bone biology and terminology

■ Describe mechanical properties of human bone

■ Discuss the clinical relevance of understanding bone properties

Research on the mechanical properties of bone continues. The review provided in this chapter is meant to offer the
reader only an introductory understanding of bone's mechanical properties. The mechanical properties of bone are not
nearly as well understood as those of the engineering materials mentioned in Chapter 2; bone is more complex than

Chapter 3 I BIOMECHANICS OF BONE

steel, for example. Many of the references cited are perhaps "old" in the context of the relatively young field of bio¬
medical engineering. In the case of bone, however, old does not mean "invalid." Much of the original work from the
1970s and 1980s is still valuable to students, clinicians, and researchers in biomechanics. The more recent work on the
mechanical properties of bone involves details of bone behavior that are beyond the scope of this book, for example
numerical models of the micromechanics of bone remodeling, or investigating the complex interactions within a net¬
work of trabeculae [e.g., 7,10,12,17,19]. Indeed, it would require a substantial monograph to justly review all of the
work on bone properties. Readers who are interested in a specific aspect of bone behavior are encouraged to consult
the primary literature.

BRIEF REVIEW OF BONE BIOLOGY,
STRUCTURE, AND CHEMICAL
COMPOSITION

The mechanical properties of bone are determined primarily
by the structural components of bone. Bone contains two prin¬
cipal structural components: collagen and hydroxyapatite
(HA). Collagen is an organic material that is found in all of the
body’s connective tissue (see Chapters 5 and 6). Organic com¬
ponents of bone make up approximately 40% of the bone’s dry
weight, and collagen is responsible for about 90% of bone’s
organic content. The collagen in bone is primarily type I col¬
lagen (bone = b + one, or I). Other types of collagen are
found in other connective tissues; for example, types II, IX,
and X are known as cartilage-specific collagens because they
seem to be found only in cartilage [6]. An excellent research
project for the reader is to determine the number of different
types of collagen and where they are all located.

The inorganic, or mineral, components make up approxi¬
mately 60% of the bone’s dry weight. The primary mineral
component is HA, which is a calcium phosphate-based min¬
eral: Ca 10 (PO 4 ) 6 (OH) 2 . The HA crystals are found primarily
between the collagen fibers. HA is related to, but distinct
from, the Ca minerals found in marine corals (they are calcium
carbonates). Pure HA is a ceramic and can be found in crys¬
tal form as a mineral (e.g., apatite). Because HA is a ceramic,
bone can be expected to have ceramic-like properties. For
example, ceramics are generally brittle, tolerating little defor¬
mation before fracture. Ceramics and bone are also relatively
strong in compression but weak in tension.

The structure of human bone changes with age. The bone
in children is different from the bone in adults. Immature
bone is composed of woven bone. In woven bone, the carti¬
lage fibers are more or less randomly distributed (as they are
in skin). The random distribution of fibers gives some
strength in all directions (i.e., no preferred direction), but
woven bone is not as strong as mature bone. Woven bone of
children is also more flexible than adult bone, presumably to
provide resilience for all the falls and tumbles of childhood.

As bone matures, cells in the bone, called osteoclasts,
essentially dig tunnels in the bone. Other cells, called
osteoblasts, line the tunnels with type I collagen. The collagen
is then mineralized with HA. The mineralization may be con¬
trolled by cells called osteocytes, which are “older” osteoblasts
that have been trapped in the collagen matrix. The osteocytes

play a role in controlling the extracellular calcium and
phosphorus. The result of all the osteoclast, osteoblast, and
osteocyte activity is a series of tubes, called haversian canals,
which are lined with layers of bone (lamellae) and are ori¬
ented in the primary load-bearing direction of the bone (e.g.,
along the long axis of a femur). The haversian canals, also
known as osteons, represent the structural units of bone. The
hollow osteons are also passageways for blood vessels and
nerves in the bone. Other passageways, which tend to be per¬
pendicular to the haversian canals, called Volkmanns canals,
allow the blood vessels to connect across the bone and form a
network throughout the bone (Fig. 3.1) [3,8,13].

The first set of osteons to develop in mature bone are
called the primary osteons. Throughout life, however, the
osteoclasts/blasts/cytes remain active, and new haversian
canals are constantly formed. The new osteons are formed on
top of the old and are called secondary osteons. Haversian
canals are formed by secondary osteons.

Once the woven bone has been replaced by the system of
osteons, the bone is considered mature. There are two
primary types of mature human bone: cortical bone and
cancellous bone. Cortical bone (also known as compact
bone) is hard, dense bone. Cancellous bone (also known as
spongy or trabecular bone) is not as dense as cortical bone

Osteocytes

Figure 3.1: Schematic drawing showing the microstructural
organization of bone.

Part I I BIOMECHANICAL PRINCIPLES

Figure 3.2: Schematic drawing of a femur, a "long bone" of the
body. The force (P) applied to the femur through the joint is off¬
set from the center line of the bone's long axis. The offset cre¬
ates bending moments (M) in the bone.

and is filled with spaces. The bone between the spaces can be
thought of as small beams that are called trabeculae. Cortical
bone is found, for example, in the midshaft of the femur, and
cancellous bone is found in the interior of the femoral head.
In the late 19th centuiy, Wolff observed that bone, especially
cancellous bone, is oriented to resist the primary forces to
which bone is subjected. Wolff suggested that “the shape of

bone is determined only by static loading” [13]. Today, it
is clear that dynamic loading of bone plays an important, if
not pivotal, role in the bone’ s structure. Thus, Nigg and
Grimston suggest restating Wolffs Law as “Physical laws are
a major factor influencing bone modeling and remodeling.
[13]” A less formal, though perhaps more elegant and to the
point, statement of Wolff’s law is “Form follows function.”

Most readers are familiar with the general shape of a long
bone, such as a human femur or tibia (Fig. 3.2). Where two
bones connect and mb against each other, the bone is coated
with articular cartilage, which allows very low friction articula¬
tion at the joints (where two bones come together). Note that
not all joints are mobile; for example, the bones of the skull are
not supposed to move relative to each other. In children, the
bone grows from the ends out, in the metaphysis and epiph¬
ysis. The regions where bone actually grows are called growth
plates. If a child’s growth plate is damaged, the bone may stop
growing altogether and will not reach a normal length.

The previous description of bone structure and biology
gives only the briefest introduction to allow readers to become
familiar with some of the language and architecture of bone.
Readers are urged to investigate one or more of the references
at the end of this chapter to leam more of the details of bone.

MECHANICAL PROPERTIES OF BONE

The mechanical properties of bone vary with both the type of
bone (e.g., cancellous vs. cortical) and with the location of the
bone (e.g., rib vs. femur). Because of the wide variety of prop¬
erties, there is no “standard value” for the strength or elastic
modulus of bone, for example. The analysis and examination
of bone mechanics thus requires that the student or investiga¬
tor research the unique properties of the specific bones that
are in question. Knowing only a few of the fundamental prop¬
erties of bone, however, it is possible to predict general behav¬
ior, since the focus here is the structural function of bone.

In a general sense, the important properties of a particular
bone may be deduced by carefully considering the bone’s
function(s). For example, the femur carries all of the body’s
weight during each step a person makes while walking. The
weight comes largely from above, so the bone must be both
stiff (high Young’s modulus) and strong in compression in the
direction of the long axis. The bone must be stiff so that for
each step, the bone does not compress like a spring. Bone
would be an ineffective structural material if, for each step
taken, the legs shortened by an inch or two. Bone would also
be ineffective if, when loaded more severely than it is loaded
in standard walking (e.g., in running or jumping), the bone
fractured or was crushed in compression. Examination of the
femur’s functional demands reveals that the bone must be
strong in compression. However, if most of the apparent load
is compressive, must bone also resist tension? Ultimately, how
strong does bone need to be?

Biomechanical studies of joint forces show that the force on
the femur varies with activity. For example, simply standing on

Chapter 3 I BIOMECHANICS OF BONE

one leg can result in a force on the femur of 1.8-2.7 times body
weight. Leg lifting in bed can result in a force of 1.5 times body
weight, and walking at a fairly good pace can exert a force of up
to 6.9 times body weight [1]! But, an individual weighs only one
body weight, so where does this “extra” weight come from?
Chapter 1 explains the analytical approach to determining the
forces in the muscles applied to the joints during activity. That
chapter reveals that muscles with their small moment arms
must generate large forces to balance the large moments pro¬
duced by external loads such as body weight. Figure 3.2
demonstrates that the femoral head is offset medially from the
centerline of the femoral shaft. The offset implies that mechan¬
ical moments are applied to the bone (by muscles) to balance
the body. Dynamic activities like walking and running demand
even larger muscle moments because the body is accelerating.
Chapter 48 discusses the forces generated during normal loco¬
motion. The additional moments from the muscles result in
increased joint forces. So, a bone is required not only to sup¬
port the body weight, but also to sustain loads that are poten¬
tially several times the body weight during normal activity.

The preceding discussion reveals that the femur must
withstand large compression loads. The femur also helps
illustrate why bone must withstand tensile loads. Because the
load on the femoral head is off center, (eccentric), the offset
creates a bending moment in the bone. In Chapter 2, the
equation in Box 2.7 shows that an applied bending moment
creates both compression and tension in a beam or bone.
Thus, according to mechanical principles, the stresses in
the bone are determined by the superposition (addition) of
the compressive and bending loads. On the medial side of the
femur, for example, the compressive axial load and compres¬
sive bending loads add, so that stresses are predominantly
compressive. On the lateral side, in contrast, the compressive
axial loads and the tensile bending loads are opposite, and
tensile stresses may occur. Under particularly intense loading
conditions (e.g., a skiing accident), the tensile forces on a
bone may be substantial and indeed can lead to fracture.
Depending on the type of accident, torsional forces that are
generated can also lead to severe fractures. Thus, bone must
be able to withstand both compressive and tensile loads (as
well as torsion).

Clinical Relevance

FRACTURES RESULTING FROM DIFFERENT KINDS
OF LOADING: Bones are subjected to different forms of
loading and fail under different conditions. Long bones such
as the tibia and femur sustain large compressive loads at
the joint but commonly fail as the result of torsional forces,
such as those applied to the tibia in a spiral fracture
described in Chapter 2. Vertebral fractures, on the other
hand , are more commonly the result of compressive forces
on the vertebral body Tensile forces produce avulsion frac¬
tures when ligaments or tendons are pulled away from their
bony attachments.

Material Properties versus Geometry

The mechanical response of materials used in building sky¬
scrapers and bridges depends on their geometry. For example,
the shape of a beam s cross section controls how much the beam
deflects under a load. Beams with a circular cross section
behave differently than I-beams of the same height, for exam¬
ple. The structural geometry of bone contributes to its mechan¬
ical response, and thus different bones have different shapes
[11]. The long bones of the legs, for example, have cross sections
that are roughly hollow cylinders. The cylindrical cross section
allows a tibia, for example, to support bending forces approxi¬
mately equally in any direction. An I-beam, on the other hand,
is good at supporting bending loads when it is upright, but when
it is sideways (like an “H”-beam), it does not support bending
loads as well. Note the orientation of the steel beams the next
time you pass under an overpass on the highway.

Anisotropy

Bone is an anisotropic material; it has different properties in
different directions. The Youngs modulus in the axial direction
of the femur, for example, differs from the Youngs mod-
/(jpmy ulus in the transverse direction (lateral to medial).

Many common structural materials, like steel, are usu¬
ally treated as isotropic materials, and a single value of
Youngs modulus or of yield stress is sufficient to analyze the
deformation or failure characteristics respectively. Bone, how¬
ever, like some other special anisotropic materials (e.g., wood),
has a structure that gives rise to differences in the mechanical
properties acting at right angles. Although the properties of
bone are different in the longitudinal and transverse directions,
it may not make any difference which transverse direction is
chosen, the properties are nearly the same in all directions
within the transverse plane (Fig. 3.3). Such a material is called

Figure 3.3: Bone can be considered a transversely isotropic mate¬
rial, which means that the properties, such as modulus of elasticity
and ultimate strength, are the same in the x and y directions
(and it makes no difference where those directions are assigned),
but differ in the z direction.

Part I I BIOMECHANICAL PRINCIPLES

EXAMINING THE FORCES BOX 3.1

AN EXPERIMENT TO DEMONSTRATE
ANISOTROPIC BEHAVIOR

The concept of anisotropy, and more specifically,
orthotropy, can be illustrated by a simple experiment.
For this experiment, find six sheets of 8V 2 x 11-in
paper and some tape. Roll each piece of paper into a
tube 8V 2 in tall, and tape it so it stays rolled. Tape all
six tubes together into a pyramid structure (Fig. 3.4).
With the taped tubes standing on end, you can place
your textbook or another book on top, and the tubes
easily carry the weight. Next lay the tubes on their
side, and place the book on top. The tubes collapse
and are crushed by the weight of the book. The stiff¬
ness and the strength are different in the different
directions, greater in the direction parallel to the
length of the rolls than in the direction perpendicu¬
lar to their lengths. Although the Young's modulus,
or the strength of the paper itself, is a constant,
when the paper has a distinct structure, similar to
that of cortical bone, properties in directions that are
at right angles to each other are different.

Figure 3.4: Demonstration of anisotropy. The strength of a
bundle of tubes (B) is different if they are loaded from the top
(C) or from the side (D).

transversely orthotropic (Box 3.1). A good first approxima¬
tion for bone is that it is transversely orthotropic.

Stress and strain are defined in Chapter 2 as force per area
and the ratio of the change in shape to the materials original
shape, respectively. They are related by Hooke s law (Box 2.5).
The relationship between stress and strain is called a consti¬
tutive equation. The constitutive equations for anisotropic or
orthotropic materials are far more complex than equation 2.6.
Two material property constants are required to relate stress
and strain for an isotropic material: its stiffness, or Youngs
modulus, E, and a measure of the amount of bulging the mate¬
rial undergoes in compression, known as Poissons ratio, v. For
an anisotropic material, 21 material property constants are
required to relate stress and strain! A general orthotropic
material is a bit simpler, requiring 9 constants. For a trans¬
versely orthotropic material such as bone, only 5 constants are
needed. Only is a relative term, of course. The constants can
be evaluated only by careful experimentation; given the geom¬
etry of bone, however, such experiments can be quite difficult.
Consequently, a complete description of bones response to
loading (i.e., the precise definition of the relationship between
stress and strain in bone) is a complex analysis, beyond the
scope of this text.

Elastic Constants of Bone

Since bone may be considered to be either orthotropic or
transversely isotropic, we must define more than the two con¬
stants (the Youngs modulus and Poissons ratio) indicated in
Box 2.5. In an elementary sense, a Youngs modulus is
required for each of the three primary directions: along the
long axis, in the radial direction, and in the circumferential
direction. For an excellent and more detailed summary of the
work on mechanical properties of bone, see Cowins book [5].
The Youngs modulus along the long axis of either human or
bovine cortical bone ranges from 17 to 27 GPa, and in the cir¬
cumferential or radial directions, ranges from about 7 to 20
GPa. The transverse stiffnesses, therefore, are roughly half
that of the longitudinal stiffness. To place these stiffnesses in
a context familiar to the reader, the Youngs modulus (a) of
steel is approximately 200 GPa; (b) of the titanium alloy used
in artificial joints (Ti6Al4V), approximately 115 GPa; (c) of
aluminum, approximately 70 GPa; and (d) of acrylic, approx¬
imately 2 GPa. The elastic constants that correspond to the
Poissons ratio are more difficult to interpret and are not
reported here.

The elastic constants of cancellous bone present an inter¬
esting challenge. The overall, or bulk, properties of a piece of
cancellous bone depend on the structure, arrangement, and
density of the component trabeculae. An individual trabecula,
or beam of bone, may actually have properties close to those
of cortical bone. When many small beams of bone are linked
together, however, in a network like cancellous bone, the
apparent properties change. Experimental studies suggest
relationships, for example, between the compressive modulus
and apparent (or measured) density of the material. Hayes

Chapter 3 I BIOMECHANICS OF BONE

gives a relationship for the compressive Youngs modulus of
cancellous bone (E) as

E = 2,915 p 3 (Equation 3.1)

where p is the apparent density [8]. Research on the elastic
constants of both cortical and cancellous bone continues.

Clinical Relevance

ARTIFICIAL JOINTS: An artificial hip joint is often held in
the cortical bone of the femur by poly(methyl methacrylate)
or PMMA bone cement, which is basically the same polymer
as Plexiglas.® Thus, the distal section of the prosthesis is really
a composite or multimaterial beam made up of an outer
shell of bone, a layer of PMMA, and a central core of
Ti6AI4V (Fig. 3.5). The modulus of elasticity ranges over
roughly two orders of magnitude, from 2 to 77 5 GPa. The
mechanical analysis of this composite system is beyond the
scope of this book, but the reader should be aware that
introducing an artificial joint into a bone changes the
mechanical environment of the bone considerably

Strength

The ultimate strength of bone is also not represented by a sin¬
gle value. The reported values vary depending on the test
method used and type of bone tested. The tensile strength of

Figure 3.5: An artificial joint introduces several different materi¬
als into a bone. The system can no longer be modeled as a beam
of homogeneous bone. The effects of the joint replacement
stem and bone cement (if used) must be accounted for.

cortical bone in the longitudinal direction varies from approx¬
imately 100 to 150 MPa or higher. The compressive strength
in the longitudinal direction is greater than the tensile
strength and varies from approximately 130 to 230 MPa or
higher [5]. For comparison, the ultimate strength for a standard
structural steel is approximately 400 MPa, for Ti6Al4V it is 900
MPa, and for bone cement it is approximately 25^10 MPa.

Fracture Toughness

Chapter 2 explains that fracture toughness is a measure of
a material’s ability to resist crack growth if a crack has
already initiated. The reported values of fracture tough¬
ness vary (depending on the study, location of bone, meth¬
ods, etc.) from about 3.3 to 6.4 MPa-m 1/2 [14,15,18]. The
range of fracture toughnesses shows about a factor of 2 dif¬
ference between the minimum and maximum reported
values. It is difficult, therefore, to assign a single value of
fracture toughness to bone, and the fracture toughness has
not been measured for many bone locations. The relative
magnitude (3-6 MPa-m 1/2 ) of bone’s fracture toughness
compared with that of other materials provides a useful
perspective. The fracture toughness of bone is compara¬
ble, for example, to that of PMMA (like Plexiglas)® with a
fracture toughness of about 1.5 MPa-m 1/2 , or a ceramic like
alumina (Al 2 O s , the basic mineral of ruby and sapphire)
with a fracture toughness of 2.7-4.8 MPa-m 1/2 . The frac¬
ture toughness of bone is low compared with that of alu¬
minum alloys (20-30 MPa-m 1/2 ), steel (70-140 MPa-m 1/2 ),
or titanium alloys (70-110 MPa-m 1/2 ). Cortical bone has a
relatively low fracture toughness because it is brittle and
does not easily absorb strain energy by deforming plasti¬
cally. Thus, bone’s fracture toughness is consistent with
that of nonbiological ceramics.

Strain Rate

Not only are the material properties of bone different in
different directions, but also the properties depend on the
strain rate, or how fast the bone is loaded; that is, bone is
viscoelastic. In general, both the modulus of elasticity and
the strength of bone increase with loading rate. For exam¬
ple, models based on experimental results predict that the
longitudinal elastic modulus of bone can vary by as much as
15% because of the differences in strain rates encountered
in everyday activity [5]. The dependence of materials prop¬
erties on strain rate may be one of the body’s protective
mechanisms; bone is able to withstand greater stresses dur¬
ing traumatic, rapid loading when needed. Recall that bone
has two primary constituent materials: collagen and HA
crystals. Collagen imparts the viscoelastic component to
bone behavior (see Chapter 2). Bone’s viscoelastic behavior
is an example in which the behavior of bone differs from
standard structural ceramics.

Part I I BIOMECHANICAL PRINCIPLES

Figure 3.6: Torsional Fracture. The radiograph illustrates the typi¬
cal spiral oblique fractures of the tibia and fibula resulting from
a torsional injury such as a skiing accident.

Clinical Relevance

EFFECTS OF LOADING RATE IN BONE: Skiers know that
a fall comes quickly and suddenly. Most skiers are able to
get up and continue to ski down the mountain after even
seemingly horrendous fails. Part of the explanation is the
viscoelastic property of bone; which allows bones to with¬
stand larger loads when those loads are applied at a rapid
rate. However ; there are limits to such protection , and when
the loads exceed the strength of the bone, the bone frac¬
tures. Many skiers sustain tibial fractures when they fall.
Before the advent of modern binding release mechanisms ,
skiers catching their skis could subject their leg bones to
rapid torsional loading (Fig. 3.6).

Understanding how bone responds to different loading rates
helps to explain the kind of fracture an individual sustains.
Fractures resulting from extremely high velocity projectiles such
as bullets are characterized by bony fragments from the shat¬
tered bone. Fractures from slower-velocity loading events such
as falls typically consist of only two or three fragments.

CHANGES IN MECHANICAL PROPERTIES
WITH AGE AND ACTIVITY

and 92 years old, several of the mechanical properties tested
decreased with age [21]. For example, the Youngs modulus,
predicted by a least squares linear curve fit to the experimen¬
tal data, decreased by about 2.3% for every 10 years after age
35, beginning with a value of 15.2 GPa (which is reasonably
consistent with values summarized by Cowin [5]). The resist¬
ance of bone to fracture, as measured by the fracture tough¬
ness, decreased with age at a rate of about 4% per 10 years
(from 6.4 MPa-m 1/2 ), and the bending strength decreased by
about 3.7% per 10 years (from 170 MPa) [21]. The decreases
in properties may be the result of changes in the bones min¬
eral content or perhaps changes in the bones structure.
Additional studies are needed to fully understand how the
mechanical properties of bone change with age.

Activity translates into increased loading on the bone. When
bone is loaded or stressed, it tends to build up; bone becomes
denser with use. When activity decreases and the loads on the
bone decrease, the bone loses mass through remodeling [2].
One of the concerns with extended voyages in the low-gravity
environment of space (e.g., human flights to Mars or long-term
stays on a permanent space station) is that bone is not loaded
as it is on Earth, and the bone mass decreases [4,9].
Investigators sometimes induce osteoporosis, or an osteo¬
porotic state, by restricting bone loading or mimicking low
gravity in experimental animal models [e.g., 16,20].

Osteoporosis is a disease or condition that is more common in
older people. It is the loss of bone density caused by the failure
of osteoblasts to lay down new bone in the holes created by
osteoclasts. Osteoporosis is thought to be mediated by hor¬
mones, for example, in the case of postmenopausal women.

Clinical Relevance

PREVENTING AND TREATING OSTEOPOROSIS: Loss
of bone mass (osteoporosis) through inactivity or disease
increases the risk of fractures. Postmenopausal women are
particularly susceptible to osteoporosis and thus are at
increased risk of fractures. Weight-bearing exercises such as
walking are an important component of maintaining bone
health by increasing the loads on bones and stimulating
bone growth. Girls and young women should be encour¬
aged to exercise to enhance bone strength long before they
might begin to lose bone mass during the peri- and post¬
menopausal years. Similarly , the National Aeronautics and
Space Agency (NASA) continues to seek the optimal training
and nutrition program that will allow astronauts to main¬
tain bone mass during extended periods in a microgravity
environment.

FRACTURE HEALING

Both bone geometry and the fundamental material properties
of bone appear to change with age, although the subject has
not been studied in depth. In a study of bones between 35

When a bone breaks, the body initiates a cascade of events
to repair the injury. The first step is similar to an inflamma¬
tory response: the bone bleeds at the fracture and possibly

Chapter 3 I BIOMECHANICS OF BONE

the surrounding tissues, and a blood clot forms. The cells
that repair the fracture gather at the fracture site. In about
2 weeks, a callus begins to form. A callus is the precursor to
the calcified bone. When the callus calcifies, it becomes
woven bone, much like the immature bone in children. The
woven bone undergoes remodeling; that is, the osteoclasts
tunnel holes, and the osteoblasts lay down collagen to fill
them in and create the haversian systems of mature or
lamellar bone. The remodeling process continues long after
the bone has healed. Eventually, the bone reforms itself into
its natural shape (e.g., a hollow cylinder for a tibia or femur),
and the intermedullary space is filled again with bone mar¬
row. The woven bone of the callus is not as strong as the
mature bone, but is more flexible and is more isotropic than
lamellar bone. This appears to be the body’s mechanism to
allow the bone to heal and to reduce the potential for
damaging the new bone. Almost all of the data on the
strength of the callus during bone healing comes from
animal models, and it is difficult to determine material prop¬
erty values for healing bone in humans. We do know, quali¬
tatively, that the strength of the callus increases as the bone
mineral density increases, that is only logical; however, the
elastic constants and tensile/compressive strengths are not
known.

Clinical Relevance

A LIMB-LENGTHENING PROCEDURE: A fascinating
example of an "artificial" use of the bone healing process
is in limb-lengthening. Limb-lengthening was pioneered by
a Russian physician named Gavril Ilizarov. Today: the
Ilizarov procedure is used to lengthen unnaturally short
bones caused by either a congenital condition or growth
plate damage, to correct bone deformities, and to help
heal difficult fractures. The Ilizarov procedure makes use
of an external mechanical construct that is fixed to the
bone by percutaneous pins and wires (Fig. 3.7). In the
case of a lengthening, the surgeon first attaches the frame
and then breaks the bone. The bone is gradually pulled
apart over the course of several days; in most instances,
the patient makes adjustments on the external fixator to
separate the ends of the broken bone. In what can only
be called a wonder of nature, new bone begins to grow in
the gap as the bone is distracted. The new bone, called a
callus , is like the woven bone that forms during "stan¬
dard" fracture healing. The method pioneered by Ilizarov
uses the patient's own weight to help bone healing. Over
the course of treatment, the bone gradually bears more of
the patient's weight. The weight bearing stimulates blood
flow in the new bone and thereby stimulates bone forma¬
tion. As the patient progresses, the bone hardens until it
can fully support the patient's weight. The frame is then
removed.

Figure 3.7: An Ilizarov fixator. The Ilizarov construct is applied
externally to stabilize fractures and lengthen limbs. (Photo cour¬
tesy of James J. McCarthy, MD, Shriners Hospitals for Children,
Philadelphia, PA)

SUMMARY

Bone serves as mechanical support, as a system of levers to
transmit forces, and as protection for vital organs in the
human body. Mechanically, bone is an anisotropic material; it
has different mechanical properties in different directions.
Bone is a composite material, consisting primarily of collagen
and HA mineral. In many respects, because of the HA, bone
behaves like a ceramic material: it is stronger in compression
than in tension, and it exhibits brittle fracture. The collagen
imparts some viscoelastic behavior, as well as increased ten¬
sile strength. Several of the referenced texts (especially 5 and
8) give an excellent overview of the properties of bone. The
reader is strongly urged to consult the primary publications
that report on the mechanical properties, however, when a
specific property of a specific bone is needed for analysis.

References

1. An K-N, Chao EYS, Kaufman KR: Analysis of muscle and joint
loads. In: Basic Orthopaedic Biomechanics. Mow VC, Hayes
WC, eds. New York: Raven Press, 1991; 1-50.

2. Bikle DD, Halloran BP: The response of bone to unloading. J
Bone Miner Metab 1999; 17: 233-244.

3. Brinker MR: Basic sciences, section 1, Bone. In: Review of
Orthopaedics, 3rd ed. Miller MD, ed. Philadelphia: WB
Saunders, 2000; 1-22.

4. Buckey JC Jr: Preparing for Mars: the physiologic and medical
challenges. Eur J Med Res 1999; 4: 353-356.

5. Cowin SC: The mechanical properties of cortical bone tissue.
In: Bone Mechanics. Cowin SC, ed. Boca Raton: CRC Press,
1989; 97-128.

6. Eyre DR, Wu JJ, Niyibizi C, Chun L: The cartilage collagens—
analysis of their cross-linking interactions and matrix organiza¬
tions. In: Methods in Cartilage Research. Maroudas A,
Kuettner K, eds. San Diego: Academic Press, 1990; 28-32.

7. Fenech CM, Keaveny TM: A cellular solid criterion for pre¬
dicting the axial-shear failure properties of bovine trabecular
bone. J Biomech Eng 1999; 121: 414-422.

Part I I BIOMECHANICAL PRINCIPLES

8. Hayes WC: Biomechanics of cortical and trabecular bone:
implications for assessment of fracture risk. In: Basic
Orthopaedic Biomechanics. Mow VC, Hayes WC, eds. New
York: Baven Press, 1991; 93-142.

9. Holick MF: Perspective on the impact of weightlessness on
calcium and bone metabolism. Bone 1998; 22(5 Suppl):
105S-111S.

10. Kabel J, van Rietbergen B, Odgaard A, Huiskes R: Constitutive
relationships of fabric, density, and elastic properties in cancel¬
lous bone architecture. Bone 1999; 25: 481-486.

11. Martin RB: Determinants of the mechanical properties of
bones. J Biomech 1991; 24(Suppl 1): 79-88.

12. Niebur GL, Yuen JC, Hsia AC, Keaveny TM: Convergence
behavior of high-resolution finite element models of trabecular
bone. J Biomech Eng 1999; 121: 629-635.

13. Nigg BM, Grimston SK: Bone. In: Biomechanics of the
Musculo-Skeletal System. Nigg BM, Herzog W, eds. Chichester:
John Wiley & Sons, 1994; 47-78.

14. Norman TL, Nivargikar SV, Burr DB: Resistance to crack
growth in human cortical bone is greater in shear than in ten¬
sion. J Biomech 1996; 29: 1023-1031.

15. Norman TL, Vashishth D, Burr DB: Fracture toughness of
human bone under tension. J Biomech 1995; 28: 309-320.

16. Thomas T, Vico L, Skerry TM, et al.: Architectural modifica¬
tions and cellular response during disuse-related bone loss in
calcaneus of the sheep. J Appl Physiol 1996; 80: 198-202.

17. Van Rietbergen B, Muller R, Ulrich D, et al.: Tissue stresses
and strain in trabeculae of a canine proximal femur can be
quantified from computer reconstructions. J Biomech 1999; 32:
443^51.

18. Wang X, Agrawal CM: Fracture toughness of bone using a
compact sandwich specimen: effects of sampling sites and crack
orientations. J Biomed Mater Res (Appl Biomater) 1996; 33:
13-21.

19. Weiner S, Traub W, Wagner HD: Lamellar bone: structure-
function relations. J Struct Biol 1999; 30; 126: 241-255.

20. Wimalawansa SM, Wimalawansa SJ: Simulated weightlessness-
induced attenuation of testosterone production may be respon¬
sible for bone loss. Endocrine 1999; 10: 253-260.

21. Zioupos P, Currey JD: Changes in the stiffness, strength, and
toughness of human cortical bone with age. Bone 1998; 22:
57-66.

CHAPTER

Biomechanics of Skeletal Muscle

CHAPTER CONTENTS

STRUCTURE OF SKELETAL MUSCLE.46

Structure of an Individual Muscle Fiber .46

The Connective Tissue System within the Muscle Belly.48

FACTORS THAT INFLUENCE A MUSCLE'S ABILITY TO PRODUCE A MOTION.48

Effect of Fiber Length on Joint Excursion.48

Effect of Muscle Moment Arms on Joint Excursion .50

Joint Excursion as a Function of Both Fiber Length and the Anatomical Moment Arm of a Muscle .51

FACTORS THAT INFLUENCE A MUSCLE'S STRENGTH.52

Muscle Size and Its Effect on Force Production .52

Relationship between Force Production and Instantaneous Muscle Length (Stretch) .53

Relationship between a Muscle's Moment Arm and Its Force Production.56

Relationship between Force Production and Contraction Velocity.58

Relationship between Force Production and Level of Recruitment of Motor Units within the Muscle .60

Relationship between Force Production and Fiber Type.61

ADAPTATION OF MUSCLE TO ALTERED FUNCTION.62

Adaptation of Muscle to Prolonged Length Changes .62

Adaptations of Muscle to Sustained Changes in Activity Level.63

SUMMARY .64

S keletal muscle is a fascinating biological tissue able to transform chemical energy to mechanical energy. The

focus of this chapter is on the mechanical behavior of skeletal muscle as it contributes to function and dysfunc¬
tion of the musculoskeletal system. Although a basic understanding of the energy transformation from chemi¬
cal to mechanical energy is essential to a full understanding of the behavior of muscle, it is beyond the scope of this
book. The reader is urged to consult other sources for a discussion of the chemical and physiological interactions that
produce and affect a muscle contraction [41,52,86].

Skeletal muscle has three basic performance parameters that describe its function:

■ Movement production
■ Force production
■ Endurance

The production of movement and force is the mechanical outcome of skeletal muscle contraction. The factors that
influence these parameters are the focus of this chapter. A brief description of the morphology of muscles and the

Part I I BIOMECHANICAL PRINCIPLES

physiological processes that produce contraction needed to understand these mechanical parameters are also presented
here. Specifically the purposes of this chapter are to

■ Review briefly the structure of muscle and the mechanism of skeletal muscle contraction

■ Examine the factors that influence a muscle's ability to produce a motion

■ Examine the factors that influence a muscle's ability to produce force

■ Consider how muscle architecture is specialized to optimize a muscle's ability to produce force or joint motion

■ Demonstrate how an understanding of these factors can be used clinically to optimize a person's performance

■ Discuss the adaptations that muscle undergoes with prolonged changes in length and activity

STRUCTURE OF SKELETAL MUSCLE

The functional unit that produces motion at a joint consists
of two discrete units, the muscle belly and the tendon that
binds the muscle belly to the bone. The structure of the mus¬
cle belly itself is presented in the current chapter. The struc¬
ture and mechanical properties of the tendon, composed of
connective tissue, are presented in Chapter 6. The muscle

belly consists of the muscle cells, or fibers, that produce the
contraction and the connective tissue encasing the muscle
fibers. Each is discussed below.

Structure of an Individual Muscle Fiber

A skeletal muscle fiber is a long cylindrical, multinucleated
cell that is filled with smaller units of filaments (Fig. 4.1). These

Figure 4.1: Organization of muscle. A progressively magnified view of a whole muscle demonstrates the organization of the filaments
composing the muscle.

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

filamentous structures are roughly aligned parallel to the mus¬
cle fiber itself. The largest of the filaments is the myofibril,
composed of subunits called sarcomeres that are arranged end
to end the length of the myofibril. Each sarcomere also con¬
tains filaments, known as myofilaments. There are two types of
myofilaments within each sarcomere. The thicker myofila¬
ments are composed of myosin protein molecules, and the
thinner myofilaments are composed of molecules of the pro¬
tein actin. Sliding of the actin myofilament on the myosin
chain is the basic mechanism of muscle contraction.

THE SLIDING FILAMENT THEORY OF MUSCLE
CONTRACTION

The sarcomere, containing the contractile proteins actin and
myosin, is the basic functional unit of muscle. Contraction of a
whole muscle is actually the sum of singular contraction events
occurring within the individual sarcomeres. Therefore, it is
necessary to understand the organization of the sarcomere.
The thinner actin chains are more abundant than the myosin
myofilaments in a sarcomere. The actin myofilaments are
anchored at both ends of the sarcomere at the Z-line and proj¬
ect into the interior of the sarcomere where they surround a
thicker myosin myofilament (Fig. 4.2). This arrangement of
myosin myofilaments surrounded by actin myofilaments is
repeated throughout the sarcomere, filling its interior and giv¬
ing the muscle fiber its characteristic striations. The amount of
these contractile proteins within the cells is strongly related to
a muscles contractile force [6,7,27].

Contraction results from the formation of cross-bridges
between the myosin and actin myofilaments, causing the actin

Myosin tW

Figure 4.2: Organization of actin and myosin within a muscle
fiber. The arrangement of the actin and myosin chains in two
adjacent sarcomeres within a fiber produces the characteristic
striations of skeletal muscle.

chains to “slide” on the myosin chain (Fig. 4.3). The tension of
the contraction depends upon the number of cross-bridges
formed between the actin and myosin myofilaments. The num¬
ber of cross-bridges formed depends not only on the abun¬
dance of the actin and myosin molecules, but also on the
frequency of the stimulus to form cross-bridges.

Contraction is initiated by an electrical stimulus from the
associated motor neuron causing depolarization of the muscle
fiber. When the fiber is depolarized, calcium is released into
the cell and binds with the regulating protein troponin. The
combination of calcium with troponin acts as a trigger, caus¬
ing actin to bind with myosin, beginning the contraction.
Cessation of the nerves stimulus causes a reduction in cal¬
cium levels within the muscle fiber, inhibiting the cross¬
bridges between actin and myosin. The muscle relaxes [86]. If

Figure 4.3: The sliding filament model. Contraction of skeletal muscle results from the sliding of the actin chains on the myosin chains.

Part I I BIOMECHANICAL PRINCIPLES

stimulation of the muscle fiber occurs at a sufficiently high
frequency, new cross-bridges are formed before prior inter¬
actions are completely severed, causing a fusion of suc¬
ceeding contractions. Ultimately a sustained, or tetanic,
contraction is produced. Modulation of the frequency and
magnitude of the initial stimulus has an effect on the force of
contraction of a whole muscle and is discussed later in this
chapter.

The Connective Tissue System within
the Muscle Belly

The muscle belly consists of the muscle cells, or fibers, and the
connective tissue that binds the cells together (Fig. 4.4). The
outermost layer of connective tissue that surrounds the entire
muscle belly is known as the epimysium. The muscle belly is
divided into smaller bundles or fascicles by additional connec¬
tive tissue known as perimysium. Finally individual fibers
within these larger sheaths are surrounded by more connec¬
tive tissue, the endomysium. Thus the entire muscle belly is
invested in a large network of connective tissue that then is
bound to the connective tissue tendons at either end of the
muscle. The amount of connective tissue within a muscle and
the size of the connecting tendons vary widely from muscle to

Figure 4.4: Organization of the connective tissue within muscle.
The whole muscle belly is invested in an organized system of
connective tissue. It consists of the epimysium surrounding
the whole belly, the perimysium encasing smaller bundles of
muscle fibers, and the endomysium that covers individual
muscle fibers.

muscle. The amount of connective tissue found within an
individual muscle influences the mechanical properties of that
muscle and helps explain the varied mechanical responses of
individual muscles. The contribution of the connective tissue
to a muscle s behavior is discussed later in this chapter.

FACTORS THAT INFLUENCE A MUSCLE S
ABILITY TO PRODUCE A MOTION

An essential function of muscle is to produce joint movement.
The passive range of motion (ROM) available at a joint
depends on the shape of the articular surfaces as well as on
the surrounding soft tissues. However the joints active ROM
depends on a muscle s ability to pull the limb through a joints
available ROM. Under normal conditions, active ROM is
approximately equal to a joints passive ROM. However there
is a wide variation in the amount of passive motion available
at joints throughout the body. The knee joint is capable of
flexing through an arc of approximately 140°, but the
metacarpophalangeal (MCP) joint of the thumb usually is
capable of no more than about 90° of flexion. Joints that
exhibit large ROMs require muscles capable of moving the
joint through the entire range. However such muscles are
unnecessary at joints with smaller excursions. Thus muscles
exhibit structural specializations that influence the magnitude
of the excursion that is produced by a contraction. These spe¬
cializations are

• The length of the fibers composing the muscle

• The length of the muscles moment arm.

How each of these characteristics affects active motion of a
joint is discussed below.

Effect of Fiber Length on Joint Excursion

Fiber length has a significant influence on the magnitude of
the joint motion that results from a muscle contraction. The
fundamental behavior of muscle is shortening, and it is this
shortening that produces joint motion. The myofilaments in
each sarcomere are 1 to 2 pm long; the myosin myofila¬
ments are longer than the actin myofilaments [125,149].
Thus sarcomeres in humans are a few micrometers in
length, varying from approximately 1.25 to 4.5 pm with mus¬
cle contraction and stretch [90-92,143]. Each sarcomere
can shorten to approximately the length of its myosin mole¬
cules. Recause the sarcomeres are arranged in series in a
myofibril, the amount of shortening that a myofibril and,
ultimately, a muscle fiber can produce is the sum of the
shortening in all of the sarcomeres. Thus the total shorten¬
ing of a muscle fiber depends upon the number of sarcom¬
eres arranged in series within each myofibril. The more
sarcomeres in a fiber, the longer the fiber is and the more it
is able to shorten (Fig. 4.5). The amount a muscle fiber can
shorten is proportional to its length [15,89,155]. A fiber can
shorten roughly 50 to 60% of its length [44,155], although

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Figure 4.5: The relationship between fiber length and shortening
capacity of the whole muscle. A muscle with more sarcomeres in
series (A) can shorten more than a fiber with fewer sarcomeres in
series (B).

there is some evidence that fibers exhibit varied shortening
capabilities [15].

The absolute amount of shortening a fiber undergoes is a
function of its fiber length. Similarly, the amount a whole
muscle can shorten is dictated by the length of its constituent
fibers. An individual whole muscle is composed mostly of
fibers of similar lengths [15]. However there is a wide varia¬
tion in fiber lengths found in the human body, ranging from a
few centimeters to approximately half a meter [86,146]. The
length of the fibers within a muscle is a function of the archi¬
tecture of that muscle rather than of the muscle s total length.
The following describes how fiber length and muscle archi¬
tecture are related.

ARCHITECTURE OF SKELETAL MUSCLE

Although all skeletal muscle is composed of muscle fibers, the
arrangement of those fibers can vary significantly among
muscles. This fiber arrangement has marked effects on a
muscle s ability to produce movement and to generate force.
Fiber arrangements have different names but fall into two
major categories, parallel and pennate [42] (Fig. 4.6). In
general, the fibers within a parallel fiber muscle are approxi¬
mately parallel to the length of the whole muscle. These mus¬
cles can be classified as either fusiform or strap muscles.
Fusiform muscles have tendons at both ends of the muscle so
that the muscle fibers taper to insert into the tendons. Strap

BIPENNATE
Rectus femoris m.

Figure 4.6: Muscle architecture. A. Muscles with parallel fibers include fusiform (biceps brachii) and strap (sartorius) muscles. B. Pennate
muscles include unipennate (flexor pollicis longus), bipennate (rectus femoris), and multipennate (subscapularis).

Part I I BIOMECHANICAL PRINCIPLES

muscles have less prominent tendons, and therefore their
fibers taper less at both ends of the whole muscle. Parallel
fiber muscles are composed of relatively long fibers, although
these fibers still are shorter than the whole muscle. Even the
sartorius muscle, a classic strap muscle, contains fibers that
are only about 90% of its total length.

In contrast, a pennate muscle has one or more ten¬
dons that extend most of the length of the whole muscle.
Fibers run obliquely to insert into these tendons. Pennate
muscles fall into subcategories according to the number of
tendons penetrating the muscle. There are unipennate,
bipennate, and multipennate muscles. A comparison of
two muscles of similar total length, one with parallel fibers
and the other with a pennate arrangement, helps to illustrate
the effect of fiber arrangement on fiber length (Fig. 4.7). The
muscle with parallel fibers has longer fibers than those found
in the pennate muscle. Because the amount of shortening
that a muscle can undergo depends on the length of its fibers,
the muscle with parallel fibers is able to shorten more than
the pennate muscle. If fiber length alone affected joint excur¬
sion, the muscle with parallel fibers would produce a larger
joint excursion than the muscle composed of pennate fibers

Figure 4.7: The relationship between muscle architecture and
muscle fiber length. The fibers in a muscle with parallel fibers
are typically longer than the fibers in a muscle of similar overall
size but with pennate fibers.

[90]. However, a muscles ability to move a limb through an
excursion also depends on the length of the muscle s moment
arm. Its effect is described below.

Effect of Muscle Moment Arms
on Joint Excursion

Chapter 1 defines the moment arm of a muscle as the per¬
pendicular distance between the muscle and the point of
rotation. This moment arm depends on the location of the
muscle s attachment on the bone and on the angle between
the line of pull of the muscle and the limb to which the mus¬
cle attaches. This angle is known as the angle of application
(Fig. 4.8). The location of an individual muscles attachment
on the bone is relatively constant across the population.
Therefore, the distance along the bone between the muscle s
attachment and the center of rotation of the joint can be
estimated roughly by anyone with a knowledge of anatomy
and can be measured precisely as well [57,81,95,151]. This

Figure 4.8: Angle of application. A muscle's angle of application
is the angle formed between the line of pull of the muscle and
the bone to which the muscle attaches.

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Figure 4.9: The relationship between a muscle's moment arm and excursion. The length of a muscle's moment arm affects the excursion
that results from a contraction. A. Movement through an angle, 0, requires more shortening in a muscle with a long moment arm than
in a muscle with a short moment arm. B. The arc subtended by an angle, 0, is larger in a large circle than in a small circle.

distance is related to the true moment arm by the sine of the
angle of application, 0, which can also be estimated or meas¬
ured directly

A muscle s moment arm has a significant effect on the
joint excursion produced by a contraction of the muscle. A
muscle with a short moment arm produces a larger angular
excursion than another muscle with a similar shortening
capacity but with a longer moment arm. Principles of basic
geometry help explain the relationship between muscle
moment arms and angular excursion. Given two circles of
different sizes, an angle, 0, defines an arc on each circle
(Fig. 4.9). However, the arc of the larger circle is larger
than the arc of the smaller circle. Thus the distance trav¬
eled on the larger circle to move through the angle 0 is
greater than that on the smaller circle. Similarly, a muscle
with a long moment arm must shorten more to produce the
same angular displacement as a muscle with a short
moment arm [76,77].

Joint Excursion as a Function of Both
Fiber Length and the Anatomical Moment
Arm of a Muscle

The preceding discussion reveals that both a muscles fiber
length and its moment arm have direct effects on the amount
of excursion a muscle contraction produces. These effects can
be summarized by the following:

• Because muscle fibers possess a similar relative shortening
capability, longer fibers produce more absolute shortening
than shorter fibers.

• Because muscles with parallel fibers generally have longer
fibers than pennate muscles, whole muscles composed of
parallel fibers have a larger shortening capacity than whole
muscles of similar length composed of pennate fibers.

• Muscles with shorter anatomical moment arms are capa¬
ble of producing greater angular excursions of a joint than
muscles of similar fiber length with larger anatomical
moment arms.

It is interesting to see how these characteristics are com¬
bined in individual muscles. Muscles combine these seem¬
ingly opposing attributes in various ways, resulting in diverse
functional capacities. It appears that some muscles, like the
gluteus maximus, possess both long fibers and relatively
short moment arms. Such muscles are capable of producing
relatively large joint excursions [62]. Others, like the bra-
chioradialis muscle at the elbow, combine relatively long
muscle fibers with large moment arms [89]. The long fibers
enhance the muscle s ability to produce a large excursion.
However, the large moment arm decreases the muscles
ability to produce a large excursion. This apparent contra¬
diction is explained in part by the recognition that the fac¬
tors that influence production of movement, muscle
architecture and anatomical moment arm, also influence
force production capabilities in a muscle. Muscles must find
ways to balance the competing demands of force production
and joint excursion. The ratio of a muscles fiber length to its
moment arm is a useful descriptor of a muscle s ability to
produce an excursion and its torque-generating capability
[99]. This ratio helps surgeons determine appropriate donor
muscles to replace dysfunctional ones.

Part I I BIOMECHANICAL PRINCIPLES

Clinical Relevance

CONSIDERATIONS REGARDING TENDON
TRANSFERS: Muscle fiber arrangement and muscle
moment arms are inherent characteristics of a muscle and
normally change very little with exercise or functional use.
However ; surgeons commonly transfer a muscle or muscles to
replace the function of paralyzed muscles [15,16]. Successful
restoration of function requires that the surgeon not only
replace the nonfunctioning muscle with a functional muscle
but also must ensure that the replacement muscle has an
excursion-generating capacity similar to that of the original
muscle. This may be accomplished by choosing a structurally
similar muscle or by surgically manipulating the moment arm
to increase or decrease the excursion capability [155].

For example, the flexor carpi radialis muscle at the wrist is
a good substitute for the extensor digitorum muscle of the
fingers in the event of radial nerve palsy. The wrist flexor has
long muscle fibers and, therefore, the capacity to extend the
fingers through their full ROM. In contrast, the flexor carpi
ulnaris, another muscle of the wrist, has very short fibers and
lacks the capacity to move the fingers through their full
excursion. Thus the functional outcome depends on the sur¬
geon's understanding of muscle mechanics, including those
factors that influence the production of motion.

FACTORS THAT INFLUENCE A MUSCLE S
STRENGTH

Strength is the most familiar characteristic of muscle per¬
formance. However, the term strength has many different
interpretations. Understanding the factors affecting strength
requires a clear understanding of how the term is used. The
basic activity of muscle is to shorten, thus producing a tensile
force. As noted in Chapter 1, a force also produces a
moment, or a tendency to rotate, when the force is exerted
at some distance from the point of rotation. The ability to
generate a tensile force and the ability to create a moment
are both used to describe a muscle s strength. Assessment of
muscle strength in vivo is typically performed by determin¬
ing the muscle s ability to produce a moment. Such assess¬
ments include determination of the amount of manual
resistance an individual can sustain without joint rotation,
the amount of weight a subject can lift, or the direct meas¬
urement of moments using a device such as an isokinetic
dynamometer. In contrast, in vitro studies often assess mus¬
cle strength by measuring a muscle s ability to generate a ten¬
sile force. Of course the muscle s tensile force of contraction
and its resulting moment are related by the following:

M = r X F (Equation 4.1)

where M is the moment generated by the muscle s tensile
force (F) applied at a distance (r, the muscles moment arm)
from the point of rotation (the joint axis). Therefore, muscle

strength as assessed typically in the clinic by the measure¬
ment of the moment produced by a contraction is a function
of an array of factors that influence both the tensile force of
contraction, F, and the muscles moment arm, r [54]. To
obtain valid assessments of muscle strength and to optimize
muscle function, the clinician must understand the factors
that influence the output of the muscle. All of the following
factors ultimately influence the moment produced by the
muscles contraction. Some affect the contractile force, and
others influence the muscle s ability to generate a moment.
The primary factors influencing the muscle s strength are

• Muscle size

• Muscle moment arm

• Stretch of the muscle

• Contraction velocity

• Level of muscle fiber recruitment

• Fiber types composing the muscle

Each of the factors listed above has a significant effect on the
muscle s moment production. An understanding of each fac¬
tor and its role in moment production allows the clinician to
use these factors to optimize a person performance and to
understand the alteration in muscle performance with pathol¬
ogy. The effects of size, moment arm, and stretch are most
apparent in isometric contractions, which are contractions
that produce no discernable joint motion. Consequently, the
experiments demonstrating these effects usually employ iso¬
metric contractions. However, the reader must recognize that
the effects are manifested in all types of contraction. Each
factor is discussed below.

Muscle Size and Its Effect on Force
Production

As noted earlier in this chapter, the force of contraction is
a function of the number of cross-links made between the
actin and myosin chains [1,39]. The more cross-links
formed, the stronger the force of contraction. Therefore,
the force of contraction depends upon the amount of actin
and myosin available and thus on the number of fibers a
muscle contains. In other words, the force of contraction is
related to a muscles size [67,126]. In fact, muscle size is
the most important single factor determining the tensile
force generated by a muscle’s contraction [44,60].
Estimates of the maximal contractile force per unit of mus¬
cle range from approximately 20 to 135 N/cm 2 [15,22,
120,155]. These data reveal a wide disparity in the esti¬
mates of the maximum tensile force that muscle can pro¬
duce. Additional research is needed to determine if all
skeletal muscle has the same potential maximum and what
that maximum really is.

Although the estimates presented above vary widely, they
do demonstrate that the maximum tensile force produced by
an individual muscle is a function of its area. However, the
overall size of a muscle may be a poor indication of the num¬
ber of fibers contained in that muscle. The relationship

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

between muscle size and force of contraction is complicated
by the muscles architecture. The anatomical cross-sec¬
tional area of the muscle is the cross-sectional area at the
muscle s widest point and perpendicular to the length of the
whole muscle. In a parallel fiber muscle this cross-sectional
area cuts across most of the fibers of the muscle (Fig. 4.10).
However, in a pennate muscle the anatomical cross-sectional
area cuts across only a portion of the fibers composing the
muscle. Thus the anatomical cross-sectional area underesti¬
mates the number of fibers contained in a pennate muscle
and hence its force production capabilities.

The standard measure used to approximate the number of
fibers of a whole muscle is its physiological cross-sectional
area (PCSA). The PCS A is the area of a slice that passes
through all of the fibers of a muscle [15]. In a parallel fiber

Figure 4.10: The relationship between muscle architecture and
muscle size. A. The anatomical cross-sectional area of a muscle is
the area of a slice through the widest part of the muscle per¬
pendicular to the muscle's length. It is similar in a parallel fiber
muscle and a pennate muscle of similar overall size. B. The
physiological cross-sectional area of a muscle is the area of a slice
that cuts across all of the fibers of the muscle. It is quite different
for a parallel fiber muscle and a pennate muscle.

muscle the PCSA is approximately equal to the anatomical
cross-sectional area. However, in a pennate muscle the PCSA
is considerably larger than its anatomical cross-sectional area.
The PCS As of two muscles of similar overall size demonstrate
the influence of muscle architecture on force production.
Although their anatomical cross-sectional areas are very simi¬
lar, the pennate muscle has a much larger PCSA. Thus if all
other factors are equal, the pennate muscle is capable of gen¬
erating more contraction force than the muscle with parallel
fibers [64,90,114].

The angle at which the fibers insert into the tendon also
influences the total force that is applied to the limb by a pen¬
nate muscle. This angle is known as the angle of pennation.
The tensile force generated by the whole muscle is the vector
sum of the force components that are applied parallel to the
muscles tendon (Fig. 4.11). Therefore, as the angle of penna¬
tion increases, the tensile component of the contraction force
decreases. However, the larger the pennation angle is, the
larger the PCSA is [2]. In most muscles the pennation angle
is 30° or less, and thus pennation typically increases the ten¬
sile force produced by contraction [86,146]. Resistance train¬
ing increases the fibers’ angle of pennation (and the muscle s
PCSA). This increase appears to result from increases, or
hypertrophy, in the cross-sectional area of individual muscle
fibers [2,13].

Muscle architecture demonstrates how muscles exhibit spe¬
cializations that enhance one performance characteristic or
another. Long fibers in a muscle promote the excursion-pro¬
ducing capacity of the muscle. However, spatial constraints of
the human body prevent a muscle with long fibers from having
a very large cross-sectional area and hence a large force-pro¬
duction capacity. On the other hand, muscles with a large
PCSA can be fit into small areas by arranging the fibers in a
pennate pattern. However, the short fibers limit the excursion
capacity of the muscle. Thus fiber arrangement suggests that
pennate muscles are specialized for force production but have
limited ability to produce a large excursion. Conversely, a mus¬
cle with parallel fibers has an improved ability to produce an
excursion but produces a smaller contractile force than a pen¬
nate muscle of the same overall size. Thus the intrinsic struc¬
tural characteristics of a muscle help define the performance of
the muscle by affecting both the force of contraction and the
amount of the resulting joint excursion. These intrinsic factors
respond to an increase or decrease in activity over time
[27,64,119,145]. However, instantaneous changes in the mus¬
cle also result in large but temporary responses in a muscle s
performance. These changes include stretching the muscle and
altering its moment arm. These effects are described below.

Relationship between Force Production
and Instantaneous Muscle Length
(Stretch)

Since the strength of muscle contraction is a function of the
number of cross-links made between the actin and myosin
chains within the sarcomeres, alterations in the proximity of

Part I I BIOMECHANICAL PRINCIPLES

Figure 4.11: The pull of a pennate muscle. The overall tensile
force (F m ) of a muscle is the vector sum of the force of contrac¬
tion of the pennate fibers (F f ).

the actin and myosin chains also influence a muscle s force of
contraction. The maximum number of cross-links between
the actin and myosin myofilaments and hence the maximum
contractile force in the sarcomere occurs when the full length
of the actin strands at each end of the sarcomere are in con¬
tact with the myosin molecule [34,50,125] (Fig. 4.12). This
length is operationally defined as the resting length of the

Figure 4.12: The length-tension curve of a sarcomere. The
length-tension curve of a sarcomere demonstrates how the
length of the sarcomere influences its force production.

muscle. The sarcomere can shorten slightly from this point,
maintaining the maximum cross-linking. However, increased
shortening causes the actin strands from each end of the sar¬
comere to interfere with each other. This reduces the number
of available sites for cross-bridge formation, and the force of
contraction decreases. Similarly, when the sarcomere is
stretched from its resting length, contact between the actin
and myosin myofilaments decreases, and thus the number of
cross-links that can be made again diminishes. Consequently,
the force of contraction decreases.

Investigation of the effects of stretch on the whole muscle
reveals that the muscle s response to stretch is affected both
by the behavior of the sarcomere described above and by the
elastic properties of the noncontractile components of the
muscle, including the epimysium, perimysium, endomysium,
and tendons [43,45,53,121]. The classic studies of the
length-tension relationships in muscle were performed by
Blix in the late 19th century but have been repeated and
expanded by others in the ensuing 100 years [43,45,
88,90,121]. These studies, performed on whole muscle, con¬
sistently demonstrate that as a muscle is stretched in the
absence of a contraction, there is some length at which the
muscle begins to resist the stretch (Fig. 4.13). As the stretch
of the muscle increases, the muscle exerts a larger pull against
the stretch. This pull is attributed to the elastic recoil of the
passive structures within the muscle, such as the investing
connective tissue. These components are known as the par¬
allel elastic components. The tendons at either end of the
muscle also provide a force against the stretch. These are
described as the series elastic components.

The combined effects of muscle contraction and stretch of
the elastic components are represented mechanically by a
contractile element in series and in parallel with the elastic
components (Fig. 4.14). The response of both the contractile
and elastic components together is examined by measuring
the resistance to increasing stretch while simultaneously stim¬
ulating the muscle to induce a contraction. Such experiments
reveal that when the muscle is very short, allowing no passive
recoil force, stimulation produces a small contractile force. As
the stretch increases and stimulations continue, the tension in

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Figure 4.13: The length-tension curve of a whole muscle. The
length-tension curve of a whole muscle demonstrates how mus¬
cle length affects the force production of the whole muscle. The
contractile, or active, component; the passive component prima¬
rily due to the connective tissue; and the total muscle tension all
are affected by the stretch of the muscle.

the muscle increases. In the middle region of stretch, the
muscle s force plateaus or even decreases, even with stimula¬
tion. This plateau occurs at approximately the resting length
of the muscle. With additional stretch, the tension in the
whole muscle begins to increase again and continues to
increase with further stretch. By subtracting the results of the
passive test from the results of the combined test, the con¬
tribution of the active, or contractile, component to muscle

Figure 4.14: A mechanical model of the contractile and elastic
components of a muscle. A muscle's contractile (actin and
myosin) and elastic (connective tissue) components are often
modeled mechanically as a combination of a contractile element
(CE) with springs that represent the elastic elements that are both
in series (SE) and in parallel (PE) with the contractile component.

tension is determined. The active contribution to muscle ten¬
sion in the whole muscle is similar to the length-tension rela¬
tionship seen in the individual sarcomeres. These results
demonstrate that while the contractile contribution to muscle
tension peaks in the midregion of stretch, the passive compo¬
nents of the muscle make an increasing contribution to force
after the midrange of stretch. Thus the overall tension of the
muscle is greatest when the muscle is stretched maximally.

It is important to recognize that the experiments described
above are performed on disarticulated muscles. Consequently,
the extremes of shortening and lengthening tested are non-
physiological. An intact human muscle functions somewhere
in the central portion of the length-tension curve, although
the precise shape of the length-tension curve varies across
muscles [45,152]. The response to stretch depends on the
architecture of the individual muscle as well as the ratio of
contractile tissue to connective tissue in the muscle [45]. In
addition, the exact amount of stretch and shortening sustained
by a muscle depends on the individual muscle and the joint.
Muscles that cross two or more joints undergo more shorten¬
ing and lengthening than muscles that span only one joint. The
force output of such multijointed muscles is influenced signif¬
icantly by the length-tension relationship [56,123].

Clinical Relevance

THE LENGTH-TENSION RELATIONSHIP OF
MUSCLES IN VIVO: Weakness is a common impairment
in individuals participating in rehabilitation. Sometimes
individuals are too weak to be able to move the limb
much at all. By positioning the patient's limb so that the
contracting muscles are functioning in the stretched posi¬
tion i, the clinician enhances the muscle's ability to generate
tension. For example; hyperextension of the shoulder
increases elbow flexion strength by stretching the biceps
brachii. Conversely , placing muscles in a very shortened
position decreases their ability to generate force. Muscles
of the wrist and fingers provide a vivid example of how
the effectiveness of muscles changes when they are length¬
ened or shortened (Fig. 4.15). It is difficult to make a force¬
ful fist when the wrist is flexed because the finger flexor
muscles are so short they produce insufficient force. This
phenomenon is known as active insufficiency. Inspection
of the wrist position when the fist is clenched normally
reveals that the wrist is extended , thereby stretching the
muscles , increasing their contractile force; and avoiding
active insufficiency.

The classic length-tension relationship described so far
has been studied by altering the length of a muscle passively
and then assessing the strength of contraction at the new
length. More recent studies have investigated the effects of

Part I I BIOMECHANICAL PRINCIPLES

Figure 4.15: The effects of muscle length on performance. A.
When the wrist is in flexion it is difficult to flex the fingers fully
because the finger flexors are so shortened. B. When the wrist
is in extension, the fingers readily flex to make a fist since the
finger flexors are lengthened.

length changes on isometric strength while the muscle is
actively contracting. These studies consistently demonstrate
that the traditional length-tension relationships are amplified
if the length changes occur during contraction. Specifically, if
a contracting muscle is lengthened and then held at its
lengthened position, the force generated at the lengthened
position is greater than the strength measured at that same
position with no preceding length change [55,128]. Similarly,
shortening a muscle as it contracts produces more strength
reduction than placing the relaxed muscle at the shortened
position and then measuring its strength [124,128]. Many vig¬
orous functional activities occur utilizing muscle contractions
that consist of a lengthening then shortening contraction
cycle [102]. Such a pattern of muscle activity appears to uti¬
lize the length-tension relationship to optimize a muscles
ability to generate force.

A muscle s length, and therefore its force of contraction,
changes as the joint position changes. However, the length
of the muscle is only one factor that changes as the
joint position changes. The moment arm of the
V®*/ muscle also varies with joint position. The influence
of a muscles moment arm on muscle performance is
described below.

Clinical Relevance

STRETCH-SHORTENING CYCLE OF MUSCLE
CONTRACTION IN SPORTS: The strength enhancement
that comes from lengthening a contracting muscle prior to
using it to produce motion is visible in countless activities,
particularly in sports. For example the wind-up that pre¬
cedes a throw or the backswing of a golf swing serves to
stretch the muscles that will throw the ball or swing the
golf club. The shoulder medial rotators are stretched prior
to the forward motion of the throw , and the shoulder
abductors and lateral rotators of the left arm are stretched
prior to the forward motion of the golf swing for a right-
handed golfer. Similarly the start of a running sprint
event is characterized by a brief stretch of the piantar
flexors , knee extensors and hip extensors before these
same muscles shorten to push the runner down the
track. The stretch of all of these muscles occurs as
they are contracting and consequently amplifies even
more the strength gains resulting from the stretch
FjHPj itself. (See the jumping activity in Chapter 4
laboratory.)

Relationship between a Muscle s
Moment Arm and Its Force Production

As noted earlier, a muscle s ability to rotate a joint depends
upon the muscle s force of contraction and on its moment
arm, the perpendicular distance from the muscle force to
the point of rotation [125]. The previous discussion reveals
that muscle size and the stretch of the muscle have a signif¬
icant impact on the force of contraction. However, the mus¬
cles moment arm is critical in determining the moment
generated by the muscle contraction. The larger the
moment arm, the larger the moment created by the muscle
contraction. The relationship between a muscles moment
arm and its angle of application is described earlier in the
current chapter. The moment arm is determined by the sine
of the angle of application and the distance between
the muscles attachment and the joints axis of rotation
(Fig. 4.16). The muscles moment arm is maximum when
the muscle’s angle of application is 90°, since the sine of 90°
equals 1. A muscle with a large moment arm produces a
larger moment than a muscle with a shorter moment arm if
both muscles generate equal contractile forces (Fig. 4.17).
The moment arms of some muscles such as the hamstrings
change several centimeters through the full ROM of the
joint, while others such as the flexor digitorum profundus
demonstrate very little change (Fig. 4.18) [57,70,71,81,
113,135,151]. Therefore, a muscles ability to produce a
moment varies with the joint position.

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Figure 4.16: Moment arm of a muscle. A muscle's moment arm
(I) is easily calculated using the muscle's angle of application (0)
and the distance (d) from the muscle attachment to the axis of
rotation.

I = d x sin 0

INTERACTION BETWEEN A MUSCLE'S MOMENT
ARM AND ITS LENGTH WITH CHANGING JOINT
POSITIONS

It is easy to observe the positions that shorten or lengthen a
muscle. For example, elbow flexion lengthens the elbow
extensor muscles and shortens the elbow flexors. Although
somewhat less obvious, a knowledge of anatomy allows the
clinician to estimate the effects of joint position on a muscle’s
angle of application and thus on its moment arm. The angle
of application of the biceps brachii is almost zero with the
elbow extended and increases to over 90° with the elbow
flexed maximally In this case, the muscle’s moment arm is at
a minimum when the muscles length is at a maximum. In
contrast, the angle of application is greatest when the length
is shortest. The optimal angle of application, 90°, occurs
when the elbow is flexed to approximately 100° of elbow flex¬
ion [4,113]. Thus the muscles ability to generate a large con¬
tractile force as a result of stretch is maximum in the very
position in which the muscle’s ability to produce a moment is
smallest by virtue of its moment arm. Consequently, the
biceps produces peak moments in the midrange of elbow
flexion where neither the muscle’s length nor angle of appli¬
cation is optimal. The relative contribution of moment arm

Figure 4.17: The effect of moment arm of a muscle on the mus¬
cle's performance. A muscle with a short moment arm (1^ gener¬
ates a smaller moment than a muscle with a longer moment arm
(l 2 ) that generates the same contraction force.

and muscle length to a muscle’s ability to produce a moment
varies among the muscles of the body and depends on the
individual characteristics of each muscle and joint [62,82,
87,100,112,148].

In a series of elegant experiments Lieber and colleagues
assessed the combined effects of muscle size, moment arm,
and length on the ability of the primary wrist muscles, the
flexor carpi ulnaris, flexor carpi radialis, extensor carpi
ulnaris, and extensor carpi radialis longus and extensor carpi
radialis brevis to produce a joint torque in the directions of
wrist flexion, extension, and radial and ulnar deviation
[88,94,95]. These investigations reveal that the influence of
moment arms and muscle lengths differs markedly among
these muscles of the wrist. The output from the wrist exten¬
sor muscles correlates well with the muscles’ moment arms,
suggesting that their output depends largely on their
moment arms and is less influenced by muscle length. In
contrast, the output of the wrist flexors is nearly maximum
over a large portion of the wrist range, suggesting that both
moment arm and muscle length have significant impact on
the muscles’ performance.

Part I I BIOMECHANICAL PRINCIPLES

Figure 4.18: Changes in muscle moment arms. A. The hamstrings'
moment arm at the knee is small with the knee extended and
much larger with the knee flexed. B. The moment arm of a ten¬
don of the flexor digitorum profundus at the finger changes lit¬
tle with the fingers extended or flexed.

Clinical Relevance

JOINT POSITION S INFLUENCE ON MUSCLE
STRENGTH: Joint position is likely to have a dramatic
effect on the output from a muscle contraction , since joint
position affects both the stretch and the moment arm of a
muscle. The exact influence is revealed through careful test¬
ing and varies across muscles and joints. Similarly , only
careful investigation provides an explanation for the precise
nature of the relationship between joint position and muscle
force. However ; a valid clinical assessment of strength
requires that the joint position at which strength is assessed

be maintained for each subsequent test. The clinician must
consider the effects of joint position on muscle output when
measuring strength and also when designing intervention
strategies to improve muscle function. Unless the effects of
muscle moment arm and muscle length are held constant
changes in strength resulting from intervention cannot be
distinguished from changes resulting from the mechanical
change in the muscle.

The following scenario provides a helpful demonstration.

In the initial visit to a patient treated at home ’ the clinician
measures hip flexion strength while the patient is sitting in a
wheelchair. Weakness is identified ' and exercises are provided.
On the next visit 2 days later ; the clinician finds the patient in
bed and so measures hip flexion strength in bed with the hip
extended. Hip flexion strength is greater at this measurement
than in the previous measurement. The astute therapist rec¬
ognizes that the apparent increase in strength may be attrib¬
utable to the change in position , since muscle hypertrophy as
a result of exercise is unlikely after only 2 days. Research
demonstrates that the hip flexors are strongest with the hip
close to extension where the muscles are in a lengthened
position (Chapter 39). It is noteworthy to recognize that in this
position the angle of application is relatively small as well
suggesting that muscle length is a larger influence on hip
flexion strength than is angle of application.

Relationship between Force Production
and Contraction Velocity

The chapter to this point has examined the influence of mus¬
cle factors on force production only in isometric contractions,
contractions with no visible change in muscle length.
However in nonisometric contractions, the direction and
speed of contraction influence the muscle s output. Speed of
movement and its direction are described together by the
vector quantity velocity. This section examines the effects of
contraction velocity on muscle output. Both the direction and
the magnitude of the velocity are important influences and
are discussed individually below.

EFFECTS OF THE MAGNITUDE OF THE
CONTRACTION VELOCITY ON FORCE
PRODUCTION IN MUSCLE

Contractile velocity of a muscle is determined usually by the
macroscopic change in length per unit time. Thus an isomet¬
ric contraction has zero contraction velocity. It is important
to recognize that on the microscopic level there is a change in
length of the muscle even in an isometric contraction. In
contrast, a concentric contraction, also known as a short¬
ening contraction, is defined as a contraction in which there
is visible shortening of the muscle [37]. Thus a concentric
contraction has a positive contraction velocity.

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Figure 4.19: The relationship between contractile force and the
velocity of contraction in isometric and concentric contractions. A
plot of contractile force and the velocity of contraction from iso¬
metric ( F) to concentric contractions shows that the strength of
the contraction decreases with increasing contractile velocity.

The relationship between contractile force and speed of
contraction in isometric and shortening contractions has been
studied for most of the 20th century and is well understood
[36,38,68,75,122,141,147]. A plot of a muscles force of con¬
traction over contractile velocity for isometric and concentric
contractions reveals that contractile force is maximum when
contraction velocity is zero (isometric contraction) and
decreases as contraction velocity increases (Fig. 4.19). Thus
an isometric contraction produces more force than a concen¬
tric contraction of similar magnitude. Similarly, a rapid short¬
ening contraction produces less force than a slow shortening
contraction.

Clinical Relevance

EXAMINING MUSCLE STRENGTH IN THE CLINIC: Both
isometric and concentric contractions are used in the clinic
to assess strength. For example ’ one form of the standard¬
ized manual muscle testing procedures examines the force
of an isometric contraction at the end of range, while
another form measures the force of a concentric contraction
through the full ROM to grade a muscle's force [59].
Similarly, clinicians use isokinetic dynamometers to measure
both isometric and concentric strength. Each of these tests is
valid, and there is a correlation among maximum force at
various contraction velocities [74,118]. However, it is impor¬
tant for clinicians to recognize that the absolute force pro¬
duced depends on the testing mode. If all other factors of
muscle performance are constant, the isometric contractions
produce greater forces than the concentric forces.

Judgments regarding the adequacy of an individual's
strength must consider the effects of contraction velocity.

Velocity of contraction

Figure 4.20: The relationship between contractile force and the
velocity of contraction in isometric, concentric, and eccentric con¬
tractions. A plot of contractile force and the velocity of contrac¬
tion from eccentric to concentric contractions shows that an
eccentric contraction is stronger than either isometric (F 7 ) or con¬
centric contractions.

EFFECTS OF THE DIRECTION OF CONTRACTION
ON FORCE PRODUCTION IN MUSCLE

As noted earlier, both the magnitude and direction of the con¬
traction influence a muscle s performance. A contraction that
occurs as a muscle visibly lengthens is called an eccentric
contraction. Eccentric contractile strength is less well
understood than isometric and concentric strength, at
least in part because it is difficult to study lengthen¬
ing contractions over a large spectrum of speeds in
intact muscles. Despite this limitation, many studies have
been completed and provide important information regard¬
ing the comparative contractile force of eccentric contrac¬
tions. A plot of muscle tension over the whole spectrum of
contraction velocities reveals that eccentric contractions pro¬
duce more force than either isometric or concentric contrac¬
tions [28,36,46,58,61,78,80,117,127,132,140,154] (Fig. 4.20).
Maximum eccentric strength is estimated to be between 1.5
and 2.0 times maximum concentric strength [127,144]. The
plot of muscle force as a function of contraction velocity also
reveals that the effect of the magnitude of contraction veloc¬
ity on force production plateaus in an eccentric contraction
[28,36,91].

Clinical Relevance

POST-EXERCISE MUSCLE SORENESS: Studies indicate
that delayed-onset muscle soreness (DOMS) typically is asso¬
ciated with exercise using resisted eccentric exercise [11,40].
Although this phenomenon has not been thoroughly
explained, one possible explanation is that a muscle

(continued)

Part I I BIOMECHANICAL PRINCIPLES

(Continued)

generates greater forces in maximal eccentric contractions
than in maximal concentric contractions. Thus the DOMS
may be the result of excessive mechanical loading of the
muscle rather than an intrinsic difference in physiology of
the eccentric contraction.

It is important to note that the length-tension relationship in
muscle demonstrated earlier in the current chapter persists
regardless of the direction or speed of the contraction. As a
result, the shape of the plots of muscle force through the
ROM are similar, regardless of velocity [75,79] (Fig. 4.21).

Relationship between Force Production
and Level of Recruitment of Motor Units
within the Muscle

Earlier in the current chapter it is reported that the strength
of the cross-links between actin and myosin is influenced by
the frequency of stimulation by the motor nerve. Examination
of the function of a whole muscle reveals a similar relationship.
A whole muscle is composed of smaller units called motor
units. A motor unit consists of the individual muscle fibers
innervated by a single motor nerve cell, or motoneuron. The
force of contraction of a whole muscle is modulated by the fre¬
quency of stimulation by the motor nerve and by the number
of motor units active. A single stimulus of low intensity from
the motor nerve produces depolarization of the muscle and a
twitch contraction of one or more motor units. As the fre¬
quency of the stimulus increases, the twitch is repeated. As in
the single fiber, if the stimulus is repeated before the muscle

Figure 4.21: Comparison of eccentric, isometric, and concentric
muscle strengths with changing muscle length. A comparison of
eccentric, isometric, and concentric muscle strengths through an
ROM reveals that the force of an eccentric contraction is greater
than the force of an isometric contraction, which is greater than
the force of a concentric contraction, regardless of the length of
the muscle.

relaxes, the twitches begin to fuse, and a sustained, or tetanic,
contraction is elicited. As the intensity of the stimulus
increases, more motor units are stimulated, and the force of
contraction increases. Thus a muscle is able to produce maxi¬
mal or submaximal contractions by modifying the characteris¬
tics of the stimulus from the nerve.

The amount of activity of a muscle is measured by its elec¬
tromyogram (EMG). The EMG is the electrical activity
induced by depolarization of the muscle fibers. In an isomet¬
ric contraction, there is a strong relationship between the
electrical activity of the muscle, its EMG, and the force of
contraction. As isometric force increases, the EMG also
increases [24,30,31,78,130,136,137,142]. This relationship is
logical, since the force of contraction is a function of the num¬
ber of cross-links formed between the actin and myosin
chains and thus a function of the number of muscle fibers
contracting. The EMG reflects the number of active fibers as
well as their firing frequency [8,10,12,26,134]. However, the
relationship of the muscle s EMG and its force of contraction
is more complicated when the muscle is free to change length
and the joint is free to move.

This chapter demonstrates that the size and stretch of the
muscle, the muscles moment arm, and the velocity of con¬
traction all contribute to the force produced by contraction.
The EMG merely serves to indicate the electrical activity in
a muscle. Thus a larger muscle produces a larger EMG pat¬
tern during a maximal contraction than a smaller muscle per¬
forming a maximal contraction, since there are more motor
units firing in the larger muscle. However, within the same
muscle, a maximal eccentric contraction elicits an EMG pat¬
tern similar to that produced during a maximal concentric
contraction, even though the force of contraction is greater
in the eccentric contraction [78,127]. In the case of maximal
contractions, the muscle recruits approximately the same
number of motor units regardless of the output. The magni¬
tude of the force output from concentric and eccentric con¬
tractions varies primarily because of the mechanical effects
of contraction velocity.

Clinical Relevance

ASSESSMENT OF PEAK STRENGTH: The basic premise
of strength assessment is that the test subject is producing a
maximal contraction; that is; the subject is maximally
recruiting available motor units. The validity and reliability
of muscle testing depends upon the tester's ability to moti¬
vate the individual to produce a maximal contraction. A
classic study of the reliability of manual muscle testing
reveals that an important factor explaining the lack of relia¬
bility is that some testers failed to elicit a maximal contrac¬
tion , erroneously grading a submaximal contraction [65].
Encouraging a subject to produce a maximal effort requires
both psychological and mechanical skills that are developed
with knowledge and practice but are essential to valid and
reliable measures of strength.

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

Maximal contractions are assumed to activate all of the
motor units of the muscle. In young healthy adults this
appears to be the case; that is they can typically activate
98-100% of the available motor units [103]. In contrast indi¬
viduals who have pain or who are chronically inactive may
be unable to fully activate the muscle, even though they are
attempting to perform a maximal voluntary contraction
(MVC) and appear to have an intact neuromuscular system
[63,106,138]. These individuals exhibit activation failure
in which, despite their best efforts and in the presence of
intact muscles and nerves, they are unable to recruit all of
the available motor units of the muscle. It is important that
the clinician be able to determine if muscle weakness is the
result of morphological changes in the muscles or nerves or
activation failure.

Clinical Relevance

ACTIVATION FAILURE IN INDIVIDUALS WITH
OSTEOARTHRITIS: Individuals with either hip or knee
osteoarthritis exhibit activation failure in the involved joints
[63,138]. This failure is also described as arthrogenic inhi¬
bition, suggesting, that joint pain inhibits full muscle activa¬
tion. Yet similar activation failure is found in individuals
1 year following total knee replacements when pain is no
longer a complaint. Traditional exercises appear to have lit¬
tle effect on the activation failure, but more dynamic func¬
tional exercises have produced improved recruitment in
patients with knee osteoarthritis [63]. Neuromuscular electri¬
cal stimulation also reduces activation failure. Identifying
activation failure as a cause of muscle weakness may alter
the intervention strategies used to improve strength.

In a submaximal contraction, the muscle recruits enough
motor units to produce the necessary muscle force. A muscle
that is lengthened or positioned with a large moment arm is
said to be at a mechanical advantage. It can produce the
same moment with less recruitment and consequently
a smaller EMG than when the muscle is at a mechan¬
ical disadvantage, positioned at a shortened length, or
with a small moment arm [66,109]. When a muscle is at a
mechanical advantage or when it is stronger, it needs fewer
motor units to generate a moment; when the muscle is at a
mechanical disadvantage or is weaker, it must recruit more
motor units to generate the same moment [9,30,108].

This literature review demonstrates that EMG reflects the
relative activity of a muscle rather than providing a direct
measure of the force of that muscles contraction. The litera¬
ture is filled with studies of the EMG activity of muscles dur¬
ing function. These studies are used to explain the role of
muscles during activity. Reference to such articles is made
frequently throughout this textbook. However, caution is
needed when interpreting these studies, since EMG reflects

only the relative activity of a muscle. Muscle size and
mechanical advantage affect the recorded electrical activity.
There are also several technical factors that influence the
magnitude of EMG produced during muscle contraction.
These include the type and size of the recording electrodes
and the signal-processing procedures. These issues are
beyond the scope of this book, but they serve as a warning to
the clinician that interpretation of EMG and comparisons
across studies must proceed with caution. To improve the
generalizability of EMG data, analysis of the electrical activ¬
ity of a muscle typically involves some form of normalization
of the data. A common normalizing procedure is to compare
the activity of a muscle to the EMG produced by a maximal
voluntary contraction (MVC). The basic premise in this
approach is that an MVC requires maximal recruitment of a
muscle s motor units, which then produces a maximum elec¬
trical signal. This maximal activity is used as the basis for com¬
paring the muscles level of activity in other activities.
Processing of the electrical signal also affects the interpreta¬
tion of the signal [8]. A discussion of the issues involved in the
analysis of EMG data is beyond the scope of this book.
However, the reader is urged to use EMG data cautiously
when analyzing the roles of individual muscles.

Relationship between Force Production
and Fiber Type

The last characteristic of muscle influencing the force of con¬
traction to be discussed in this chapter is the type of fibers
composing an individual muscle. Different types of muscle
fibers possess different contractile properties. Therefore,
their distribution within a muscle influences the contractile
performance of a whole muscle. However, because human
muscles are composed of a mix of fiber types, fiber type has
less influence on the force-producing capacity of a muscle
than do the factors discussed to this point.

There are a variety of ways to categorize voluntary muscle
fibers based on such characteristics as their metabolic
processes, their histochemical composition, and their pheno¬
type. Although each method examines different properties,
each identifies groups ranging from fatigue-resistant fibers
with slow contractile properties to rapidly fatiguing cells with
faster contractile velocities [153]. A common cataloging sys¬
tem based on metabolic properties classifies most human mus¬
cle fibers as type I, type Ha, or type lib fibers. Characteristics
of these three fiber types are listed in Table 4.1. For the pur¬
poses of the current discussion, a closer examination of the
mechanical properties of these fibers is indicated. In general,
the contractile force of a type lib fiber is greater than that of
a type I fiber [14]. Thus muscles composed of more type lib
fibers are likely to generate larger contractile forces than a
comparable muscle consisting of mostly type I fibers [110].
Type I fibers are innervated by small-diameter axons of the
motor nerve. They are recruited first in a muscle contraction.
Type lib fibers are innervated by large axons and are recruited
only after type I and type Ha fibers. Type lib fibers are
recruited as the resistance increases [105,107].

Part I I BIOMECHANICAL PRINCIPLES

TABLE 4.1: Basic Performance Characteristics of Types

1, lla, and lib Muscle Fibers

lla

Contraction velocity

Slow

Moderately fast

Fast

Contractile force

Low

Variable

High

Fatigability

Fatigue resistant

Somewhat fatigue resistant

Rapidly fatiguing

The velocity of contraction also differs among fiber types
[3,14]. Consequently the force-velocity relationship also
varies among the fiber types. Data from human muscles sug¬
gest that type lib fibers exert larger forces at higher velocities,
while type I fibers have slower maximal contractile velocities
as well as lower peak forces [14]. Thus muscles with a pre¬
ponderance of type II fibers have a higher rate of force pro¬
duction and a higher contractile force than muscles with
more type I fibers [1].

Postural muscles typically are composed largely of type I
fibers, while muscles whose functions demand large bursts
of force consist of more type II fibers [1,133]. However, as
already noted, human muscles contain a mixture of fiber
types [32,33,104,107]. Therefore, the contractile properties
of whole muscles reflect the combined effects of the fibers
types. Consequently, the other factors influencing force
production such as muscle size and mechanical advantage
appear to have a larger influence on contractile force [25].
However, muscle fibers demonstrate different responses to
changes in activity and thus play a significant role in muscle
adaptation. The adaptability of muscle is discussed briefly
below.

ADAPTATION OF MUSCLE TO ALTERED
FUNCTION

Muscle is perhaps the most mutable of biological tissues. A
discussion of the mechanical properties of muscle cannot be
complete without a brief discussion of the changes in these
mechanical properties resulting from changes in the demands
placed on muscle. The following provides a brief discussion of
the changes in muscle that occur in response to sustained
changes in

• Muscle length

• Activity level

Understanding the effects of sustained changes in muscle
length or activity level is complicated by the recognition
that these factors are often combined in investigations.
Studies assessing the effects of length changes often use
immobilization to apply the length change. Consequently,
the muscles respond to both the altered length and
decreased activity. As a result, a complete understanding of
the influence of these factors on muscle function continues
to elude investigators. The following briefly reviews the
current state of knowledge of muscles’ adaptation to altered
function.

Adaptation of Muscle to Prolonged
Length Changes

The relationship between stretch of a muscle and its force of
contraction is presented in detail elsewhere in this chapter.
This relationship is a function of both the contractile and non-
contractile components of muscle. However, it also is impor¬
tant to ponder the effect of prolonged length change on the
length-tension relationship. Since muscles are organized in
groups of opposing muscles, when one muscle is held on
stretch, another muscle is held in a shortened position.
Therefore, it is important to consider a muscle s response to
both prolonged lengthening and prolonged shortening. The
vast majority of studies examining alterations in muscle
resulting from prolonged length changes use immobilization
procedures to provide the length change. Therefore, the
reader must exert caution when attempting to generalize
these results to other cases such as postural abnormalities that
do not involve immobilization.

CHANGES IN MUSCLE WITH PROLONGED
LENGTHENING

In general, prolonged stretch of a muscle induces protein syn¬
thesis and the production of additional sarcomeres [48,49,139,
150,153]. The muscle hypertrophies, and as a result, peak
contractile force is increased with prolonged stretch. The
addition of sarcomeres in series increases the overall length of
the muscle fibers. This remodeling appears important in
allowing the muscle to maintain its length-tension relation¬
ship. There also is evidence of changes in the metabolic char¬
acteristics of muscle cells subjected to prolonged stretch.
Some muscles exhibit changes in mRNA consistent with a
transition from type II to type I fibers [153].

Although hypertrophy is the typical muscle response to
prolonged stretch, studies report more varied responses
among individual muscles. Changes in muscle mass, peak
strength, and even gene expression with prolonged stretch
vary across muscles and appear to depend upon the muscle s
fiber type composition and its function [86,96].

CHANGES IN MUSCLE HELD IN A SHORTENED
POSITION FOR A PROLONGED PERIOD

Investigation into the effects of prolonged shortening also
demonstrates a complex response. Prolonged shortening pro¬
duced by immobilization appears to accelerate atrophy, and
muscles demonstrate a loss of sarcomeres [48,139,153]. Some
muscles immobilized in a shortened position also show

Chapter 4 I BIOMECHANICS OF SKELETAL MUSCLE

evidence of a transition toward type II fibers. Yet a study exam¬
ining the effects of shortening without immobilization reports
an increase in sarcomeres [77]. Results of this study suggest
that tendon excursion may be a stronger factor than the short¬
ening itself in determining the muscles remodeling. In addi¬
tion, like prolonged stretch, prolonged shortening yields
different responses in different muscles [86].

Clearly, complete understanding of the factors inducing
muscle adaptation requires further investigation. The studies
reported here demonstrate that the adaptability of muscle to
prolonged length changes is complex and depends on many
factors besides the specific change in length. Yet these stud¬
ies do consistently demonstrate changes that seem directed,
at least in part, at maintaining a safe and functional
length-tension relationship in each muscle [86,125,139].

Clinical Relevance

PROLONGED LENGTH CHANGES IN MUSCLE AS THE
RESULT OF POSTURAL ABNORMALITIES: Postural
abnormalities reportedly produce prolonged lengthening of
some muscles and prolonged shortening of other muscles
[69]. This has led to the belief that abnormal posture pro¬
duces changes in muscle strength. Studies have attempted
to identify such changes in strength and changes in the
length-tension relationships of muscles that appear to be
affected by postural abnormalities [23,116]. However ; these
studies fail to demonstrate a clear change in strength attrib¬
utable to length changes. Yet clinicians continue to treat
abnormal postural alignment with strengthening and
stretching exercises. Although current studies neither prove
nor disprove the existence of clinically measurable changes
in muscle as the result of prolonged length changes; they
emphasize the need for clinicians to use caution in assum¬
ing relationships between postural alignment and muscular
strength.

Adaptations of Muscle to Sustained
Changes in Activity Level

Muscles basic response to changes in activity level is well
known: increased activity results in hypertrophy and
increased force production, and decreased activity leads to
atrophy and decreased force production. Of course the exact
response is far more complicated than this. The response
depends on the nature of the activity change and on the
nature of the muscle whose activity is altered.

Resistance exercise leads to muscle hypertrophy and
increased strength in both men and women of virtually all
ages [18,72,83,119,145]. Strengthening exercises in humans
produce an increase in the cross-sectional area (CSA) of both
type I and type II fibers, although there is evidence that there
is a greater increase in the CSA of type II fibers

[25,27,61,97,101,115]. In addition, animal studies reveal that
protein synthesis is consistent with a transition from type lib
fibers to type I fibers [6,86].

In contrast, decreased activity produces a decrease in CSA
and loss of strength [47,85,115]. One study reports a 13%
decrease in some lower extremity strength in 10 healthy sub¬
jects who underwent only 10 days of non-weight-bearing
activity [9]! Disuse atrophy is apparent in both type I and type
II fibers. In addition, there is evidence supporting a transition
from type I fibers to type II fibers [5,6,96].

Although the preceding discussion demonstrates general
patterns of muscle response to changes in activity level, the
response is actually quite muscle dependent [85,86]. One
study reports a 26% loss in plantarflexion strength with no sig¬
nificant loss in dorsiflexion strength in healthy individuals fol¬
lowing 5 weeks of bed rest [85]. Animal studies show similar
differences among muscle groups [19,86]. Other mechanical
factors such as stretch also affect a muscles response to
reduced activity [96].

Clinical Relevance

DISUSE ATROPHY IN PATIENTS: Patients who have
spent prolonged periods in bed are likely to demonstrate
significant loss of strength resulting directly from the inactiv¬
ity and unrelated to other simultaneous impairments or
comorbidities. However ; the effects of inactivity may be
manifested differently in the various muscle groups of the
body. The clinician must be aware of the likely loss of
strength and also must consider the possible loss of muscu¬
lar endurance that may result from a transition from type I
to type II muscle fibers. In addition , the clinician also must
screen carefully for these changes to identify those muscle
groups that are most affected by disuse.

Astronauts and cosmonauts experience a unique case of
disuse atrophy resulting from their time spent in a micro¬
gravity environment. As with bed rest, microgravity induces
atrophy of both Type I and Type II with evidence of a transi¬
tion toward more Type II fibers [29]. Motor unit recruitment
also appears altered. The resulting muscle weakness and
decreased muscular endurance present significant challenges
for protracted space travel and particularly for re-entry to
earths gravitational field.

Clinical Relevance

EXERCISING IN SPACE: The International Space Lab is
designed to allow prolonged stays in space and may serve
as an intermediate stop for travelers to farther locations

(continued)

Part I I BIOMECHANICAL PRINCIPLES

(Continued)

such as Mars. However ; unless these space travelers can
exercise sufficiently to prevent the loss of muscle function
that currently accompanies space travel travel to outer
space will remain limited to a select few individuals who can
tolerate these changes. Exercise and rehabilitation experts
who devise exercise equipment and regimens for microgravity
use will find a very interested audience at the National
Aeronautics and Space Agency (NASA) and other interna¬
tional space agencies.

AGING AS ANOTHER MODEL OF ALTERED ACTIVITY

Loss of strength is a well-established finding in aging adults
[17,83,93,98,119,131]. This loss of strength is attributed to a
decreased percentage of, and greater atrophy in, type II
fibers [73,84,129]. As in the other adaptations of muscle
described above, changes in muscle with age vary across mus¬
cles [20]. Some muscle groups appear to be more susceptible
to age-related change; others seem impervious to such
changes. Again these data reveal that the clinician must assess
strength in the aging individual. However, the clinician must
also take care to identify those muscle groups that are weak¬
ened and those that are relatively unaffected, to target the
intervention specifically for optimal results.

Clinical Relevance

DECREASED STRENGTH WITH AGING: Decreased func¬
tional ability is a frequent finding with aging. Although
many factors contribute to diminished function with age,
investigations demonstrate a relationship between dimin¬
ished functional ability and decreased strength [21,23,51].
Similarly, increasing strength in elders improves functional
ability [35,111]. One of the challenges in rehabilitation is to
identify successful strategies to prevent or reduce strength
loss and preserve functional ability in the aging population.

SUMMARY

This chapter reviews the basic mechanisms of muscle short¬
ening and discusses in detail the individual factors that influ¬
ence a muscles ability to produce motion and to generate
force. The primary factors influencing a muscle s ability to pro¬
duce joint motion are the length of the muscle fibers within
the muscle and the length of the muscles moment arm.
Muscle strength, including its tensile force of contraction and
its resulting moment, is a function of muscle size, muscle
moment arm length, stretch of the muscle, contraction veloc¬
ity, fiber types within the muscle, and amount of muscle fiber
recruitment. Each factor is described and examples are pro¬
vided to demonstrate how an understanding of the factor can
be used in the clinic to explain or optimize performance. The

discussion also demonstrates that often as one factor is
enhancing a performance characteristic another factor may be
detracting from that performance. The final output of a mus¬
cle is the result of all of the factors influencing performance.
Thus to understand the basis for a patients performance, the
clinician must be able to recognize how the individual factors
influencing muscle performance change as joint position and
motion change.

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CHAPTER

Biomechanics of Cartilage

JOSEPH M. MANSOUR, PH.D.

CHAPTER CONTENTS

COMPOSITION AND STRUCTURE OF ARTICULAR CARTILAGE.70

MECHANICAL BEHAVIOR AND MODELING .72

MATERIAL PROPERTIES .72

RELATIONSHIP BETWEEN MECHANICAL PROPERTIES AND COMPOSITION.75

MECHANICAL FAILURE OF CARTILAGE.76

JOINT LUBRICATION.78

MODELS OF OSTEOARTHRITIS.78

EXERCISE AND CARTILAGE HEALTH .80

SUMMARY .80

T he materials classed as cartilage exist in various forms and perform a range of functions in the body.

Depending on its composition, cartilage is classified as articular cartilage (also known as hyaline), fibrocarti-
lage, or elastic cartilage. Elastic cartilage helps to maintain the shape of structures such as the ear and the tra¬
chea. In joints, cartilage functions as either a binder or a bearing surface between bones. The annulus fibrosus of the
intervertebral disc is an example of a fibrocartilaginous joint with limited movement (an amphiarthrosis). In the freely
moveable synovial joints (diarthroses) articular cartilage is the bearing surface that permits smooth motion between
adjoining bony segments. Hip, knee, and elbow are examples of synovial joints. This chapter is concerned with the
mechanical behavior and function of the articular cartilage found in freely movable synovial (diarthroidal) joints.

In a typical synovial joint, the ends of opposing bones are covered with a thin layer of articular cartilage (Fig. 5.1). On the
medial femoral condyle of the knee, for example, the cartilage averages 0.41 mm in rabbit and 2.21 mm in humans [2].
Normal articular cartilage is white, and its surface is smooth and glistening. Cartilage is aneural, and in normal mature
animals, it does not have a blood supply. The entire joint is enclosed in a fibrous tissue capsule, the inner surface of which
is lined with the synovial membrane that secretes a fluid known as synovial fluid. A relatively small amount of fluid is
present in a normal joint: less than 1 mL, which is less than one fifth of a teaspoon. Synovial fluid is clear to yellowish
and is stringy. Overall, synovial fluid resembles egg white, and it is this resemblance that gives these joints their name,
synovia , meaning "with egg."

Cartilage clearly performs a mechanical function. It provides a bearing surface with low friction and wear, and because
of its compliance, it helps to distribute the loads between opposing bones in a synovial joint. If cartilage were a stiff
material like bone, the contact stresses at a joint would be much higher, since the area of contact would be much
smaller. These mechanical functions alone would probably not be sufficient to justify an in-depth study of cartilage
biomechanics. However, the apparent link between osteoarthritis and mechanical factors in a joint adds a strong impe¬
tus for studying the mechanical behavior of articular cartilage.

Part I I BIOMECHANICAL PRINCIPLES

Figure 5.1: Schematic representation of a synovial joint. Articular
cartilage forms the bearing surface on the ends of opposing bones.
The space between the capsule and bones is exaggerated in the
figure for clarity.

The specific goals of this chapter are to

■ Describe the structure and composition of cartilage in relation to its mechanical behavior

■ Examine the material properties of cartilage, what they mean physically, and how they can be determined

■ Describe modes of mechanical failure of cartilage

■ Describe the current state of understanding of joint lubrication

■ Describe the etiology of osteoarthritis in terms of mechanical factors

Before proceeding through this chapter, the reader should be familiar with the basic concepts and terminology
introduced in Chapters 1 and 2.

COMPOSITION AND STRUCTURE
OF ARTICULAR CARTILAGE

Articular cartilage is a living material composed of a relatively
small number of cells known as chondrocytes surrounded by
a multicomponent matrix. Mechanically, articular cartilage is
a composite of materials with widely differing properties.
Approximately 70 to 85% of the weight of the whole tissue is
water. The remainder of the tissue is composed primarily of
proteoglycans, collagen, and a relatively small amount of
lipids. Proteoglycans consist of a protein core to which
glycosaminoglycans (chondroitin sulfate and keratan sulfate)
are attached to form a bottlebrush-like structure. These
proteoglycans can bind or aggregate to a backbone of

hyaluronic acid to form a macromolecule with a weight up to
200 million daltons [76] (Fig. 5.2). Approximately 30% of the
dry weight of articular cartilage is composed of proteoglycans.
Proteoglycan concentration and water content vary through
the depth of the tissue. Near the articular surface, proteogly¬
can concentration is relatively low, and the water content is
the highest in the tissue. In the deeper regions of the carti¬
lage, near subchondral bone, the proteoglycan concentration
is greatest, and the water content is the lowest [67,74].
Collagen is a fibrous protein that makes up 60 to 70% of the
dry weight of the tissue. Type II is the predominant collagen
in articular cartilage, although other types are present in
smaller amounts [25]. Collagen architecture varies through
the depth of the tissue.

Chapter 5 I BIOMECHANICS OF CARTILAGE

Figure 5.2: A proteoglycan aggregate showing a collection of
proteoglycans bound to a hyaluronic acid backbone. Proteoglycans
are the bottlebrush-like structures consisting of a protein core
with side chains of chondroitin sulfate and keratan sulfate.
Negatively charged sites on the chondroitin and keratan sulfate
chains cause this aggregate to spread out and occupy a large
domain when placed in an aqueous solution.

The structure of articular cartilage is often described in
terms of four zones between the articular surface and the sub¬
chondral bone: the surface or superficial tangential zone, the
intermediate or middle zone, the deep or radiate zone, and the
calcified zone (Fig. 5.3). The calcified cartilage is the boundary
between the cartilage and the underlying subchondral bone.
The interface between the deep zone and calcified cartilage is
known as the tidemark. Optical microscopy (e.g., polarized
light), scanning electron microscopy, and transmission electron
microscopy have been used to reveal the structure of articular
cartilage [10,11,36,37,53,76,111]. While each of these methods
suggests somewhat similar collagen orientation for the superfi¬
cial and deep zones, the orientation of fibers in the middle zone
remains controversial.

Using scanning electron microscopy to investigate the struc¬
ture of cartilage in planes parallel and perpendicular to split
lines, Jeffery and coworkers [37] have given some new insights
into the collagen structure (Fig. 5.3). Split lines are formed by
puncturing the cartilage surface at multiple sites with a circular
awl. The resulting holes are elliptical, not circular, and the long
axes of the ellipses are aligned in what is called the split line
direction. In the plane parallel to a split line, the collagen is
organized in broad layers or leaves, while in the plane orthogo¬
nal to the split lines the structure has a ridged pattern that is
interpreted as the edges of the leaves (Fig. 5.3). In the calcified
and deep zones, collagen fibers are oriented radially and are
arranged in tightly packed bundles. The bundles are linked by
numerous fibrils. From the upper deep zone into the middle

Figure 5.3: Cross sections cut through the thickness of articular cartilage on two mutually orthogonal planes. These planes are oriented
parallel and perpendicular to split lines on the cartilage surface. The background shows the four zones of the cartilage: superficial,
intermediate, radiate, and calcified. The foreground shows the organization of collagen fibers into "leaves" with varying structure and
organization through the thickness of the cartilage. The leaves of collagen are connected by small fibers not shown in the figure.

Part I I BIOMECHANICAL PRINCIPLES

zone, the radial orientation becomes less distinct, and collagen
fibrils form a network that surrounds the chondrocytes. In the
superficial zone, the fibers are finer than in the deeper zones,
and the collagen structure is organized into several layers. An
amorphous layer that does not appear to contain any fibers is
found on the articular surface.

Scanning electron microscopy is also used to investigate
the structure of osteoarthritic cartilage [16,53]. These investi¬
gations demonstrate two primary structural changes associated
with degeneration: rolling of delaminated sheets into fronds,
and formation and propagation of large cracks. In delaminated
regions of tissue below the superficial layer, and in large fis¬
sures, cracks appear to propagate by separation or peeling of
parallel collagen fibrils, rather than fracture of fibrils [53].
This observation suggests a mechanism that binds fibrils into
a parallel structure [9,53].

Clinical Relevance

STRUCTURE: On a structural level osteoarthritis is character¬
ized by surface fibrillation, fissures, and eventual removal of
cartilage from underlying bone. Scanning electron microscopy
provides a detailed picture of specific structural changes that
occur in osteoarthritis. Recognizing that peeling of collagen
fibrils in delaminated and fissured regions may involve failure
of a component that "glues" fibrils together ; could lead to new
procedures for treating osteoarthritis.

MECHANICAL BEHAVIOR AND MODELING

The mechanical behavior of articular cartilage is deter¬
mined by the interaction of its predominant compo¬
nents: collagen, proteoglycans, and interstitial fluid. In
an aqueous environment, proteoglycans are polyanionic; that
is, the molecule has negatively charged sites that arise from its
sulfate and carboxyl groups. In solution, the mutual repulsion
of these negative charges causes an aggregated proteoglycan
molecule to spread out and occupy a large volume. In the car¬
tilage matrix, the volume occupied by proteoglycan aggregates
is limited by the entangling collagen framework. The swelling
of the aggregated molecule against the collagen framework is
an essential element in the mechanical response of cartilage.
When cartilage is compressed, the negatively charged sites on
aggrecan are pushed closer together, which increases their
mutual repulsive force and adds to the compressive stiffness
of the cartilage. Nonaggregated proteoglycans would not be
as effective in resisting compressive loads, since they are not
as easily trapped in the collagen matrix. Damage to the colla¬
gen framework also reduces the compressive stiffness of the
tissue, since the aggregated proteoglycans are contained less
efficiently.

The mechanical response of cartilage is also strongly tied
to the flow of fluid through the tissue. When deformed,
fluid flows through the cartilage and across the articular sur¬
face [56]. If a pressure difference is applied across a section

of cartilage, fluid also flows through the tissue [66]. These
observations suggest that cartilage behaves like a sponge,
albeit one that does not allow fluid to flow through it easily.

Recognizing that fluid flow and deformation are interde¬
pendent has led to the modeling of cartilage as a mixture of
fluid and solid components [74-76]. This is referred to as the
biphasic model of cartilage. In this modeling, all of the solid¬
like components of the cartilage, proteoglycans, collagen, cells,
and lipids are lumped together to constitute the solid phase of
the mixture. The interstitial fluid that is free to move through
the matrix constitutes the fluid phase. Typically, the solid
phase is modeled as an incompressible elastic material, and
the fluid phase is modeled as incompressible and inviscid, that
is, it has no viscosity [75]. Under impact loads, cartilage
behaves as a single-phase, incompressible, elastic solid; there
simply isn’t time for the fluid to flow relative to the solid matrix
under rapidly applied loads. For some applications, a
viscoelastic model is used to describe the behavior of
cartilage in creep, stress relaxation, or shear. Although
the mathematics of modeling cartilage is outside the
scope of this chapter, some examples illustrate the fundamen¬
tal fluid-solid interaction in cartilage.

Clinical Relevance

BIPHASIC MODEL OF CARTILAGE: Mathematical simula¬
tions , using the biphasic model for cartilage\ show that when
a compressive load is applied to a joint pressure in the inter¬
stitial fluid', not stress in the solid matrix, supports a signifi¬
cant portion of the load [2]. If the load is maintained over
hundreds of seconds, fluid pressure decreases, and stress in
the solid matrix increases. The biphasic model shows that
fluid pressure shields the solid matrix from the higher level of
stress that it would experience if cartilage were a simple elas¬
tic material without significant interaction of its fluid and solid
components. In osteoarthritic cartilage that is more permeable
than normal, stress shielding by fluid pressurization is dimin¬
ished, and more stress is transferred to the solid matrix.

Biphasic behavior is sometimes described using an analogy
of a balloon that is tightly packaged within a cardboard box.
Pressure in the balloon allows the box to support more com¬
pressive load than it could if it were empty. If the pressure in
the balloon is reduced, more stress is transferred to the box.

MATERIAL PROPERTIES

Modeling cartilage as an isotropic biphasic material requires
two independent material constants for the solid matrix, and
one for fluid flow. As with a simple elastic material, multiple
material constants can be determined for the solid matrix (the
elastic, aggregate, shear, and bulk moduli, and Poissons ratio)
but only two of these are independent. The constant associated
with fluid flow is called the permeability. Typically, these
constants are determined from confined or unconfined com¬
pression. Shear tests are also used to determine the intrinsic
material properties of the solid matrix.

Chapter 5 I BIOMECHANICS OF CARTILAGE

Constant load

Figure 5.4: Schematic drawing of an apparatus used to perform
a confined compression test of cartilage. A slice of cartilage is
placed in an impervious, fluid-filled well. The tissue is loaded
through a porous plate. In the configuration shown, the load is
constant throughout the test, which can last for several thousand
seconds. Since the well is impervious, flow through the cartilage
will only be in the vertical direction and out of the cartilage.

A confined compression test is one of the commonly used
methods for determining material properties of cartilage
(Fig. 5.4). A disc of cartilage is cut from the joint and placed
in an impervious well. Confined compression is used in either
a “creep” mode or a “relaxation” mode. In the creep mode, a
constant load is applied to the cartilage through a porous
plate, and the displacement of the tissue is measured as a
function of time. In relaxation mode, a constant displacement
is applied to the tissue, and the force needed to maintain the
displacement is measured.

In creep mode, the cartilage deforms under a constant load,
but the deformation is not instantaneous, as it would be in a
single-phase elastic material such as a spring. The displacement
of the cartilage is a function of time, since the fluid cannot

Figure 5.5: Typical displacement of cartilage tested in a confined
compression creep test. A constant load is applied to the carti¬
lage, and the displacement is measured over time. Initially, the
deformation is rapid, as relatively large amounts of fluid are
exuded from the cartilage. As the displacement reaches a con¬
stant value, the flow slows to zero. Two material properties are
determined from this test.

escape from the matrix instantaneously (Fig. 5.5). Initially, the
displacement is rapid. This corresponds to a relatively large
flow of fluid out of the cartilage. As the rate of displacement
slows and the displacement approaches a constant value, the
flow of fluid likewise slows. At equilibrium, the displacement is
constant and fluid flow has stopped. In general, it takes several
thousand seconds to reach the equilibrium displacement.

By fitting the mathematical biphasic model to the meas¬
ured displacement, two material properties of the cartilage are
determined: the aggregate modulus and permeability. The
aggregate modulus is a measure of the stiffness of the tissue at
equilibrium when all fluid flow has ceased. The higher the
aggregate modulus, the less the tissue deforms under a given
load. The aggregate modulus of cartilage is typically in the
range of 0.5 to 0.9 MPa [3]. There is no analogous material
constant for solid materials, but using the aggregate modulus
and representative values of Poissons ratio (described below),
the Youngs modulus of cartilage is in the range of 0.45 to 0.80
MPa. For comparison, the Youngs modulus of steel is 200
GPa and for many woods is about 10 GPa parallel to the grain.
These numbers show that cartilage has a much lower stiffness
(modulus) than most engineering materials.

In addition to the aggregate modulus, the permeability of
the cartilage is also determined from a confined compression
test. The permeability indicates the resistance to fluid flow
through the cartilage matrix. Permeability was first introduced
in the study of flow through soils. The average fluid velocity
through a porous sample (v ) is proportional to the pressure
gradient (Vp) (Fig. 5.6). The constant of proportionality (k) is
called the permeability. This relationship is expressed by
Darcy’s law, as shown in Box 5.1.

Fluid-filled chamber

High pressure (P 2 )

iUUl

— Articular
cartilage

s®

_ Dm*Ai io aIoIa

tttttt

Low pressure (Pp

Fluid-filled chamber

rorous piaie

> Direction of fluid flow -

Figure 5.6: Schematic representation of a device used to measure
the permeability of cartilage. A slice of cartilage is supported on
a porous plate in a fluid-filled chamber. High pressure applied
to one side of the cartilage drives fluid flow. The average fluid
velocity through the cartilage is proportional to the pressure
gradient, and the constant of proportionality is called the
permeability.

Part I I BIOMECHANICAL PRINCIPLES

In SI units, the permeability of cartilage is typically in the
range of 10 -15 to 10 -16 m 4 /Ns. If a pressure difference of
210,000 Pa (about the same pressure as in an automobile tire)
is applied across a slice of cartilage 1 mm thick, the average
fluid velocity will be only 1 • 10 _s m/s, which is about 100 mil¬
lion times slower than normal walking speed.

Permeability is not constant through the tissue. The per¬
meability of articular cartilage is highest near the joint surface
(making fluid flow relatively easy) and lowest in the deep zone
(making fluid flow relatively difficult) [65-67]. Permeability
also varies with deformation of the tissue. As cartilage is com¬
pressed, its permeability decreases [49,63]. Therefore, as a
joint is loaded, most of the fluid that crosses the articular sur¬
face comes from the cartilage closest to the joint surface.
Under increasing load, fluid flow will decrease because of the
decrease in permeability that accompanies compression.

Clinical Relevance

VARIABLE PERMEABILITY: Deformation-dependent per¬
meability may be a valuable mechanism for maintaining
load sharing between the solid and fluid phases of cartilage.
If the fluid flowed easily out of the tissue, then the solid
matrix would bear the full contact stress, and under this
increased stress; it might be more prone to failure.

Constant

force

Rigid porous indenter

Articular cartilage

Displacement of
cartilage surface

Bone-

Figure 5.7: Schematic representation of an apparatus used to
perform an indentation test on articular cartilage. Unlike the
confined compression and most permeability tests, the cartilage
remains attached to its underlying bone, which provides a more
natural environment for testing. A constant load is applied to a
small area of the cartilage through a porous indenter. The dis¬
placement of the cartilage is similar to that shown in Figure 5.6.
Three material properties are determined from this test.

An indentation test provides an attractive alternative to
confined compression [29,31,42,60,73,106] (Fig. 5.7). Using
an indentation test, cartilage is tested in situ. Since discs of
cartilage are not removed from underlying bone, as must be
done when using confined compression, indentation may
be used to test cartilage from small joints. In addition, three
independent material properties are obtained from one
indentation test, but only two are obtained from confined
compression. Typically, an indentation test is performed
under a constant load. The diameter of the indenter varies
depending on the curvature of the joint surface, but generally
is no smaller than 0.8 mm. Under a constant load, the
displacement of the indenter resembles that for confined
compression and requires several thousand seconds to reach
equilibrium. By fitting the biphasic model of the test to the
measured indentation, the aggregate modulus, Poisson s ratio,
and permeability are determined. Poissons ratio is typically
less than 0.4 and often approaches zero. This finding is a sig¬
nificant departure from earlier studies, which assumed that
cartilage was incompressible and, therefore, had a Poissons
ratio of 0.5. This assumption was based on cartilage being
mostly water, and water may often be modeled as an incom¬
pressible material. However, when cartilage is loaded, fluid
flows out of the solid matrix, which reduces the volume of the
whole cartilage. Recognizing that cartilage is a mixture of a
solid and fluid leads to the whole tissue behaving as a com¬
pressible material, although its components are incompressible.

The interpretation of Poissons ratio used here is somewhat
different from the commonly used definition, the ratio of
transverse to axial strain. However, a material that has a
Poissons ratio of 0.5, as commonly defined, will deform as if
it is incompressible; that is, its volume will not change. The
relationship between Poissons ratio equal to 0.5 and incom¬
pressibility applies only to small deformations. Rubber and
many other polymeric materials are commonly modeled as
incompressible.

The equilibrium displacement is determined by the aggre¬
gate modulus and Poissons ratio. The permeability influences
the rate of deformation. If the permeability is high, fluid can
flow out of the matrix easily, and the equilibrium is reached
relatively quickly. A lower permeability causes a more gradual
transition from the rapid early displacement to the equilibrium.
These qualitative results are helpful for interpreting data
from tests of normal and osteoarthritic cartilage.

Clinical Relevance

PERMEABILITY OF OSTEOARTHRITIC CARTILAGE:

The lower modulus and increased permeability of
osteoarthritic cartilage result in greater and more-rapid
deformation of the tissue than normal. These changes may
influence the synthetic activity of the chondrocytes, which
are known to respond to their mechanical environment.
[13,114,123].

Chapter 5 I BIOMECHANICS OF CARTILAGE

Pure shear provides a means for evaluating the intrinsic
properties of the solid matrix. Small torsional displacements
of cylindrical samples (which produce pure shear), result in
no volume change of the cartilage to drive fluid flow.
Furthermore, the interstitial fluid is water. It has low viscosity
and does not make an appreciable contribution to resisting
shear. Therefore, the resistance to shear is due to the solid
matrix. Tests of cartilage in shear show that the matrix
behaves as a viscoelastic solid [27-29]. Mathematical models
of cartilage deformation also suggest that the matrix may
behave as a viscoelastic solid [59,103].

RELATIONSHIP BETWEEN MECHANICAL
PROPERTIES AND COMPOSITION

In addition to the qualitative descriptions given above,
quantitative correlations between the mechanical properties
of cartilage and glycosaminoglycan content, collagen con¬
tent, and water content have been established. The com¬
pressive stiffness of cartilage increases as a function of the
total glycosaminoglycan content [45] (Fig. 5.8). In contrast,
there is no correlation of compressive stiffness with collagen
content. In these cases, compressive stiffness is measured in
creep, 2 seconds after a load is applied to the tissue.
Permeability and compressive stiffness, as measured by the
aggregate modulus, are both highly correlated with water
content. As the water content increases, cartilage becomes
less stiff and more permeable [1] (Fig. 5.9). Note that the
inverse of permeability is plotted in Figure 5.9B. This is
done for convenience, since the permeability becomes very
large as the water content increases.

1 uu

HcT

^ 140-

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•

• • •

•

^ i nn

•

• •

•

• _

CD 1 uu
c

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• •

•

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•

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£ 20

-1

1-1

-1

1-1

i-1-1-1-1-1

60 80 100 120 140 160

Total glycosaminoglycan content
(|ig/mg dry weight)

Figure 5.8: Correlation of compressive stiffness with the total
glycosaminoglycan concentration. As the total glycosaminogly¬
can concentration decreases, the compressive stiffness also
decreases.

Figure 5.9: A. Correlation of the aggregate modulus with water
content of articular cartilage. A regression line obtained from
tests of a large number of samples is plotted. As the water con¬
tent increases, the aggregate modulus decreases. B. Correlation
of the inverse of permeability with water content. A regression
line obtained from tests of a large number of samples is plotted
As the water content increases, the permeability increases.

Clinical Relevance

MATERIAL PROPERTIES OF CARTILAGE: The relation¬
ships between material properties and water content help to
explain early cartilage changes in animal models of
osteoarthritis. Proteoglycan content and equilibrium stiffness
decrease and the rate of deformation and water content
increases in these models [50,71]. Decreasing proteoglycan
content allows more space in the tissue for fluid. An increase
in water content correlates with an increase in permeability.
Increasing permeability allows fluid to flow out of the tissue
more easily, resulting in a more rapid rate of deformation.

(continued)

Part I I BIOMECHANICAL PRINCIPLES

(Continued)

Using confined compression , indentation , tension , and
shear tests; the mechanical properties of cartilage can be
determined. These properties are necessary for any analysis
of stress in the tissue. However ; material properties do not
give any indication of the failure of cartilage. For example,
simply knowing the value of aggregate modulus or Poisson's
ratio is not sufficient to predict if cartilage will develop the
cracks, fissures , and general wear that are characteristic of
osteoarthritis. Various loading conditions have been used to
gain better insight into the failure properties of cartilage.

MECHANICAL FAILURE OF CARTILAGE

A characteristic feature of osteoarthritis is cracking, fibrilla¬
tion, and wear of cartilage. This appears to be a mechanically
driven process, and it motivates numerous investigations
aimed at identifying the stresses and deformations responsible
for the failure of articular cartilage. Since cartilage is an
anisotropic material, we expect that it has greater resistance to
some components of stress than to others. For example, it
could be relatively strong in tension parallel to collagen fibers,
but weaker in shear along planes between leaves of collagen.

Studying the tensile properties of cartilage illustrates its
anisotropy, inhomogeneity, some surprising age-dependent
changes in mechanical behavior, and additional collagen-
proteoglycan interaction. Tensile tests of cartilage are per¬
formed by first removing the cartilage from its underlying
bone. This sheet of cartilage is sometimes cut into thin slices
(200-500 fxm thick) parallel to the articular surface, using a
microtome. Dumbbell-shaped specimens are cut from each
slice with a custom-made cookie cutter.

A particularly thorough study of the tensile properties of
cartilage shows that samples oriented parallel to split lines
have a higher tensile strength and stiffness than those perpen¬
dicular to the split lines. In skeletally mature animals (closed
physis), tensile strength and stiffness decrease from the surface
to the deep zone. In contrast, tensile strength and stiffness
increase with depth from the articular surface in skeletally
immature (open physis) animals [96].

The relative influence of the collagen network and proteo¬
glycans on the tensile behavior of cartilage depends on the rate
of loading [100]. When pulled at a slow rate, the collagen net¬
work alone is responsible for the tensile strength and stiffness
of cartilage. At high rates of loading, interaction of the colla¬
gen and proteoglycans is responsible for the tensile behavior;
proteoglycans restrain the rotation of the collagen fibers when
the tissue is loaded rapidly.

Tensile failure of cartilage has been of particular interest,
since it was generally believed that vertical cracks in cartilage
were initiated by relatively high tensile stresses on the articu¬
lar surface. More-recent computational models of joint con¬
tact show that the tensile stress on the surface is lower than
originally thought, although tensile stress still exists within the

35 -|

1 20 40 60 80 100

Age in years

Figure 5.10: Comparison of the tensile failure stress of cartilage
from the hip and talus. There is a statistically significant drop in
the failure stress, as a function of age, for cartilage from the hip,
but not for cartilage from the talus. Interestingly, there is a rela¬
tively high occurrence of osteoarthritis in the hip compared with
that in the ankle (talus).

cartilage [20-22]. It now appears that failure by shear stress
may dominate. Studies of the tensile failure of cartilage are
primarily concerned with variations in properties among
joints, the effects of repeated load, and age.

Kempson and coworkers report a decrease in tensile failure
stress with age for cartilage from hip and knee [40,41,43, 44].
However, they find no appreciable age-dependent decrease in
tensile failure stress for cartilage from the talus (Fig. 5.10).

Clinical Relevance

INCIDENCE OF OSTEOARTHRITIS AT THE ANKLE:

There is a low incidence of osteoarthritis in the ankle com¬
pared with the hip or knee. The maintenance of tensile
strength of cartilage from the ankle may play a role in the
reduced likelihood of degeneration in this joint.

Repeated tensile loading (fatigue) lowers the tensile
strength of cartilage as it does in many other materials. As the
peak tensile stress increases, the number of cycles to failure
decreases (Fig. 5.11 ) [120-122]. For any value of peak stress,
the number of cycles to failure is lower for cartilage from older
than younger individuals.

Repeated compressive loads applied to the cartilage sur¬
face in situ also cause a decrease in tensile strength, if a suffi¬
cient number of load cycles are applied [68]. Following
64,800 cycles of compressive loading, there is no change in
the tensile strength of cartilage, but after 97,200 cycles, ten¬
sile strength is reduced significantly. Surface damage is not
found in any sample. This shows that damage may be induced

Chapter 5 I BIOMECHANICS OF CARTILAGE

Figure 5.11: The effects of repeated tensile loading on the tensile
strength of cartilage. As the tensile loading stress increases,
fewer cycles of loading are needed to cause failure. Age is also
an important factor. Cartilage from older individuals fails at a
lower stress than that from younger people. Regression lines fit
to multiple tests are plotted.

within the tissue before any signs of surface fibrillation are
apparent.

How do the number of cycles used in this test relate to
human activity? Running a 26-mile marathon with a 6-foot
step length corresponds to 11,440 cycles of load on each leg.
Bicycling for 4 hours with a cadence of 90 revolutions per
minute corresponds to 21,600 cycles per leg.

Some caution must be exercised when evaluating the
effects of repeated loading on cartilage. In many cases failure
occurs under large strain applied to samples removed from
underlying bone. The strain to failure may be greater than that
experienced in vivo. In addition, the properties of most bio¬
logical materials change with the applied strain; the collagen
network becomes aligned with the direction of the tensile
strain, and the material becomes strongly anisotropic. Lastly,
repeated loading of dead tissue does not include any biologi¬
cal response, and therefore may not give a complete picture of
the effects of loading.

Rather than assume that tensile stress is responsible for fib¬
rillation of the articular surface, the feasibility of several crite¬
ria is considered in a combined experimental and computa¬
tional approach to cartilage failure [4-6]. Dropping three
different-sized spherical indenters (2, 4, and 8 mm) onto the
articular surface produces three different states of stress and,
in some instances, a crack through the surface. Based on the
stresses in the cartilage in each test and the presence or
absence of a crack, a regression is used to determine the con¬
dition that is most likely to cause a crack to develop. The max¬
imum shear stress in the cartilage is the most likely predictor
of crack formation based on the location of the crack with
respect to the calculated stresses.

Since cartilage is loaded in compression, the idea of failure
by shear stress may seem unrealistic. Shear stresses do exist in
cartilage, although the orientation of these stresses is not

Compressive

force

Compressive

force

Compressive

force

Shear

force

Figure 5.12: Illustration of shear stress in a simple loading condi¬
tion. A. A free body diagram of a bar loaded in compression.

B. A free body diagram of the same bar cut perpendicular to the
load at an arbitrary location. On the cut surface, the resultant
force must be Pto maintain equilibrium. C. The same bar cut at
an arbitrary angle. Again, to be in equilibrium the resultant
force parallel to the bar must be equal to P. This force can
always be decomposed into components parallel and perpendi¬
cular to the cut. The component parallel to the cut is a shear
force that gives rise to a shear stress on the inclined surface.

always obvious. To illustrate this, imagine a loading situation
that is simpler than a joint, namely a straight bar loaded in
compression (Fig. 5.12). If the bar is cut by a plane perpendi¬
cular to its length, then the resultant force on the cross section
must also be compressive and equal to the applied force to
maintain equilibrium. Now imagine the bar is cut at a 45°
angle to its length (the exact angle is not important). The
resultant force must still be equal to the applied force.
Resolving the resultant force into components parallel and
perpendicular to the cut surface gives rise to a shear force and
a normal force. The shear stress (force per unit area) comes
from the shear force acting over the inclined cut area of the
bar. The same concept applies in any loading situation, includ¬
ing the cartilage in a synovial joint. However, in a synovial joint
the stresses are multiaxial, not uniaxial as in the bar.

Radin and coworkers also show that cartilage failure could
be induced by shear stress [86]. However, they are particularly
interested in failure at the cartilage-bone interface, not the
articular surface. Motivation for this investigation comes from
postmortem studies that show cracks at the cartilage-bone
interface and the recognition that under rapid loading, carti¬
lage behaves as an incompressible elastic material, that is, its
Poissons ratio is 0.5. The relatively compliant, but incom¬
pressible cartilage experiences large lateral displacement (due
to its high Poissons ratio) when loaded in compression,
but this expansion is constrained by the stiff underlying bone
(Fig. 5.13). Under these conditions, high shear stress develops
at the cartilage-bone boundary.

Most studies of cartilage failure are based directly on the
values of ultimate stress or strain. An alternative is to use

Part I I BIOMECHANICAL PRINCIPLES

Compressive force

Figure 5.13: Under impulsive compressive loads, the cartilage
experiences a relatively large lateral displacement due to its high
Poisson's ratio. This expansion is restrained by the much stiffer
subchondral bone, causing a high shear stress at the cartilage
bone interface.

parameters that more directly represent the propagation of a
crack in a loaded material sample. The feasibility of using two
methods to determine fracture parameters of cartilage is eval¬
uated extensively by Chin-Purcell and Lewis (Fig. 5.14) [14].
The so-called J integral is a measure of the fracture energy
dissipated per unit of crack extension. As used, the J integral
also assumes that a crack propagates in the material, as
opposed to deformation or flow of the material, which results
in a more ductile failure. Since cracks may not propagate in
soft biological materials, a tear test is also evaluated. The tear
test yields a fracture parameter similar to the J integral. As
with tensile-stress-based ideas of failure, it is necessary to
apply large strains to cause failure of the samples: these
strains may be far greater than those found in any in vivo load¬
ing conditions. To date, the application of these fracture
parameters is limited to the normal canine patella.

edge notch test

Figure 5.14: Sample shape and load application for the modified
single-edge notch and trouser tear tests. Each test yields a specific
measure of fracture, the energy required to propagate a crack in
the material.

JOINT LUBRICATION

Normal synovial joints operate with a relatively low coeffi¬
cient of friction, about 0.001 [54,69,113]. For comparison,
Teflon sliding on Teflon has a coefficient of friction of about
0.04, an order of magnitude higher than that for synovial
joints. Identifying the mechanisms responsible for the low
friction in synovial joints has been an area of ongoing research
for decades. Both fluid film and boundary lubrication mech¬
anisms have been investigated.

For a fluid film to lubricate moving surfaces effectively, it
must be thicker than the roughness of the opposing surfaces.
The thickness of the film depends on the viscosity of the fluid,
the shape of the gap between the parts, and their relative
velocity, as well as the stiffness of the surfaces. A low coeffi¬
cient of friction can also be achieved without a fluid film
through a mechanism known as boundary lubrication. In this
case, molecules adhered to the surfaces are sheared rather
than forming a fluid film.

It now appears that a combination of fluid film lubrication
and boundary lubrication are responsible for the low friction
in synovial joints [55,90,92].

Numerous mechanisms for developing a lubricating
fluid film on the articular surface have been postulated
[19,38,62,69,116,118,119]. If cartilage is modeled as a
rigid material, it is not possible to generate a fluid film of
sufficient thickness to separate the cartilage surface rough¬
ness. However, models that include deformation of the
cartilage and its surface roughness have shown that a suffi¬
ciently thick film can be developed [38]. This is known as
microelastohydrodynamic lubrication. Deformation also
causes fluid flow across the cartilage surface, which modi¬
fies the film thickness, although there is some question as
to the practical importance of this component of flow
[32,33,38].

Boundary lubrication of the articular surface appears to be
linked to a glycoprotein fraction in synovial fluid known as
lubricin [17,24,26,30,69,85,90,93,94,98,101,108-110]. Recent
evidence suggests that lubricin may be a carrier for lubricating
molecules known as surface active phospholipids that may
provide the boundary lubricating property of synovial joints
[101]. Surface active phospholipids are believed to be bound¬
ary lubricants not just in synovial joints, but in other parts of
the body such as the pleural space.

MODELS OF OSTEOARTHRITIS

Animal models are used to provide a controlled environment
for studying the progression of osteoarthritis. Although
osteoarthritis may be induced by numerous means, models
based on disruption of the mechanical environment of the
joint, either by surgical alteration of periarticular structures
or by abnormal joint load, are commonly used [34,35,
72,83,89,91,104].

Chapter 5 I BIOMECHANICS OF CARTILAGE

Surgical resection of one or combinations of the anterior
cruciate ligament, the medial collateral ligament, and a partial
medial meniscectomy produce osteoarthritis of the knee.
These models are thought to produce an unstable joint, but
kinematic studies show varying degrees of deviation from nor¬
mal joint kinematics.

Small differences in kinematics between control and oper¬
ated knees (anterior cruciate ligament release and partial
medial meniscectomy) are reported in rabbit [64]. At 4 weeks
after surgery, there is a statistically significant change in the
maximum anterior displacement of the knee, but anterior
displacement is not significantly different from normal at 8 or
12 weeks after surgery. The most notable kinematic changes
are in external rotation at 8 weeks and adduction at 4, 8, and
12 weeks after surgery. In dog, which has a more extended
knee, greater anterior-posterior drawer is found after anterior
(cranial) cruciate ligament release [48,115]. The relatively
small changes in kinematics in unstable joints (particularly in
rabbit) suggests that altered forces and possibly sensory input
may be more important than joint displacements in the devel¬
opment of osteoarthritis [39].

Repetitive impulse loading also produces osteoarthritis in
animal joints [87,89,91]. An advantage of this model is that
it is more controlled than surgical models; the force applied
to the limb is known and can be altered. This model has
demonstrated the effect of loading rate on the development
of osteoarthritis. Impulsively applied loads were found to
produce osteoarthritis, while higher loads applied at a lower
rate do not. The importance of impulsive loading to the
development of osteoarthritis also appears in humans; per¬
sons with knee pain, but no history to suggest its origin, load
their legs more rapidly at heel strike than persons without
knee pain.

Although biochemical, metabolic, and mechanical assays
have been used to evaluate the properties of cartilage from
animal models of osteoarthritis, this chapter concentrates on
the mechanical properties of cartilage. Following resection
of the anterior cruciate ligament in dog, tensile stiffness,
aggregate modulus, and shear modulus are lower than those
in cartilage from unoperated control joints [102].
Permeability increases significantly 12 weeks after surgery.
There is a significant increase in water content of samples
from the medial tibial plateau and the lateral condyle and
femoral groove.

In summary, various mechanical alterations of a joint lead
to the development of osteoarthritis. The kinematic instabil¬
ity induced by surgical alterations may be small, suggesting
that altered forces are primarily responsible for the devel¬
oping osteoarthritis. Models based solely on abnormal joint
loading support the view that alterations in force can lead to
osteoarthritis. Following resection of the anterior cruciate
ligament, cartilage is less stiff in both compression and
shear, and fluid flows more easily through the tissue in joints
with osteoarthritis. This implies greater displacement of
osteoarthritic cartilage than normal (decreased stiffness) and
a greater rate of deformation (increased permeability).

Clinical Relevance

OSTEOARTHRITIS: Osteoarthritis is a leading cause of disabil¬
ity in developed countries [15]. In the United States; it is sec¬
ond to cardiovascular disease as the most common cause
of disability [82]. Despite the widespread occurrence of
osteoarthritis , it is difficult to study in human populations.
Early physical symptoms such as fibrillation and cracking of
the articular surface cannot be detected by an individual
since cartilage is aneural. Insults to the cartilage may take
years to progress to the point at which symptoms are
detected by the surrounding joint structures and underlying
bone. Although numerous epidemiological studies of
osteoarthritis have been performed , they have been
described as "disappointing" since they have not led to an
explanation of the mechanisms underlying the development
of osteoarthritis [82]. However; what seems to be clear is
that the development of osteoarthritis depends on a combi¬
nation of factors, including age, sex, heredity, joint mechan¬
ics and cartilage biology, and biochemistry [18,61,70].

Although it is not an inescapable consequence of aging,
osteoarthritis is more prevalent in the elderly [77,81]. In the
United States, approximately 80% of people over the age of
65 and essentially everyone over the age of 80 has
osteoarthritis, although it is uncommon before the age of
40. After 55 years of age, osteoarthritis is more common in
women than men. Typically the distal interphalangeal, first
carpometacarpal, and knee joints are the first joints affected
[77]. However, specific links between aging and osteoarthri¬
tis are not known.

Excessive mechanical loading may also predispose joints
to osteoarthritis. Workers in physically strenuous occupa¬
tions (coal miners) have a higher incidence of osteoarthritis
than those in less strenuous lines of work (office workers)
[82]. Interestingly, evidence of osteoarthritis of the shoulder
and elbow is found in relatively young individuals in ancient
populations dependent on hunting [82]. However, strenuous
work may not be the only risk factor for osteoarthritis, since
people who use pneumatic drills or physical education
teachers do not have an increased risk of osteoarthritis [82].

Radin argues that it is not the magnitude of the load, but
rather the loading rate that is the determining factor in the
development of osteoarthritis. Osteoarthritis develops only
when impulsive loads are applied; that is, the load reaches its
maximum value over a relatively short time. This is demon¬
strated in animal models using externally applied loads, and
in sheep walking on soft and hard surfaces [84,88,89,91,104].
The role of impulsive rather than more slowly applied loads is
also supported by tests in humans. Individuals with knee pain
who are diagnosed as "prearthrotic" have a higher loading
rate at heel strike than normal subjects [86]. These studies
suggest that particular activities alone do not necessarily
predispose an individual to osteoarthritis. Rather, the way in

(continued)

Part I I BIOMECHANICAL PRINCIPLES

(Continued

which the activity is performed may be the factor that deter¬
mines if osteoarthritis will develop.

Obesity is also correlated with an increasing risk of devel¬
oping osteoarthritis, particularly in the tibio-femoral,
patellofemoral, and carpometacarpal joints [15]. Although
increased weight would be expected to increase the load on
joints of the lower extremity , and possibly predispose an indi¬
vidual to osteoarthritis, obesity has no direct mechanical
effect on the carpometacarpal joint.

EXERCISE AND CARTILAGE HEALTH

Participation in certain sports also appears to increase the
risk of developing osteoarthritis. Based on a review of
existing literature, Saxon et al. [99] concluded that activi¬
ties that involve torsional loading, fast acceleration and
deceleration, repetitive high impact, and high levels of par¬
ticipation appear to increase the risk of developing
osteoarthritis. Track and field events, racket sports, and
soccer are among the sports that are linked to a higher risk
of osteoarthritis. Swimming and cycling are not linked with
an increased risk of developing osteoarthritis at the hip,
although cycling may be related to osteoarthritis of the
patella.

Injuries to the anterior cruciate ligament, collateral lig¬
ament, or meniscus are implicated in the development of
osteoarthritis in the knee [51]. Loss of the anterior cruci¬
ate ligament may impair sensory function and protective
mechanisms at the knee. Disruption of internal joint struc¬
tures may alter joint alignment and the areas of cartilage
that are loaded. If ligament damage results in a loss of joint
stability, then joint loads may be increased by active mus¬
cle contraction trying to stabilize the joint. Partial or total
meniscectomy can also be expected to increase the stress
on the joint since the joint force is concentrated over a
smaller area [117].

Despite an increased risk of developing osteoarthritis from
excessive or abnormal joint loading, some level of loading or
exercise appears to be beneficial for joint heath. In an in vivo
study with 37 healthy human volunteers, Tiderius et al. [112]
show that glycosaminoglycan content in medial and lateral
femoral condyle cartilage is lower in sedentary subjects than
those who exercise regularly. After an exercise regimen there
is also an increase in glycosaminoglycan content in the knee of
patents at risk for developing osteoarthritis [95]. These latter
two studies use an MRI imaging technology known as
dGEMRIC [7, 8] to quantitatively measure glycosaminoglycan
content in vivo. They show a biochemical adaptation to exer¬
cise, although there appears to be no adaptation of cartilage
morphology to exercise as determined by tissue mass [23].
Since exercise can enhance production of matrix molecules, it
may seem reasonable to expect that it can have a positive
effect on joint health.

Clinical Relevance

EXERCISE THERAPY: Smidt et al. [105] review the literature
on the effectiveness of exercise therapy for patients with disor¬
ders of the musculoskeletal nervous, respiratory , and cardio¬
vascular systems. They conclude that among musculoskeletal
disorders; exercise therapy is effective in patients with
osteoarthritis of the knee', and subacute and chronic low back
pain. They also find indications that exercise therapy is effec¬
tive for patients with osteoarthritis of the hip and ankylosing
spondylitis. However ; existing evidence is not sufficient to sup¬
port or refute the effectiveness of exercise therapy for neck
and shoulder pain or repetitive strain injury , and they con¬
clude that exercise therapy is not effective for patients with
acute low back pain. Exercise in people with osteoarthritis is
shown to have positive effects on several outcome measures
such as pain, strength, self-reported disability , observed dis¬
ability in walking, and self-selected walking and stepping
speed [46,80]. Although mild to moderate exercise is often
recommended', the optimal therapeutic exercise protocol for
people with osteoarthritis is not known [46,58,80].

Although exercise that is actively controlled by a person
with osteoarthritis has multiple positive effects, passive joint
motion may be superior for healing of articular cartilage
defects. Using surgically created defects in an animal mode1,
Salter et al. [97] show a significantly higher healing rate in
joints subjected to continuous passive motion (44% of defects
healed) than those subjected to intermittent active motion (5%
of defects healed) or immobilization (3% of defects healed).

It should not be surprising that active or passive joint
motion can have positive effects on articular cartilage. As
established earlier in this chapter, cartilage is a fluid-saturated
deformable porous material. Active or passive joint motion
results in cartilage deformation, and hydrostatic pressure and
movement of interstitial fluid in the cartilage matrix. A large
body of research, not reviewed in this chapter, shows that car¬
tilage matrix production is sensitive to applied hydrostatic
pressure or deformation. Varying pressure or deformation
applied within defined amplitude and frequency ranges can
enhance or inhibit matrix production [12,47,52,78,79,
114,123]. The effects of active and passive exercise on the
health of articular cartilage are consistent with accepted mod¬
els and carefully controlled experimental results.

SUMMARY

In summary, articular cartilage provides an efficient load-
bearing surface for synovial joints that is capable of function¬
ing for the lifetime of an individual. The mechanical behavior
of this tissue depends on the interaction of its fluid and solid
components. Numerous factors can impair the function of
cartilage and lead to osteoarthritis and a painful and nonfunc¬
tional joint. Mechanical factors are strongly implicated in the

Chapter 5 I BIOMECHANICS OF CARTILAGE

development of osteoarthritis, although exact mechanisms
are still not known. Exercise has both beneficial and detri¬
mental effects on cartilage. It produces positive biochemical
changes and reduces pain and increases function in people
with arthritis. Conversely, sports injuries are significant con¬
tributors to osteoarthritis.

References

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CHAPTER

Biomechanics of Tendons
and Ligaments

MARGERY A. LOCKARD, P.T., PH.D.

CHAPTER CONTENTS

STRUCTURE OF CONNECTIVE TISSUE .85

Composition of Tendons and Ligaments.85

MECHANICAL PROPERTIES .87

Determination of Stress and Strain.87

Stress-Strain Curve for Tendons and Ligaments.88

Modes of Failure.90

Effects of Physical Conditions on Mechanical Properties.90

Biological Effects on Mechanical Properties.92

RESPONSE OF TENDONS AND LIGAMENTS TO IMMOBILIZATION .94

Immobilization and Remobilization of Normal Connective Tissue.94

Immobilization and Mobilization in Healing Connective Tissue.96

RESPONSE OF TENDONS AND LIGAMENTS TO STRESS ENHANCEMENT .98

SUMMARY .99

T endons and ligaments are dense connective tissue structures that connect muscle to bone and bone to bone,
respectively. Both are located in and around the joints of the body, and as a result, they are both subjected to
large distractive or tensile loads. These are the structures that are largely responsible for providing joint sta¬
bility during movement and function. Tendons and ligaments are biologically active structures, and as a result, injuries,
aging, and abnormal conditions such as joint immobilization produce alterations in their composition and structure.
These changes in structure affect the mechanical properties of tendons and ligaments as well as the functioning of the
joints with which they are associated. It is important for clinicians who are treating patients with tendon and ligament
injuries to understand these structural and mechanical changes so that the treatments selected will stimulate positive
tissue adaptations and improve joint and overall function.

The specific goals of this chapter are to

■ Describe the components and organization of dense regular connective tissues, particularly tendons and ligaments
■ Discuss the mechanical behavior of tendons and ligaments in response to tensile loads
■ Describe physical factors affecting the mechanical properties of tendons and ligaments
■ Describe biological factors affecting the mechanical properties of tendons and ligaments
■ Discuss the response of tendons and ligaments to immobilization and remobilization

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

■ Describe the mechanical properties of tendons and ligaments during healing

■ Describe the effects of stress enhancement on the mechanical properties of tendons and ligaments

Prior to reading this chapter, the reader should understand the mechanical properties of viscoelastic tissues that are
described in Chapter 2.

STRUCTURE OF CONNECTIVE TISSUE

Tendons and ligaments are composed of connective tissue.
Connective tissues provide and maintain form in the body,
functioning mechanically to connect and bind cells and organs
together, thus giving support to the body Connective tissues
are typically classified into three major groups: connective tis¬
sue proper, supporting connective tissues, and specialized con¬
nective tissues. Supporting connective tissues include bone
and cartilage. The histological and mechanical properties of
these tissues are described in Chapters 3 and 5. Specialized
connective tissues include adipose tissue and hematopoietic
tissue. Connective tissue proper is described as loose or dense.
Loose, or areolar, connective tissue is the “packing material”
that is found within and between muscle sheaths, supporting
epithelial tissue, and encircling neurovascular bundles. Loose
connective tissue is very delicate and not very resistant to
stress or strain. Dense connective tissue is less flexible and
more resistant to stress. The dermis of skin is classified as
dense, irregular connective tissue since the fiber bundles are
disorganized and lack specific orientation. Tendons and liga¬
ments are classified as dense, regular connective tissue. Fiber
bundles in tendons and ligaments are densely packed and are
oriented parallel to one another as well as to frequently
applied forces. This arrangement makes them particularly well
adapted to resisting traction or tensile forces.

Composition of Tendons and Ligaments

Like all dense connective tissues, tendons and ligaments are
composed of two major compartments, cells and extracellular
matrix. The primary cell type in tendons and ligaments is the
fibrocyte, also called the fibroblast, when it is actively manu¬
facturing proteins. Cells, however, make up only about 20%
of the total tissue volume. Fibroblasts manufacture and
secrete the components of the extracellular matrix that makes
up the remaining 80%. The extracellular matrix is composed
of fibers (collagen and elastin) and the ground substance.
The ground substance is the gelatinous material that fills the
spaces between cells and fibers. It is composed of non-
collagenous structural glycoproteins (fibronectin), proteogly¬
cans (decorin, biglycan), and water (Fig. 6.1).

EXTRACELLULAR MATRIX: FIBERS

The fibrous component of tendons and ligaments is com¬
posed primarily of collagen, giving tendons and ligaments
their white appearance. Collagen, which has strength similar
to that of steel, is manufactured in the rough endoplasmic

reticulum of fibroblasts. It is composed of amino acids that
are assembled into long polypeptide chains in which every
third residue is glycine. Three polypeptide chains become
attached together to form a triple helix called procollagen.
Procollagen, an organic crystal, is secreted from the fibroblast
into the extracellular matrix, end components are cleaved,
and the slightly shorter molecule is now called tropocollagen.
Tropocollagen molecules in the extracellular space polymer¬
ize into collagen microfibrils, which in turn aggregate into
subfibrils, fibrils, and finally fibers [64] (Fig. 6.2).
Tropocollagen molecules are initially attracted to each other
by weak hydrogen, hydrophobic, hydrophilic, and covalent
bonds. Once aggregation into microfibrils has occurred,
changes in the intra- and intermolecular attachments
progress from less, to more, stable bonding. As the collagen
matures, there is an increase in both the density and the sta¬
bility of bonding, which results in increased tissue strength
and stiffness [6]. This is a brief overview of how collagen is
manufactured in connective tissues. A more detailed descrip¬
tion can be found in a histology textbook.

The amino acid sequence of the polypeptide chains that
constitute the tropocollagen molecules is not always the same.

Figure 6.1: Biochemical composition of normal rabbit medial
collateral ligament. This graph demonstrates the proportion of
components in ligament (based on analysis of rabbit ligament).

Part I I BIOMECHANICAL PRINCIPLES

Figure 6.2: Structural composition of ligament and tendon. A. The hierarchical organization of tendon and ligament
from tropocollagen molecule to gross structure. Fibers and their crimp pattern can be seen with optical microscopy.
The fibril levels can only be visualized with electron microscopy. B. The formation of microfibrils from end-to-end
and lateral aggregation of tropocollagen molecules. Microfailures occur at the amorphous junctions at the ends of
tropocollagen molecules.

As a result, many types of genetically distinct collagens have
been identified, each with a different chemical composition
and mechanical properties. Nineteen different collagen types
have been identified. Tendons and ligaments primarily have
type I (approximately 70%, dry weight), with a small amount
of type III (3-10%), and trace amounts of types V, X, VII, and
XIV [92]. However, the proportion of collagen types present
varies among specific tendons and ligaments. For example,
tendons typically have very little type III collagen; ligaments
have a greater proportion. The cruciate ligaments of the knee
have a greater proportion of type III collagen than medial col¬
lateral ligaments. Granulation tissue has a very high propor¬
tion of type III collagen. Variations in collagen type
composition may contribute to variations in the mechanical
behavior of different ligaments and tendons.

Clinical Relevance

GENETIC DISORDERS AFFECTING COLLAGEN: Ehiers-
Danlos syndrome (EDS) is a genetically inherited connective
tissue disorder resulting from defective collagen types I, II,

III, or V. There are several types of EDS, each with a unique
clinical presentation. However, in all types, various genetic
abnormalities cause defective collagen that is unable to
form fibrils properly. As a result, connective tissues with this
defective collagen are very weak. Some patients suffer from
joint hypermobility, subluxations, and dislocations. Other
symptoms due to the defective connective tissues include
mitral valve prolapse, gastrointestinal tract problems, ten¬
dency to bruise easily, and slow-healing skin wounds.

(continued)

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

(Continued)

Another genetic disorder that results from defects of a
gene that codes for collagen type I is osteogenesis imper¬
fecta. Mutations result in failure to form the collagen triple
helix correctly , which interferes with subsequent fibril forma¬
tion. The result is brittle bones , manifested clinically in vary¬
ing degrees of severity and producing multiple low-load
fractures.

Elastin fibers constitute a much smaller proportion of the
fibrous composition of tendons and ligaments. Tendons have
very little elastin. The proportion of elastin to collagen fibers
varies among ligaments. However, in most ligaments of the
joints, as in tendons, collagen is present in a much higher
proportion than elastin. The ligamentum flavum, an inter¬
laminar ligament in the spine, however, has more elastin than
collagen [66].

EXTRACELLULAR MATRIX: GROUND SUBSTANCE

The ground substance, or nonfibrous part of the extracellular
matrix, is composed of structural glycoproteins, proteogly¬
cans, and water. Structural glycoproteins contain a large pro¬
tein fraction and a small carbohydrate component. These
glycoproteins, such as fibronectin, thrombospondin, tenascin-
C, and undulin, play an important role in the adhesion of cells
to fibers and other extracellular matrix components [43].

Although proteoglycans constitute less than 1% of a ten¬
dons or ligament’s total dry weight, they play a key role in lig¬
ament and tendon functioning (Fig. 6.1). Proteoglycans are
large, complex macromolecules with a protein core to which
one or more glycosaminoglycans (GAGs) are covalently
attached. GAGs are linear molecules of repeating disaccha¬
ride units, which are bound to the protein core at one end
and radiate from it to form a “bottlebrush” configuration (see
Fig. 5.2). The concentration of GAGs is considerably smaller
in tendon and ligament than in cartilage. However, due to
their high charge density and charge-to-charge repulsion
force, proteoglycan molecules are stiffly extended and thus
contribute to tendons’ and ligaments’ ability to resist com¬
pression and tensile forces [43]. The polar nature of these
molecules also attracts and holds water within the connective
tissues. This hydrophilic characteristic helps to maintain ten¬
don and ligament extensibility in response to tensile forces.
For example, wet tendon is able to elongate easily in
response to a distraction force, while dry tendon loses com¬
pliance [100]. The hydrophilic property of proteoglycans also
enables rapid diffusion of water-soluble molecules and
migration of cells within the extracellular matrix of the ten¬
don or ligament [43].

Proteoglycans also help to regulate and maintain the
structural organization of the tissue by providing support
and spacing for the cellular and fibrous connective tissue
components. Attachments between GAGs and collagen
fibers occur in connective tissue, which contributes to the

aggregation of collagen into fiber bundles and to tissue
strength. The crimp pattern (wavy appearance of collagen
fibers in dense regular connective tissue) has been attrib¬
uted to the attachments of GAGs to collagen [9]. Examples
of GAGs include chondroitin sulfate, dermatan sulfate, and
hyaluronic acid, although dermatan sulfate is usually most
common in tendons and ligaments. Examples of proteogly¬
cans common in tendons and ligaments include decorin and
biglycan.

MECHANICAL PROPERTIES

The mechanical properties of tendons and ligaments are
measured by subjecting tissue preparations to uniaxial tensile
loads to failure. Tissue preparations typically consist of
bone-ligament-bone complexes or tendon. Data collected
from these tests are used to create load-deformation curves
by plotting the externally applied load against the correspon¬
ding amount of elongation of the tissue. These load-defor¬
mation curves represent the structural properties of the tissue
tested (Fig. 6.3). As described in Chapter 2, load-deforma¬
tion curves can be converted to stress-strain curves that
mathematically describe the mechanical properties of the
tendon or ligament tested. These mechanical properties
depend on the composition of the tissue, the orientation of
the collagen fibers, and the interaction between collagen and
the components of the ground substance. This method of
determining ligament and tendon mechanical properties,
however, requires extraction of whole tissues. Thus, studies
that employ these methods use tissues from a variety of ani¬
mal models or human tissue removed during surgery (e.g.,
degenerated ligaments removed for insertion of total knee
replacement prosthetic components).

More recently tendon strain has been measured in vivo in
humans using real-time ultrasound scanning methods. These
emerging methods allow researchers to study the effects of
various factors, such as aging and exercise, on tendon mechan¬
ical properties directly in humans during movement [57].

Determination of Stress and Strain

Tensile strain is defined as the elongation per unit length of
a material in response to a tensile load. It is represented by
the formula strain = (1 — 1 )/l , where 1 is the length before
the tensile load is applied, and 1 is the length after the load
has been applied. Thus, strain has no units and is usually
expressed as a percentage. Length can be measured directly
in extracted tissues by placing markers on the soft tissue in
the region to be studied. It can also be measured by using
devices such as linearly variable differential transformers
(LVDT), which are instruments that measure voltage
change during elongation and convert this change to a cor¬
responding change in length. This method can be used to
collect data in vivo during joint movement, but requires
invasive methods to attach the device directly to the tissue

Part I I BIOMECHANICAL PRINCIPLES

Deformation (mm)
(elongation)

Figure 6.3: A typical load-deformation curve for a
bone-ligament-bone complex. When tensile forces
(loads) are applied to a bone-ligament-bone complex,
a load-deformation curve can be drawn to represent
its structural properties.

[13,92]. Noninvasive in vivo ultrasound methods are also
used to measure tendon elongation, strain, and tissue stiff¬
ness during joint movement, but require the subject to be
stationary [30].

Tensile stress is defined as the externally applied tensile
load per cross-sectional area of the tendon or ligament tested.
It is represented by the formula stress = F/A, where F is the
amount of the externally applied distraction force and A is
the cross-sectional area of the material tested. It is usually
expressed as newtons per square millimeter. Although this is
a simple relationship, accurate determination of the cross-
sectional area of the structure can be difficult. Various meth¬
ods to determine area are used, ranging from low-tech calipers
to measure the width and thickness of the specimen to sophis¬
ticated noncontact laser methods [92]. This method requires
tissue explants from animal models or human tissues dis¬
carded at surgery.

The forces or stresses within tendon and ligaments are also
measured in vivo during movement using devices that are
placed in or around the midsubstance of the tissue of interest.
Examples of these instruments include buckle transducers,
fiberoptic sensors, and other implantable force probes [30].

Stress-Strain Curve for Tendons
and Ligaments

A stress-strain curve typical for a tendon or ligament is drawn
in Figure 6.4. Five major regions can be identified on the
stress-strain curve of a tendon or ligament. These regions are
called the toe region, the linear or elastic region, the pro¬
gressive failure or plastic region, the region of major fail¬
ure, and complete failure [84].

The first region is the toe region. In this region there is very
little increase in stress as the tissue elongates. Strain is also
very low (1.2-1.5%). The stresses that produce strains in the
toe region have been equated to those applied by an evaluator

during clinical ligament stress tests. In tendons, toe region
stress is sufficient to straighten collagen s crimp pattern and is
equivalent to the force produced by a maximum tetanic con¬
traction of the corresponding muscle (Fig. 6.5).

In the linear, or elastic, region of the curve, elongation in
response to the applied load continues to increase. Stiffness
or resistance to elongation also increases, but the relationship
between stress and strain remains consistently linear. Micro-
and macro-examination of the strain response of tendons to
tensile loading shows that loads that exceed those that pro¬
duce crimp straightening result in tissue elongation by means

Figure 6.4: Stress-strain curve for tendon or ligament. Five
regions are labeled: 1 , toe region; 2, linear, or elastic, region; 3 ,
progressive failure, or plastic, region; 4, major failure region; and
5, complete rupture region or failure.

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

Figure 6.5: Changes in the internal structure of the collagenous
tissue of tendons and ligaments in response to tensile loads.

The drawing demonstrates the changes that occur in collagenous
tissue during stretching. 1 , Toe region, in which the stretch
straightens the wavy pattern of the collagen. Regions 2 and 3
are the elastic and plastic regions, in which the crimp is elimi¬
nated, and tissue elongation occurs because of stretching of the
straightened collagen fibers. Region 4 is the site of major failure
where the ligament is still intact, but there is visible narrowing,
or necking, of the structure. Region 5 is characterized by com¬
plete failure.

of collagen fibers sliding with respect to one another [78].
When the tensile force is removed, the tendon or ligament
returns to its prestressed length and shape. However,
depending on the duration of this elastic range deformation,
additional time may be required for full recovery to the orig¬
inal prestressed length. The persistence of elongation demon¬
strates the hysteresis property of viscoelastic material. The
stress and strain that occur in tendons and ligaments because
of normal physiological motions fall in this elastic, or linear,
region of the curve and strain is estimated to be up to 6%.
Physiological strains in the anterior cruciate and medial col¬
lateral ligaments at the knee are in the range of 4-5% (Table
6.1) [12,13]. A current view is that during normal movements,
tendon elongation does not exceed 4% [43,84].

Physiological strains vary among different tendons and lig¬
aments and among different regions within the same tendon.
For example, optical methods examining the strain behavior
in normal anterior tibialis tendons reveal that the strain meas¬
ured in the region close to the muscle was five times greater
than that measured in either the mid-tendon or the region of
the tendon close to the bone [9]. Tendon fascicles from patel¬
lar tendons of young men also demonstrate regional variation
in strain. Fascicles from the anterior portion of these tendons

TABLE 6.1: Peak Strain in Human Anterior Cruciate
Ligaments during Selected Rehabilitation Activities
(n = 8-18)

Activity

Peak Strain (%)

Isometric quads contraction @15°

4.4

Squatting with sport cord

4.0

Active flexion/extension of knee with

45 N weight boot

3.8

Lachman test (150 N anterior shear load)

3.7

Squatting

3.6

Active flexion/extension (no weight)

2.8

Stationary bike

1.7

display considerably greater peak and yield stress and elastic
modulus as compared with the posterior portion of the ten¬
dons [36]. Regional variation of the strain within tendons may
also be affected by joint position. Researchers who examined
tensile strain in patellar tendons with the knee in various posi¬
tions report uniform strains throughout the tendon with the
knee in full extension. However, when the knee is flexed, the
tensile strain increases on the anterior side of the tendon and
decreases on the posterior side [3]. A better understanding of
these regional variations in the biomechanical properties and
stresses and strains within tendons and ligaments may con¬
tribute to our comprehension of tendinopathy and other
microtrauma injuries.

The slope of the elastic, or linear, portion of the
stress-strain curve is called Young’s modulus of elasticity
and is represented numerically as stress divided by strain (see
Fig. 2.6). R represents the resistance of the tissue to elonga¬
tion. When the slope of the curve is steep and the modulus is
high, the material exhibits a high degree of stiffness, or resist¬
ance to elongation. When the slope of the curve is gradual
and the modulus is low, the tissue is more compliant and eas¬
ily deformed when subjected to a tensile force (see Fig. 2.7).

The third region of the curve is the region of progressive
microfailures, also called the plastic range. The point at
which the elastic region transitions to the plastic region is
called the yield point. Plastic region tensile forces disrupt
enough collagen fibers and bonds to produce a slow unravel¬
ing of the collagen fibers and a decrease in the slope of the
stress-strain curve [78]. Thus, when the deforming force is
removed, the structure is not able to return completely to its
prestretched dimension. The tissue remains permanently
deformed, even though, to the naked eye, it appears normal
and intact. Stresses in this range could occur during an injury
that causes a ligamentous sprain. A ligament sprained to this
extent may remain overstretched or lax, causing joint instabil¬
ity and future reinjury.

When the plastic range is exceeded, the slope of the curve
flattens dramatically This is the region of major failure.
Although the tendon or ligament is still intact, there is visible
narrowing, or necking, of the structure (Fig. 6.5). Elongation

Part I I BIOMECHANICAL PRINCIPLES

of the material may occur without additional force. This stage
is followed by complete failure. The stress and strain at the
failure point are called the ultimate stress and the ultimate
strain, respectively. During physiological loading, a tendon is
subjected to tensile strains of up to 6% [78]. When an acute
stress causes an elongation of 8% or more, the tendon or lig¬
ament will probably rupture, depending on the specific struc¬
ture and the method of loading [43]. These are properties of
the tendon or ligament tissue, thus they can only be deter¬
mined if failure is through the substance of the ligament or
tendon tissue itself. However, ultimate failure in a tendon or
ligament can occur in three different ways. Failure can occur
by rupture in which there is tearing through the substance of
the tissue, by failure through the enthesis (tendon or ligament
insertional site), or by pulling away a portion of the bony
attachment of the ligament or tendon. Failure at the bony site
is called an avulsion fracture.

Modes of Failure

The nature of failure varies among different tendons and
ligaments and may be influenced by several factors includ¬
ing age or skeletal maturity, structural differences among
different ligaments and tendons, and the speed or rate at
which the elongation force is applied. Failure modes in lig¬
aments, in particular, depend on age or skeletal maturity.
For example, collateral ligaments at the knee fail by tibial
avulsion in animals with open epiphyses, whereas in animals
with closed epiphyses, failure is by ligamentous disruption
[98]. Structural differences within the ligament or tendon
and differences in the nature of their attachment to bone
also influence the method of failure. For example, in animal
studies, the collateral ligaments at the knee typically fail by
midsubstance rupture, while anterior cruciate ligaments
(ACLs) fail by tibial avulsion. Patellar tendons fail by avul¬
sion from the inferior pole of the patella [24]. However, clin¬
ical observations show that knee ligament injuries in
humans typically result in ligament failures by midsubstance
rupture. Thus, there may be species differences as well as
differences among different tendons and ligaments within a
subject.

Effects of Physical Conditions
on Mechanical Properties

The biomechanical properties of tendons and ligaments and
their behavior in response to tensile loads are affected by
physical conditions. Two conditions that have been studied
extensively include the speed, or rate, of application of the
stretching force and the temperature of the structures at the
time of stretching.

EFFECTS OF RATE OF FORCE APPLICATION

The effect of the rate of stretch, or strain rate, on the biome¬
chanical properties of tendons and ligaments has been inves¬
tigated and debated for many years. Studies using human

tendons and ligaments demonstrate that the speed of stretch
has an effect on their stress-strain curves [68,86]. It had been
suggested that the rate of strain that occurred during injury
had an effect on the nature of the resulting ligament injury.
For example, as the rate of force application increases, stiff¬
ness and ultimate load also increase, and failure is more likely
to occur by rupture. Conversely, at slow speeds, failure occurs
predominantly by avulsion [22,68]. However, data from
more-recent studies suggest that the effects of strain rate are
overestimated [92]. Ligaments tested at low, medium, and
high rates of elongation show similar biomechanical
responses. Small differences are observed in the modulus of
elasticity between the slow and medium strain rates, but the
modulus at the fast extension rate is only 30% higher
[24]. Consistent with this finding, tendons also show
moderate increases in elastic modulas and ultimate
tensile strength as strain rate increases [62]. Failure modes
(avulsion versus substance rupture) in ligaments are inde¬
pendent of strain rate but depend on age and skeletal matu¬
rity. In animals with open epiphyses, all failures occur by
avulsion, regardless of strain rate, whereas in animals with
closed epiphyses, failures occur by ligament disruption [70].

EFFECTS OF TEMPERATURE

Temperature has an important effect on the molecular and
mechanical properties of collagen. When an unrestrained ten¬
don or ligament is heated to 59-60°C, it undergoes irreversible
shrinkage [65]. This critical temperature, called the melting
temperature , is presumed to cause breaking of chemical bonds
that maintain the structure and organization of collagen fibers.

Clinical Relevance

USING HEAT TO STABILIZE THE SHOULDER: In the

young , active population , a common cause of shoulder pain
is joint instability due to capsular injury Once shoulder
instability is present the incidence of reinjury is high. As a
result surgical intervention may be required to restore the
joint stability necessary for return to an active lifestyle. Open
surgical procedures to retense lax ligaments require a
lengthy healing and rehabilitation time. As a result another
procedurethermal capsulorraphy, has been developed in
which thermal energy is used to shrink the lax capsule and
ligaments , thus restoring joint stability. Thermal energy , pro¬
duced by lasers or radiofrequency (RF) electrothermal
probes, denatures the collagen and disrupts the covalent
molecular bonds, producing a condensed collagen coil that
results in decreased tissue length. The amount of tissue
shrinkage is variable and increases with increasing applied
thermal energy , tissue temperature , and the duration of
application [55].

Arthroscopically applied thermal energy at a temperature
between 55 and 65°C is commonly used to produce the

(continued)

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

(Continued)

tissue shrinkage and associated histological changes [38].
Biomechanical analysis has demonstrated that gleno¬
humeral thermal capsulorraphy results in decreased
humeral head translation on the glenoid and reestablish¬
ment of intraarticular pressure within the shoulder joint
[71,83]. Although the joint appears to be stabilized by the
heat-tightened ligaments, studies have shown that heat-
treated ligaments have an increase in ultimate and yield
strain and a decrease in tissue stiffness [71,80]. The failure
load of shrunken ligaments is also reduced as a result of the
thermal treatment. Patients who have received this treat¬
ment and their therapists must protect the treated joints
from excessive tensile loads during the postoperative period
to allow for healing and restoration of strength in the
shrunken tissue. Researchers tested heat shrunken ligaments
to failure at 3-week intervals for 9 weeks after thermal treat¬
ment. They found that the highest failure load occurred in
the third week after treatment; that is, the treated ligaments
were the strongest at 3 weeks after treatment. However, at
9 weeks after thermal treatment, those heat shrunken liga¬
ments that had also been immobilized were significantly
weaker than they were at 3 weeks after treatment and were
also weaker than those in which motion was permitted [25].
Total immobilization seems to have a negative effect on the
healing ligament. However, the ideal timing for remobiliza¬
tion is still controversial. Joint instability in other regions
that is also treated with thermal shrinking procedures
include chronic lateral ankle instability and the medial patel¬
lar retinaculum for recurrent patellar instability [21,41].

A less severe temperature increase to 37-40°C is termed
thermal transition. When collagen in tendons and ligaments
is heated to temperatures in the thermal transition range, the
viscoelastic properties of the structure are affected, including
stress relaxation, rate of creep, and rupture strain and load
[84]. When tendons are held under tension during
load-deformation studies, stress relaxation is independent of
temperature until 37°C is reached. Above this temperature,
stress relaxation increases as temperature increases (Fig. 6.6)
[76]. Recovery from stress relaxation is also temperature
dependent. For example, when tendons heated to 37°C and
subjected to strains between 1 and 4% are allowed to cool,
they return to their original stress. However, in tendons
heated to 40°C, the change is irreversible [75].

Heating tendons to thermal transition temperatures also
increases the rate of creep. That is, it takes less time to reach
a given strain in response to application of a stretching load
when tendons are heated [91] (Fig. 6.7). Heated tendons
demonstrate increased rates of creep in response to both
cyclic stretching and constant loading. Both loading condi¬
tions produce similar increases in creep in response to the
heating [51].

The rupture load and strain of tendons are also affected by
heating. For example, tendons heated to 40°C demonstrate a

Time

Figure 6.6: The effect of temperature on the stress relaxation
curves. Stress relaxation curves for rat tail tendon at a given
strain for several temperatures above and below thermal
transition. F 0 , original force; F f force at the end of the period
of sustained strain. Note that as stress relaxation increases, the
difference between F o and F t increases. (Data from Rigby BJ, Hirai
N, Spikes JD: The mechanical behavior of rat tail tendon.

J Gen Physiol 1959; 43: 265-283.)

rupture strain of only 3-4%, compared with 8-14% for ten¬
dons at temperatures lower than 37°C. These results support
the conclusion that heating to 40°C can produce structural
damage to collagen [75]. Additionally, tendons that are heated
and then cooled while the tensile load is maintained rupture
at lower strains than those in which loading was not main¬
tained during cooling. This suggests that cooling in an elon¬
gated position inhibits reestablishment of structural bonds,
thus structurally weakening the tendon [91]. These conclu¬
sions are drawn from in vitro studies conducted on rat tail and

Temperature (°C)

Figure 6.7: Effect of temperature on tendon elongation time.

The graph demonstrates the effect of temperatures above
thermal transition on the time required to achieve a 2.6% strain
(elongation) in rat tail tendon. (Data from Warren CG, Lehman
JF, Koblanski JN: Elongation of rat tail tendon: effect of load and
temperature. Arch Phys Med Rehabil 1971; 52: 465-484.)

Part I I BIOMECHANICAL PRINCIPLES

canine tendon. A more recent in vivo study examined the
effect of immersing the legs of young men in water at 5°C or
42°C for 30 minutes. No changes in the strain behavior of
their Achilles tendons were found. These results provide
evidence that the general application of superficial heat or
cooling agents may not affect the mechanical properties of
tendons [48].

Clinical Relevance

CLINICAL RELEVANCE: USING HEAT TO INCREASE
RANGE OF MOTION: Clinicians are often confronted with
patients who have restricted joint range of motion that inter¬
feres with functional activities. Joint restriction can be pro¬
duced by dense connective tissues within muscles and in
joint capsules as well as in tendons and ligaments.

Treatment to increase joint range of motion is often focused
on increasing the length of dense connective tissues that
have shortened as a result of immobilization or healing.
Treatments involve the application of stretching forces.
Among the methods that can be used to produce stretching
are brief intense methods , such as joint mobilization or
manual osteokinematic stretching , as well as low-load , pro¬
longed stretching techniques using splints, casts, or other
devices. Increased connective tissue extensibility is helpful in
the administration of any of these stretching techniques. The
studies described above have shown that heating dense
connective tissue to about 40°C affects the bonding between
tropocollagen molecules, resulting in increased ductility. The
increased rate of creep and stress relaxation that occurs in
heated dense connective tissue should facilitate the effective¬
ness of stretching techniques. Preheating or heating applied
simultaneously with stretch allows clinicians to produce
elongation in shortened connective tissues with less force;
thus reducing the risk of injuring healing; or other adjacent ,
tissues. However ; heating also results in reduced rupture
load and strain. When 2% strain has been exceeded , colla¬
gen begins to yield. Since heated connective tissues fail at
significantly lower loads than unheated tissues; clinicians
applying stretching forces to heated connective tissues dur¬
ing treatment must carefully monitor the amount of stretch¬
ing force applied to avoid unwanted tissue tearing.

Another clinical consideration is how to achieve the tem¬
perature increase required to produce the desired effect in
the target tissue. The 40°C temperature necessary to pro¬
duce increased tissue extensibility refers to the temperature
of the collagen , not the temperature on the surface of the
body. Since most methods of heating used in the clinic
require transmission of thermal energy through skin and
perhaps overlying muscle and fat before the target tissue
can be reached ' clinicians must be knowledgeable about the
depth of penetration of the heating method selected.

Heating modalities with superficial depths of penetration ,
such as moist hot packs , may not achieve the necessary

temperature elevation in dense connective tissue structures
such as tendons and ligaments that are located beneath
overlying muscles. As a result , insufficient heating will com¬
promise treatment effectiveness.

Biological Effects on Mechanical
Properties

The biomechanical properties of tendons and ligaments are
affected by several biological factors. Biological factors dis¬
cussed in this chapter include skeletal maturity, aging, sex,
and hormones.

EFFECTS OF MATURATION AND AGING

Skeletal maturation and aging have significant effects on the
biomechanical properties of ligaments and tendons. In general,
tensile strength, load to failure, and elastic modulus all improve
rapidly during maturation until skeletal maturity (closing of the
epiphyses) is achieved. Stress relaxation and creep in response
to both static and cyclic loads are also greater in very young ani¬
mals and improve with maturation. Maximal tissue strength is
achieved around the time of skeletal maturity. It declines grad¬
ually during adulthood and into senescence [96,97]. Thus liga¬
ments and tendons from very young and old animals withstand
lower maximal tensile loads than those from young and
middle-aged adults [96] (Fig. 6.8). In addition to affecting the

Figure 6.8: Mechanical properties of the bone-ligament-bone
complexes of skeletally mature and immature rabbits. A load-
deformation curve demonstrates the mechanical properties of
the bone-ligament-bone complexes of skeletally mature and
immature rabbits. Skeletally immature rabbits have open
epiphyses; mature rabbits have closed epiphyses.

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

Age

Figure 6.9: Comparison of the strength of the insertional site and
the ligament substance during skeletal maturation in rabbit
medial collateral ligaments. The graph demonstrates the
differences in ultimate strength between insertion site and the
ligament substance of rabbit medial collateral ligaments as the
animals mature. In skeletally immature animals, failure is most
often by ligament tibial avulsion because its ultimate strength is
less than that of the ligament substance. In skeletally mature
animals, failure occurs by ligament substance rupture, which is
now weaker than the insertion site. (Adapted from Woo SL-Y,
Ohland KJ, Weiss JA: Aging and sex-related changes in the
biomechanical properties of the rabbit medial collateral ligament.
Mech Ageing Dev 1990; 56: 129-142.)

mechanical properties of ligaments, maturation also changes
the mode of failure caused by tensile loading. In young animals
with open epiphyses, the mechanism of failure that occurs
most often is avulsion fracture of bone. In mature animals with
closed epiphyses, ligament failure is more likely to occur by
midsubstance rupture [96] (Fig. 6.9).

Biochemical and histological alterations also occur during
maturation and aging and may explain some of the mechani¬
cal changes associated with aging. During maturation, colla¬
gen fibril size increases, and collagen concentration and
synthesis are greater than in adults [43]. Prior to skeletal
maturity there is a greater number of immature, reducible
collagen cross-links, which corresponds to collagen synthesis.
Adults have a higher proportion of the more stable pyridino-
line cross-links [6] (Fig. 6.10). The mechanical superiority of
adult ligaments may be related to the shift in the predominant
type of collagen cross-linking and increased fibril size. Aged
tendons and ligaments have reduced collagen concentration
and an increased number of small-diameter collagen fibrils
[82]. In addition, collagen type V, a known regulator of colla¬
gen fibril diameter, is found in aged tendons and ligaments,
but not in those of young animals [27]. Other in vitro analyses
of aged tendons show that aging is also associated with a
reduction in the collagen fibril crimp angle, an increase in
elastin content, and a reduction in extracellular water and
proteoglycan content. The net effect of these changes is a
reduction of tendon stiffness in aged tendons [59].

Figure 6.10: Age-related biochemical changes in anterior cruciate
and medial collateral ligaments of rabbits. Two-month-old
rabbits are skeletally immature (open epiphyses), while 12- and
36-month-old rabbits have closed epiphyses; 36-month-old
rabbits are considered aged or elderly. (Data from Amiel D,
Kuiper SD, Wallace D, et al: Age-related properties of medial
collateral ligament and anterior cruciate ligament: a morphologic
and collagen maturation study in the rabbit. J Gerontol 1991; 46:
B159—B165.)

Although most of these findings were taken from investi¬
gations using animal models, studies on human tissues also
demonstrate the mechanical inferiority of older tendons and
ligaments. For example, the stiffness, ultimate load, and elas¬
tic modulus of ACL specimens from young adults (age 22-35
years) are about three times higher than those from older
people [69,95]. Anterior and posterior spinal ligaments taken
from humans at the time of surgery show a strong inverse
relationship between age and tensile strength [42,60].
Elderly human patellar tendons also show more compliance
and loss of stiffness [73]. However, recent studies in both ani¬
mals and humans show that the reduction in tendon and lig¬
ament stiffness that occurs with aging can be minimized, if
not reversed, in response to low or moderately intense resist¬
ance exercise [18,49,74].

Part I I BIOMECHANICAL PRINCIPLES

Clinical Relevance

IMPLICATIONS OF RESISTANCE EXERCISE FOR
MAINTENANCE OF TENDON STRENGTH AND
STIFFNESS IN OLDER ADULTS: Older individuals who
desire to continue athletic activities often are hampered by
musculoskeletal strain injuries; which are at least partially
because of deterioration in the mechanical properties of
aging connective tissues. The protective effects of resistance
exercise on maintaining tendon and ligament stiffness and
resistance to elongation forces may reduce the likelihood of
tendon strain injuries. Additionally , because tendons are the
connective tissue structures that connect muscle to bone;
they can affect the speed of muscle contraction force trans¬
mission. The increased tendon stiffness induced by resist¬
ance exercise training is associated with faster development
of joint torque. Functional activities that require rapid pro¬
duction of joint torque, such as recovery from loss of bal¬
ance as well as athletic activities; may also improve as a
result of this resistance exercise training. Thus; resistance
exercise training has benefits to elders beyond simply
increased muscle strength.

Clinical Relevance

AFFECTS OF AGE ON FUNCTIONAL CAPACITY OF
LIGAMENTS AND TENDONS: Ligaments and tendons
help to maintain joint stability. As tendons and ligaments
age, they are less able to withstand tensile loads. As a result
they may be less effective in stabilizing a joint in response to
repetitive or high forces that occur during functional activi¬
ties. Joint instability may result in abnormal joint mechanics
during movement. This alteration in joint mechanics may
place excessive stress on joint structures and lead to degen¬
erative joint disease.

EFFECT OF HORMONES

Hormones can affect the mechanical properties of dense con¬
nective tissues also. The adrenocorticotropic hormone of the
pituitary and cortisone of the adrenal cortex both lower the
GAG content of the extracellular matrix of connective tissues.
Excessive levels of cortisol also reduce the synthesis of type I
collagen. Both of these effects reduce connective tissue
strength. Another hormone, relaxin, which is produced during
pregnancy, softens and increases the extensibility of pelvic lig¬
aments. The female sex hormone estrogen may also play a role
in determining the tensile properties of ligaments.
Observations that more female than male athletes experience
ACL injuries led some clinicians and researchers to hypothe¬
size a role for estrogen in determining ligament strength [8].
Functional estrogen and progesterone receptors have been

found in the ligaments of rabbits and humans [77].
Additionally, in vitro incubation of rabbit ACLs with estrogen
produces alterations in cell behavior, including downregula-
tion (reduced transcription) of type I collagen synthesis [54].
The mechanical behavior of ligaments is also affected by alter¬
ations in hormone concentration. For example, increasing the
concentration of circulating estrogen is associated with
reduced tensile properties in ACLs of rabbits [81]. This find¬
ing prompted others to attempt to find a relationship between
menstrual cycle phase and incidence of ACL injury [35]. A
recent study, however, shows that there are no significant dif¬
ferences in ACL ligament laxity in any of the three menstrual
phases, either before or after exercise [58]. Further, a well-
designed and controlled investigation examines the influence
of estrogen and estrogen receptors on knee ligament mechan¬
ical properties in an animal model. These authors report that
estrogen treatment has no significant effect on the viscoelastic
or tensile properties of the MCL or ACL. Thus, it does not
seem likely that estrogen plays a significant role in explaining
the high incidence of ACL injuries in women [90].

RESPONSE OF TENDONS AND
LIGAMENTS TO IMMOBILIZATION

Decreased joint mobility has significant effects on both bone
and soft tissues. Wolffs law describes the effect of mechanical
stresses on bone remodeling. This law is often restated as the
specific adaptation to imposed demand (SAID) principle,
which is used to explain remodeling in response to alterations
in external loading in soft tissues such as tendons and liga¬
ments [17]. Joint immobilization by casting or pinning is often
used to study the effects of stress reduction on tendons and
ligaments. This immobilization model enables examination of
the responses of normal soft tissues as well as soft tissues that
have been injured and are healing. The importance of under¬
standing the effects of immobilization on healing connective
tissue is obvious, since many tendon and ligament injuries are
treated with periods of rest and immobilization. Clinicians
need to decide when to begin and how much movement is
desirable to facilitate joint motion without producing negative
effects on the healing tissue. However, noninjured connective
tissues may also be immobilized. For example, bed rest or pain
following surgical procedures may result in joint immobility.
Neuromuscular conditions such as stroke or spinal cord injury
may also cause muscle paralysis, weakness, or pain resulting in
loss of joint motion. Understanding the effects of immobiliza¬
tion on healing as well as normal joint structures is important
to provide effective and safe rehabilitation treatments.

Immobilization and Remobilization
of Normal Connective Tissue

Joint immobilization, which reduces the tensile forces nor¬
mally applied to tendons and ligaments during joint move¬
ment, alters the biomechanical properties of the joint

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

structures. These alterations include reduction of the load at
failure, reduced stiffness and elastic modulus, and increased
elongation at failure load [44] (Fig. 6.11). In addition, on load-
to-failure testing, the frequency of failure by avulsion at the
insertional site, rather than by midsubstance tear, is increased
significantly in immobilized ligaments. This is related to
increased subperiosteal osteoclastic activity, which results in
subperiosteal bone resorption.

Biochemical and histological changes also occur in immobi¬
lized tendons and ligaments, which may help to explain the bio¬
mechanical changes. Both collagen synthesis and degradation
increase, resulting in increased collagen turnover. This increase
in collagen turnover is time dependent. For example, when
immobilization is limited to 9 weeks, collagen turnover
increases, but the total collagen mass does not change [7].
However, when immobilization is continued to 12 weeks, the
increased collagen turnover results in reduced collagen mass or
atrophy [4]. Ligaments subjected to extended periods of joint
immobilization also demonstrate disorganization of collagen
fiber orientation and alteration in collagen fiber size [15,28,44].
These changes may contribute to the reduced failure load and
reduced stiffness observed in immobilized tissue. Collagen
cross-linking is also affected by immobilization. Chemical
analysis of immobilized dense connective tissue shows an over¬
all increase in the quantity of collagen cross-links. However,
the greatest increase is found in the proportion of cross¬
links associated with newly synthesized collagen, indicating
the presence of increased amounts of immature collagen [2].
Water content and total GAGs decrease during immobiliza¬
tion [1]. These losses may be associated with the develop¬
ment of connective tissue contractures in immobilized dense
connective tissue structures [5].

Joint position during an immobilization period affects
the nature of the biomechanical changes that occur in both

Figure 6.11: Typical load-deformation curves from normal and
immobilized ligaments. The graph demonstrates typical load-
deformation curves for bone-ligament-bone complexes from
normal and normal immobilized joints.

tendons and ligaments. For example, immobilization of liga¬
ments with some tension results in less deterioration of their
tensile properties than does immobilization without tension
[61]. Collagen fiber disorganization occurs in tendons main¬
tained in a stress-reduced position in immobilized joints.
However, recasting the joint in a position in which the tendon
is elongated can reverse the fiber disorganization [28]. Thus,
in both ligaments and tendons, maintaining tensile load in the
tissue during joint immobilization provides some protection
from reduction of their biomechanical properties.

Stress-shielding experiments without joint immobiliza¬
tion also demonstrate the protective role of tension in main¬
taining tissue strength [37]. In these studies, patellar tendons
were either totally (100%) or partially (70%) shielded from
tensile load while animals were permitted full use of knee
joint range of motion and weight bearing. After 2 weeks of
stress shielding, bone-tendon-bone complexes were tested to
failure. Although all stress-shielded tendons show reduced
failure load, those that maintained some tensile stress were
much less affected [37] (Fig. 6.12).

Fortunately, the deleterious histological, biochemical, and
mechanical changes associated with joint immobilization are
reversible. Reestablishment of normal stresses to the tissues
restores normal structure and function. However, full recov¬
ery of the biomechanical characteristics of the complex may
take longer than the time required to produce the undesir¬
able change. Restoration of mechanical properties also
appears to vary among specific structures and among animals
of different species. For example, in a study of the ACLs of
primates, 5 months of remobilization (following 2 months of

Time (week)

Figure 6.12: Changes in maximum failure load with stress shield¬
ing. The graph demonstrates the difference in maximum failure
load of rabbit patellar tendons in response to complete (100%)
or partial (70%) stress shielding. (Data from Hayashi K:
Biomechanical studies of the remodeling of knee joint tendons
and ligaments. J Biomech 1996; 29: 707-716.)

Part I I BIOMECHANICAL PRINCIPLES

110

100

c 90

? 80
N

50-

—►2 months
immobilization

-*-5 months-*-
reconditioning

12 months

reconditioning

Stiffness

Maximum
load at
failure

Time (months)

Figure 6.13: Effect of immobilization and reconditioning on
mechanical properties of ligaments. This graph demonstrates the
effect of 2 months of knee joint immobilization on the mechani¬
cal properties of the anterior cruciate ligament in primates.
Recovery after 5 and 12 months of remobilization is only partial.
(Data from Noyes FR: Functional properties of knee ligaments
and alterations induced by immobilization: a correlative biome¬
chanical and histological study in primates. Clin Orthop 1977;

123: 210-242.)

immobilization) are required to recover about 80% of the
maximum load at failure. After 12 months of recovery, liga¬
ment strength is still only about 90% of preimmobilization
values [67] (Fig. 6.13). With rabbit medial collateral ligament
complexes, 12 weeks of immobilization requires 9 to 12 weeks
of remobilization to restore most of the biomechanical prop¬
erties of the ligaments [93]. Insertional sites, however, are
more resistant to recovery and require 3—1 months of
increased activity to reverse the detrimental effects of immo¬
bilization. In general, prolonged recovery time is required for
recovery from relatively brief periods of stress deprivation.

Because of the methods required to determine mechani¬
cal tissue properties in the past, the findings reported on the
effects of immobilization and stress shielding discussed above
come from studies that used a variety of animal models, from
rats to primates. Recent advances in ultrasound scanning
have enabled the in vivo assessment of the effects of disuse on
some of the mechanical properties of intact human tendon.
Using this methodology short-term (20 days) and long-term
(90 days), bed-rest studies show that tendon stiffness is
reduced by approximately 30% and 40%, respectively [47,72].
The adaptive responses of tendon to loss of normal mechani¬
cal loading are also studied by measuring tendon stiffness and
elastic modulus in tendons from paralyzed muscles in patients
with spinal cord injuries (SCI) of longstanding duration (1.5
to 24 years) [56]. Investigators report that tendon stiffness
and elastic modulus were lower by 77% in the subjects with
SCI as compared with healthy age-matched controls.
Although tendon length was maintained in the paralyzed
muscles, cross-sectional area (CSA) was reduced by 17%.

This reduction in tendon CSA that occurs with paralyzed
muscles differs from nonparalyzed immobilized muscles, in
which CSA is maintained. These findings concerning the
mechanical properties of tendons from paralyzed muscles
have safety implications for clinicians using electrical stimula¬
tion protocols with patients with SCI.

Clinical Relevance

TREATMENT DURING AND FOLLOWING IMMOBI¬
LIZATION: Clinicians must consider these biomechanical
changes when treating patients with joints that are immobi¬
lized for any reason. Application of safe tensile loads to
affected ligaments and tendons during immobilization may
minimize the amount of connective tissue atrophy and loss
of mechanical integrity that might otherwise occur. This can
be accomplished by very simple methods , such as having
patients perform isometric contractions of immobilized mus¬
cles to load the immobilized tendons or by moving joints
that are not being used functionally through their range of
motion.

Clinicians must also exercise caution when applying
forces during exercise to remobilize joint structures that
have been immobilized for extended periods. Immobilized
connective tissues may have developed contractures dur¬
ing the immobilization period. As a result clinicians often
select therapeutic techniques to stretch or elongate the
shortened tissues. These techniques involve application of a
tensile load in an attempt to lengthen the restricted joint
structures. In addition to being shortened , these tissues and
their bony insertion sites may be weaker and more suscep¬
tible to disruption in response to tensile loads. Thus, to
avoid inadvertent injury , when treating shortened connec¬
tive tissue post-immobilization , it may be prudent to select
techniques that apply low-load , prolonged stretch rather
than brief but intense or high-load stretch.

Immobilization and Mobilization
in Healing Connective Tissue

Healing in tendons and ligaments has four overlapping
phases [94]. These include a hemorrhagic phase in which
the gap is filled with a blood clot, and lymphocytes and
leukocytes expand the inflammatory response. In phase two,
the inflammatory phase, macrophages become the predom¬
inant cell type. They secrete growth factors that induce neo¬
vascularization and the formation of granulation tissue.
They are also chemotactic for fibroblasts and stimulate
fibroblast proliferation and the synthesis of types I, III, and
V collagen and noncollagenous proteins. The last cell type to
become prominent is the fibroblast. This event signals the
beginning of the third, or proliferative, phase of healing.
During this stage, fibroblasts produce collagen and other
matrix proteins. This usually occurs within 1 week of injury.

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

The last stage, remodeling and maturation, is marked by a
gradual decrease in the cellularity of the healed tissue. The
matrix becomes more dense and longitudinally oriented.
Collagen turnover, water content, the ratio between types I
and III collagens, and the ratio among the various collagen
cross-links begin to approach normal levels.

During the last stage of healing, various biochemical and
mechanical signals are critical in facilitating the remodeling
process. Tension or physiological loading is an important sig¬
nal necessary to trigger the changes required for the healed
ligament to recover normal tensile strength and other bio¬
mechanical properties. Certain growth factors also influence
healing and facilitate the restoration of strength. The healed
tissue continues to mature for many months but may never
attain normal morphological characteristics or mechanical
properties. The degree of recovery of normal composition,
organization, and tensile strength varies among structures
within an individual as well as among species. For example,
in the knee, ACLs do not heal after disruption, while collat¬
eral ligaments do. Additionally, tendons and ligaments heal
much more slowly than a wound in the skin. Variation in
healing time among tissues occurs for a variety of reasons,
including the amount of blood supply to the tissue and the
method by which the tissue receives nutrition and has
metabolites removed.

Biomechanical testing of healing tendons and ligaments
shows that like normal ligaments that have been immobi¬
lized, healing dense connective tissues have poorer
stress-strain characteristics, including lower failure loads
and reduced stiffness (lower elastic modulus). Healing ten¬
dons and ligaments demonstrate smaller-diameter collagen
fibers, a greater proportion of type III collagen, and a
higher proportion of reducible cross-links, indicating
immature collagen. The effect of joint immobilization ver¬
sus free joint movement on tendon and ligament healing
has been studied to determine the most beneficial method
of treatment. For many years, the clinical management of
healing tendons and ligaments included protection from
tensile loads by casting or splinting for long periods.
Current practice focuses on early mobilization during heal¬
ing, since prolonged protection of tendon and ligament
wounds from stress is detrimental to restoration of normal
strength and stress-strain characteristics. Movement within
physiological limits introduced immediately after repair
results in stronger unions than delayed mobilization or pro¬
longed immobilization [29,33,34]. Additionally, compared
with the outcome of delayed activity, early motion to heal¬
ing connective tissue results in faster healing and reduced
scar tissue adhesions [23].

As the proportion of the population classified as elderly
increases and more older adults choose surgical treatments
of musculoskeletal injuries rather than reduction of activity
level, clinicians require specific knowledge about the effects
of aging on healing in tendons and ligaments to make sound
clinical decisions about patient care. Many studies describe
the detrimental effects of aging in healing dermal wounds;

however, little is known about tendon and ligament healing in
older adults. Interestingly, at least one animal study using
healthy elderly rabbits has demonstrated that the biomechan¬
ical properties of healing tendons in old animals (age 4.8
years) are equally as good as those for young (age 1 year) rab¬
bits. It appears that age does not negatively influence the bio¬
mechanical properties of healing patellar tendon repairs [26].
In fact, the tensile strength of the healing scar tissue in ten¬
don wounds of the older rabbits was just as strong as the scar
tissue in the tendons of the young rabbits. However, more
study is required to determine interactions among the
processes of tendon and ligament healing, comorbid factors
often present in elders that are known to affect healing (e.g.,
diabetes mellitus, poor nutrition, and smoking) and exercise
on the biomechanical properties of healing tendons and liga¬
ments in elders.

Clinical Relevance

EARLY MOBILIZATION OF TENDON REPAIRS: The

most commonly used treatment approaches for postopera¬
tive management of primary tendon repairs in the hand use
early passive mobilization. During the inflammatory and
early fibroblastic stages of healing (the first 3 weeks after
surgical repair% the repaired tendon is protected from active
motion. However ; with the joints of the hand and wrist posi¬
tioned so that the repaired tendon is protected from exces¬
sive tension or elongating stresses that might rupture or
attenuate the repair site, each joint of the finger is moved
passively through its full range of motion. Healing tendons
subjected to this type of passive motion during the first
3 weeks of healing are two to three times as strong as
immobilized tendons [40]. In fact, repaired tendons that are
immobilized for 3 weeks are no stronger than immediately
after suture [32].

Growth factors also affect the outcome of healing in ten¬
dons and ligaments. Endogenous growth factors and growth
factor receptors increase during the first 2 weeks after liga¬
ment injury [50,53]. This observation led researchers to add
selected growth factors to healing ligaments. When exoge¬
nous growth factors, such as platelet-derived growth factor-
BB and transforming growth factor 1, are applied individually
to healing ligaments, tensile strength and stiffness increase
significantly [39,46]. This effect is greatest when the growth
factors are applied within 24 hours of injury [10]. Although
growth-factor-treated healing ligaments are stronger than
untreated healing ligaments, their tensile strength still does
not approach that of uninjured ligaments [10]. Continuing
study of the effects of growth factors to facilitate and enhance
tendon and ligament healing is needed as clinical applications
emerge.

Part I I BIOMECHANICAL PRINCIPLES

Clinical Relevance

TREATMENT OF TENDON AND LIGAMENT TEARS:

Not all tendons and ligaments heal in the same manner.

The healing capability of collateral and cruciate ligaments
at the knee and extrasynovial versus intrasynovial grafts for
tendon injuries have been compared [99]. Isolated injuries of
the medial collateral ligament heal reliably without surgical
intervention. Conversely, there is little chance that the cruci¬
ate ligaments will heal after disruption. As a result, when a
cruciate ligament tear results in functional joint instability,
ligament reconstruction with a connective tissue graft is usu¬
ally performed.

Reviews from the clinical literature on anterior cruciate
reconstructions report approximately a 30% incidence of
postoperative laxity [19,52]. There is concern that excessive
loading during rehabilitation may contribute to elongation of
ligament grafts [13,63]. As a result, it is important for clini¬
cians to understand the creep properties of ligament grafts
over time and the effect of joint immobilization and mobi¬
lization on these properties. Autografts for medial collateral
ligaments in rabbits are more susceptible to creep than nor¬
mal medial collateral ligament controls. Additionally, immo¬
bilization of the autografts during healing results in
increased vulnerability of the grafts to creep. Progressive
elongation of the grafts could occur over time because of
their inability to recover from the imposed creep strain, com¬
pared with normal ligaments [16]. As a result, most postoper¬
ative protocols for rehabilitation following ACL reconstruction
include early mobilization rather than extended periods of
immobilization. Moreover, to prevent excessive creep from
accumulating over time, it is important for clinicians to
understand the effect of postoperative exercises on ligament
and graft strain. Exercises that may be effective in increasing
muscle strength may apply excessive strain to the ligament
graft and thus potentially contribute to recurrence of liga¬
ment instability due to graft creep (Table 6.1) [12].

RESPONSE OF TENDONS AND
LIGAMENTS TO STRESS ENHANCEMENT

Since immobilization and reduction of stress in tendons and
ligaments has a deleterious effect on the their biomechanical
properties, researchers have become interested in the effect
of exercise and stress enhancement on dense connective tis¬
sues. Studies to determine if exercise affects normal tendons
and ligaments identified some positive effects on the strength
of ligaments and tendons and their insertion sites [20,85,
87,88]. For example, following an endurance exercise regi¬
men, trained animals have ligaments with smaller-diameter
collagen fiber bundles, higher collagen content, increased
tensile strength, and maximum load at failure. Animals that
performed nonendurance exercise did not exhibit these

adaptations [89]. However, the results of a more recent study
show that a lifelong endurance exercise training program
(420-557 weeks of training) had little or no effect on the bio¬
mechanical properties of medial collateral ligaments from
normal adult animals [98]. Thus, it appears that exercise may
only protect against the weakening effect of inactivity and not
strengthen normal tendons and ligaments. A drawback of all
of these studies is that the amount of load actually applied to
the tendons and ligaments by the exercise is not known.
Studies previously discussed in this chapter (see Effects of
Maturation and Aging) show that low and moderately intense
resisted exercise can minimize or reverse the loss of tendon
stiffness that occurs with aging [49,73]. Aerobic endurance
exercise, however, does not have the same protective effect
on aging tendons, even though an in vitro study in which
cyclic tensile strain was applied to tendon explants showed
collagen synthesis up-regulation and retention of the newly
synthesized collagen after the tensile loading had ceased
[45,79]. These findings are very interesting, but additional
study is required to help develop clinical applications.

In addition to altering the tensile load to tendons and liga¬
ments by exercise, stress enhancement in tendons and liga¬
ments is studied in animals by partial removal of tissue from
the structure [37]. For example, by cutting away both edges
of the rabbit patellar tendon, the cross-sectional area of the
remaining tendon is reduced, and the remaining tendon is
subjected to higher stress when the animal performs normal
activity (force remains the same, but cross-sectional area
decreases). Biomechanical tests performed on tendons from
animals that experienced stress enhancement by this method
show that when stress is elevated 33% above the normal
stress, there is no significant difference between the tensile

Figure 6.14: Effect of stress enhancement on tensile strength.

This graph demonstrates the effect of stress enhancement on the
ultimate tensile strength of rabbit patellar tendons. (Data from
Hayashi K: Biomechanical studies of the remodeling of knee joint
tendons and ligaments. J Biomech 1996; 29: 707-716.)

Chapter 6 I BIOMECHANICS OF TENDONS AND LIGAMENTS

strength of experimental (tendons reduced in cross-sectional
area) and control (normal) tendons. Interestingly, the cross-
sectional area of these stress-enhanced tendons increases
back to normal over 6 to 12 weeks. However, when stress is
elevated 100%, although all tendons gradually increase in
cross-sectional area, not all tendons respond in the same way
to biomechanical testing. One group of stress-enhanced ten¬
dons (group A) show no change in the stress-strain behavior,
no histological differences, and only a small change in tensile
strength. However, another group of stress-enhanced ten¬
dons (group B) show reduced tensile strength and stiffness as
well as histological changes including an increased number of
fibroblasts and breakage of collagen bundles [37] (Fig. 6.14).
These results indicate that knee joint tendons and ligaments
seem to have the ability to adapt to overstressing within a cer¬
tain range, (less than 100% overstress). However, they cannot
adapt if stress exceeds this limit [37].

Clinical Relevance

PATELLAR TENDON GRAFTS: The middle third of the
patellar tendon is commonly used as an autogenous graft
in ACL reconstruction surgery. The remaining tendon is sub¬
jected to tensile loads caused by quadriceps muscle contrac¬
tion and weight bearing. Patients do not typically experience
difficulty with quadriceps function as a result of the reduc¬
tion of the patellar tendon; however; occasionally a patient
develops patellar tendinitis. This may be related to over¬
stress to a susceptible tendon.

SUMMARY

Tendons and ligaments provide passive and dynamic stability
to joints. They are collagenous structures with a molecular
and gross structure designed to resist tensile loads. Changes
in physical and biological conditions alter the biochemical
composition, histological structure and organization, and bio¬
mechanical properties of the tissue. These changes can occur
by physical means, such as breaking of intermolecular bonds,
or by biological means. Alteration by biological means is
called remodeling. During remodeling, various factors affect
the fibroblasts within the tissue, causing a cellular response
that alters the components of the extracellular matrix and the
way in which it responds to loading. Factors that cause dete¬
rioration of the biomechanical properties of tendons and lig¬
aments include immobilization, aging, healing, and stress
shielding. The application of physiological loads during
immobilization and healing reduces the amount of deteriora¬
tion of the biomechanical properties. Normal ligaments can
withstand increased stress loading within a range (about
133% of normal), after which the biomechanical properties
of the overstressed ligaments deteriorate. Maintenance of

normal biomechanical functioning of tendons and ligaments
is important to ensure normal joint kinematics and tolerance
to loading during functional activities.

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CHAPTER

Biomechanics of Joints

MARGERY A. LOCKARD , P.T., PH.D.
CAROL A. OATIS, P.T., PH.D.

CHAPTER CONTENTS

CLASSIFICATION AND STRUCTURE OF HUMAN JOINTS.104

Diarthroses.104

JOINT MOTION.105

Classification of Motion .105

Classification of Synovial Joints.109

FACTORS INFLUENCING MOTION AT A JOINT.110

The Effect of Joint Structure on Joint Motion .110

External Forces on a Joint .112

Interactions between Joints and the External Environment.113

SUMMARY .114

J oints, the sites of motion between articulating bones, are joined to one another by various connective tissue

structures that must maintain the integrity of the junction while allowing motion between the bones. Thus, the
architectural challenge of joints is to create a balance between mobility and stability. The amount of mobility
or stability varies widely throughout the joints of the body. For example, the primary function of the joints within the
skull is to provide stability between articulating bones. Other joints, such as the glenohumeral joint of the shoulder,
allow remarkable mobility between the adjacent bones and exhibit much less stability. Most movable joints must
demonstrate a combination of stability and mobility, producing stable motion capable of supporting functional use of
the body part in which the joint is located. Thus, the structure of joints must be able to support a wide range of func¬
tions from extreme stability, permitting almost no movement at all, to maximum mobility.

The body exhibits a wide variety of joint designs and structures. Despite the variations in structure from joint to joint,
the connective tissues discussed in the preceding chapters—bone, cartilage, and dense, fibrous connective tissues—are
used in each joint design. Thus, the reader is encouraged to review the biomechanical properties of each connective
tissue type to understand how each joint component contributes to the overall function of the joint.

The specific goals of this chapter are to

■ Describe the design and general structure of human joints
■ Discuss the factors that influence the stability and mobility of the joint
■ Classify joints anatomically and biomechanically
■ Define the terminology used to describe joint motion biomechanically
■ Discuss the production and control of joint motion

103

104

Part I I BIOMECHANICAL PRINCIPLES

CLASSIFICATION AND STRUCTURE
OF HUMAN JOINTS

The broadest level of classification divides joints into two
groups on the basis of the amount of motion available at the
joint. Diarthroses permit free bone movement, while
synarthroses permit very limited or no movement at all.
Synarthroses are subclassified as synostoses, synchondroses,
and syndesmoses.

In a synostosis, bone is connected to bone by bone. No
movement takes place. In elderly persons, the sutures of the
skull are synostoses. In synchondroses and syndesmoses, bone
is connected to bone by cartilage or fibrous connective tissue,
respectively. The connective tissue connection between adja¬
cent bones is solid, allowing only a slight-to-moderate amount
of motion. A synchondrosis contains either hyaline cartilage
or fibrocartilage. The attachments of the ribs to the sternum
are synchondroses formed by hyaline cartilage. They furnish a
great deal of stability, which is necessary for the rib cage to
protect the vital organs within the chest. However, they also
permit the mobility needed to allow the chest wall to expand
and relax during ventilation.

The symphysis pubis is a synchondrosis composed of fibro¬
cartilage. This joint transmits forces between the weight¬
bearing lower extremities and the pelvis. It must withstand
high loads and thus must be very stable, permitting little
movement. However, during pregnancy, hormones soften the
fibrocartilage in the symphysis to permit movement necessary
for the baby to pass through the birth canal. Typically, carti¬
laginous joints allow more motion than fibrous joints.

In a syndesmosis, or fibrous joint, adjacent bones are
connected by a fibrous connective tissue membrane that
allows some movement but is primarily designed for stability.
The distal tibiofibular articulation is a syndesmosis in which
the shafts of the tibia and fibula are connected and stabilized
by the syndesmotic membrane. Additionally, early in life the
sutures of the skull are connected in synarthroses by fibrous
membranes. Although inherently stable, these fibrous con¬
nections allow some movement to occur. This is necessary to
allow molding of the infant s head during passage through the
birth canal and to allow for growth.

Diarthroses

Diarthroses, or synovial joints, are joints that are free to
move because there is a space between the ends of the bones
that meet in the joint. The ends of long bones are usually
united in this type of articulation. The ends of the adjacent
long bones are connected by a fibrous joint capsule that
encloses a sealed cavity called the articular cavity (see
Fig. 5.1). A synovial membrane lines the capsule and pro¬
duces synovial fluid that is contained within the cavity. The
ends of the bones within the articular cavity are covered with
a smooth layer of articular cartilage, which is usually hya¬
line cartilage. Articular cartilage has no perichondrium. The
smooth articular surfaces, coupled with the lubricating

properties of the synovial fluid, facilitate low-friction move¬
ment within these joints.

In addition to the common components of diarthroses or
synovial joints, some may contain additional structures that
protect the joint and guide movement. For example, synovial
joints may contain fibrocartilaginous menisci, or discs, or fat
pads to increase the protection of the bony surfaces from com¬
pressive loads. Fibrocartilage labra and menisci deepen con¬
cave joint surfaces, thus improving joint congruency and stabil¬
ity. Joints are protected from tensile loads, or forces that tend
to pull the surfaces apart, by both ligaments and tendons as well
as by the contraction of muscles whose tendons cross the joint.

JOINT CAPSULE AND SYNOVIAL MEMBRANE

Joint capsules have an external fibrous layer and an inner
synovial layer. The fibrous layer, composed of dense fibrous
connective tissue, attaches to the periosteum, which in turn
attaches to the subjacent bone via Sharpeys fibers, fibers
which originate in the periosteum and perforate into the
underlying bone [4,23].

Although the fibrous layer of the joint capsule has a meager
blood supply, it is richly innervated. Capsules typically receive
innervation from articular nerves that are branches of adjacent
peripheral nerves and from branches of nerves that supply
muscles controlling the joint. Several nerves usually supply
joint capsules, and their distributions tend to overlap. Joint
receptors, located in capsules, tendons, and ligaments, trans¬
mit information about the status of the joint to the central
nervous system. This afferent information is used by the cen¬
tral nervous system to coordinate muscle activity around the
joint to maintain appropriate balance between joint mobility
and stability [12]. Joint capsules contain various types of joint
receptors, which, along with receptors located in skin, other
connective tissues, and muscle, contribute to static joint posi¬
tion sense, sense of movement, direction of movement,
change in movement, and regulation of muscle tone [25,29].

Clinical Relevance

JOINT SPRAIN: When joint capsules and ligaments are
injuredas in a sprain, clinicians have observed continued
reduced functional performanceeven after all impairments
such as swelling, pain, restricted range of motion, and
decreased strength have been corrected. Further examina¬
tion often reveals reduced proprioception (position sense) at
the injured joint even after healing of the original injury
appears to be complete. Patients may continue to complain
that the joint "feels unstable," even when results of tests for
structural stability are normal. These deficits may be due to
reduced or abnormal sensory input from the joint receptors
that may also be injured during a sprain. As a result clini¬
cians may need to include balance and coordination activi¬
ties on unstable surfaces, such as a rocker board, to
improve the patient's functional abilities [6,7].

Chapter 7 I BIOMECHANICS OF JOINTS

105

Additional ligaments in areas that are subjected to great strain
may also reinforce the fibrous outer layer of the joint capsule.
Some of these reinforcing ligaments may be separate or dis¬
crete from the capsule itself. Others may be thickenings of
the capsule substance and cannot be separated from the cap¬
sule. For example, at the knee, the lateral collateral ligament
is a discrete structure separate from, and outside of, the joint
capsule, which resists varus forces, or forces that adduct the
tibia on the femur. The medial collateral ligament, however,
is a thickening of the medial aspect of the knee joint capsule
and cannot be separated from the capsule. It restrains valgus
forces, or forces that abduct the tibia on the femur.

The tendons of muscles that cross a joint may insert into
bone outside the joint capsule, such as the semitendinosus
tendon on the medial aspect of the knee. In this case the ten¬
don helps to reinforce the capsule and protect it from tensile
stresses. Tendons, however, may also attach to bone within
the joint capsule. In this case, the tendon must pierce the
joint capsule, such as the attachment of the long head of the
biceps brachii at the shoulder.

The inner layer of the joint capsule, lining the joint cavity,
is the synovial layer. This layer is more cellular than the
fibrous layer. Cells on its surface synthesize hyaluronic acid
and proteins that are secreted into the synovial fluid. These
components of synovial fluid are essential for reducing fric¬
tion and providing joint lubrication. The structure of the syn¬
ovial membrane is variable. In some joints, it simply lines the
fibrous capsule. Other joints have folds that project quite far
into the joint cavity. Folds of the synovial membrane may be
transient formations, depending on the position of the joint,
or they may be permanent villi that extend into the joint cav¬
ity. Loose or dense connective tissue and adipose tissue lie
below the surface cells of the synovium. Although the synovium
is poorly innervated compared with the fibrous capsule, it
contains numerous blood and lymphatic vessels.

Clinical Relevance

RHEUMATOID ARTHRITIS: Rheumatoid arthritis (RA) is a
disease characterized by chronic inflammatory changes in
the synovial membranes of joints. As a result of chronic
inflammation, the synovium becomes congested and edema¬
tous, infiltrated by leukocytes and inflammatory cells; and
further thickened by the proliferation of synovial cells and
hypertrophy of the synovial villi. Chronic synovitis resulting
in synovial hypertrophy can contribute to stretching of the
fibrous capsule of a joint, resulting in joint instability and,
ultimately, the joint deformities characteristic of RA.

Synovial fluid, produced by the synovial membrane, is con¬
tained within the joint capsule. Normal synovial fluid is a
clear, pale yellow, viscous fluid. Its composition is similar to
that of blood plasma, with the addition of hyaluronate and
other proteins that aid joint lubrication. Synovial fluid also

plays an important role in supplying nutrition to, and remov¬
ing metabolic wastes from articular cartilage and intraarticu-
lar fibrocartilage. This occurs by diffusion and imbibition
between the synovial fluid and the cartilage. Intermittent
compression and distraction of the joint surfaces that occur
during weight bearing and active movement of the joints
facilitate diffusion of nutrients and are required to maintain
healthy joint function [17,18].

Clinical Relevance

EARLY REMOBILIZATION FOLLOWING JOINT
INJURY: An understanding of the mechanics and pathome-
chanics of joint structure and tissues supports the current
practice of early mobilization following fractures and
sprains. Ankle sprains are frequently treated with splints that
limit motions that stress the injured ligaments, but allow
other ankle motions in order to promote normal joint lubri¬
cation during the healing period. Early mobilization mini¬
mizes the deleterious effects of immobilization and facilitates
return to normal function more quickly [24]. Safe early joint
motion during the healing process requires clinicians to
apply their knowledge of tissue and joint mechanics.

JOINT MOTION

Classification of Motion

As described in Chapter 1, the two basic types of motion are
rotation and translation. Rotation is motion about an axis,
causing points on the rotating body to travel different dis¬
tances depending upon their distance from the point of rota¬
tion (see Fig. 1.11). Translation produces a linear movement
in which all points in the body travel the same distance
regardless of their location in the body. Most cartilaginous
and fibrous joints allow translation, or linear movement.
Synovial joints, on the other hand, allow both rotation and
translation.

PLANES AND AXES OF MOTION

Rotation about an axis produces motion in a plane that is
perpendicular to that axis. Thus flexion of most joints occurs
about a medial-lateral axis and takes place in the sagittal plane
(Fig. 7.1). Similarly, abduction of most joints occurs in the
frontal plane about an anterior-posterior axis. Rotation
of most joints occurs in the transverse plane about a
longitudinal axis.

DEGREES OF FREEDOM

Another way of describing the kind of motion available at a
joint is to describe its degrees of freedom (DOF). A move¬
ment can be described completely by describing it with

106

Part I I BIOMECHANICAL PRINCIPLES

Figure 7.1: Axes of motion. Axes of motion are
perpendicular to the plane in which the seg¬
ment rotates. The flexion extension axis (X) is
medial-lateral and perpendicular to the sagittal
plane, the plane of flexion and extension. The
abduction-adduction axis (Z) is anterior-posterior
and perpendicular to the frontal plane, the
plane of abduction and adduction. The rotation
axis (Y) is usually a long axis through the length
of the bone and perpendicular to the transverse
plane, the plane of medial and lateral rotation.

respect to a coordinate system. Movement in a two-
dimensional space can be described as some combination of
translation along the x and y axes and a rotation about the 2
axis (Fig. 7.2). Thus movement in two-dimensional space is
said to have three DOF. In contrast, a body in three-dimen¬
sional space can translate along all three axes, x, y, and 2 and
also can rotate about the three axes. Thus a body moving in
three-dimensional space can have up to six DOF [5].

COMBINING TRANSLATION AND
ROTATION IN A SYNOVIAL JOINT

Although most of the movements that occur at synovial
joints are rotations, they also allow translation. This transla¬
tion is often subtle but essential to the normal motion of the
joint. To understand the combinations of motions that occur
in most synovial joints, it is necessary to understand how
an object that typically rotates may undergo simultaneous

translation. An object that demonstrates pure rotation with
no translation has a fixed axis, and the resulting movement
is described as spin (Fig. 7.3). An automobile tire that lacks
traction on ice spins with its axle fixed in space. As the tire
spins, all points on the tire eventually come in contact with
a single point on the pavement. In contrast, normal pro¬
gression of a car on the road results when the tires roll on
the surface of the road. Rolling motion causes each point
on the tire surface to contact a unique location on the road.
Finally, pure translation of the articulating surfaces is often
referred to as glide, analogous to a car in a skid, when a
single point on the tire glides over several points on the
road [19].

Many synovial joints exhibit a combination of rotation
and translation during normal motion. When knee flexion is
viewed from the tibia, the femur rolls posteriorly (Fig. 7.4).
During knee extension, the femur rolls anteriorly Early two-
dimensional research suggested that during knee flexion, the

Chapter 7 I BIOMECHANICS OF JOINTS

107

y y

A B

Figure 7.2: Degrees of freedom (DOF). A. An object moving in two-dimensional space has up to three DOF, translation along the x and
y axes and rotation in the plane of the two axes about the z axis. B. An object moving in three-dimensional space has up to six DOF,
translation about the x, y f and z axes, and rotation about the three axes.

femur also translated anteriorly, giving rise to the so-called
concave-convex rule, which suggested that the direction
of the intraarticular glide accompanying rotation could be
predicted by the shape of the moving articular surface. This
rule was used by clinicians to help determine the particular
joint glide that was needed to restore a specific limited joint
movement. More recent three-dimensional studies, however,
show that during knee flexion slight femoral translation may
occur in both anterior and posterior directions within the
same flexion movement (Fig 7.4). Other biomechanical
analyses demonstrate that many other synovial joints with
convex and concave articular surfaces also do not behave
according to the concave-convex rule. For example, studies
of the glenohumeral joint demonstrate that there is slight
superior glide of the humeral head as it rolls superiorly dur¬
ing shoulder flexion and abduction [10,26]. The humeral
head also glides posteriorly during lateral rotation and ante¬
riorly during medial rotation [10,20,26]. These studies
show that the convex humeral head rolls and glides in the
same direction during shoulder motion, contradicting the
concave-convex rule.

The metacarpal joints, also composed of concave and con¬
vex surfaces, have fixed axes of motion during flexion and
extension, with no appreciable glide [9,30]. Motion at these
joints is essentially pure rotation. Thus, it appears that the
concave-convex rule is neither correct nor clinically useful.
However, current understanding of joint motion continues to
indicate a need to restore joint glides as well as rotations when
attempting to improve joint mobility.

Synovial joints move by combining rotation and transla¬
tion or by pure rotation. The rotational movement of one

bone on another is described as the osteokinematics of
the joint. The gliding motions of the joint surfaces that
may accompany the joint rotations are described as the
arthrokinematics of the joint [19,28]. These joint glides
are usually much smaller, more-subtle movements than
the accompanying rotations and are known as component
or accessory motions of a joint. Although small, compo¬
nent motions are essential to the normal mechanics of
the joint. Inadequate glide may inhibit the restoration of
normal motion, while excessive glide may contribute to dam¬
age of the soft tissues surrounding a joint.

Clinical Relevance

JOINT MOBILIZATION: Joint mobilization is a manual
therapy technique used to restore the joint glides necessary
for normal joint range of motion (ROM). The specific mobi¬
lizations used at a joint usually are based on the normal
arthrokinematics of that joint. For exampleto increase knee
flexion and extension ROM, a clinician may work to restore
normal anterior and posterior glides of the tibia on the
femur while also stretching the surrounding soft tissues into
flexion and extension.

INSTANT CENTER OF ROTATION

Joints that exhibit pure rotation move about a fixed axis.
Joints that rotate while simultaneously gliding, are rotating

108

Part I I BIOMECHANICAL PRINCIPLES

Figure 7.3: Spin, roll, and glide. A. Spin of an object is rotation
about a fixed axis so that all of the points on the spinning object
contact a single point on the stationary surface. B. Roll of an
object is rotation about a moving axis so that all of the points on
the spinning object contact unique points on the stationary sur¬
face. C. Glide of an object is translation without any rotation, a
single point on the gliding object contacting several points on
the stationary surface.

Figure 7.4: Roll and glide of the knee. In flexion (A) the femur
rolls posteriorly. In extension (B) the femur rolls anteriorly. In
both, the femur may glide both anteriorly and posteriorly.

about an axis that moves in space, just as the car that skids is
gliding on the road surface even as the tire continues to
rotate about its axle. The two-dimensional motion of a joint
that undergoes simultaneous rotation and glide is often
described by the joints instant center of rotation (ICR)
[21,27] (Fig. 7.5). The ICR is the theoretical axis of rotation
for the joint for a given joint position. A joint that has a fixed
axis exhibits a constant ICR, but a joint that exhibits rotation
and joint glide, such as the knee, possesses multiple ICRs. A
common method for determining the ICR is pictured in
Figure 7.6. A helical axis of rotation describes the move¬
ment of a joint s axis of rotation in three-dimensional space.
A detailed discussion of helical axes is beyond the scope of
this textbook.

Figure 7.5: Instant center of rotation (ICR). The ICR is the theoret¬
ical axis of rotation at a specific joint position. It is constant in
pure rotation (A) but is variable in motions that combine
rotation and glide (B).

Chapter 7 I BIOMECHANICS OF JOINTS

109

Figure 7.6: Method to determine the instant center of rotation.
A common method to determine the ICR is to locate two points
on the moving segment and draw a line connecting two loca¬
tions of each point. The intersection of lines drawn perpendicu¬
lar to the first lines identifies the ICR. By repeating this proce¬
dure at several consecutive joint positions, the ICR is identified
through the range of motion.

Clinical Relevance

GONIOMETRY: Assessment of ROM is a common clinical
approach to identify joint impairment. A single-axis goniome¬
ter allows determination of the angular position of one limb
segment with respect to another when the device is aligned
appropriately at the joint (Fig. 7.7). An understanding of a
joint's motion helps the clinician align the goniometer cor¬
rectly to obtain an accurate measure of the joint's ROM.

Figure 7.7: A single-axis goniometer to measure joint motion.
The single-axis goniometer measures the angular position of a
joint.

Classification of Synovial Joints

Most joints in the body are freely moving diarthrodial, or syn¬
ovial joints. While sharing several structural characteristics,
they also demonstrate considerable variation in structure that
produces a broad spectrum of functional capabilities. Synovial
joints are classified anatomically by their surface shapes and
kinds of motion or biomechanically by the number of axes of
motion they possess [23,28] (Table 7.1). For example, a hinge
joint is also called a uniaxial joint because motion occurs
about a single axis. The superior radioulnar joint and the dis¬
tal interphalangeal joints of the fingers are classified as uniax¬
ial joints, although they can also be classified separately as
pivot and hinge joints, respectively. From a rehabilitation
standpoint, the mechanical classification based on axes of
motion is helpful because it identifies the kind of motion that
is normally available at a given joint.

Classification of synovial joints by the number of axes of
rotation implies that these joints allow only rotational move¬
ment. However, many synovial joints demonstrate both trans¬
lation and rotation during normal movement [28]. A uniaxial
joint that allows pure rotation has a single DOF. Most synovial
joints possess at least three DOF Joints such as the knee

TABLE 7.1: Classification of Synovial Joints

Anatomical Classification

Mechanical Classification

Example

Hinge (ginglymus)

Uniaxial

Distal interphalangeal joint

Pivot (trochoid)

Uniaxial

Superior radioulnar joint

Condyloid

Biaxial

Metacarpophalangeal joint of the fingers

Ellipsoid

Biaxial

Wrist (radiocarpal) joint

Saddle

Triaxial

Carpometacarpal joint of the thumb

Ball-and-socket

Triaxial

Glenohumeral joint

Gliding (plane or sliding)

No rotation is available

Midtarsal joint of the foot

110

Part I I BIOMECHANICAL PRINCIPLES

exhibit six DOF, demonstrating translation along, and rota¬
tion about, all three axes. For example, the following move¬
ments can occur at the knee: (1) anterior and posterior glide,
(2) medial and lateral glide, (3) superior and inferior glide, (4)
flexion and extension rotation about a medial-lateral (ML)
axis, (5) abduction and adduction rotation about an AP axis,
and (6) internal and external rotation about a vertical (longi¬
tudinal) axis.

FACTORS INFLUENCING MOTION
AT A JOINT

Joint structure and the external forces applied to the joint
together determine the type and quantity of motion that
occurs at a joint. These factors also influence the nature and
amount of internal forces required of the joints muscles and
ligaments to control the joint effectively (Fig. 7 . 8 ). The inter¬
action between a joint and its adjacent joints and the external
environment also affects the joints motion.

The Effect of Joint Structure
on Joint Motion

The structure of a joint is described by its articular surfaces
and ligamentous supporting structures. Each has a significant
effect on the motion available at a joint.

JOINT SURFACES

Both the amount and the kind of motion available at a joint
are dictated to a large extent by the shapes of its articular
surfaces. The shapes of the ends of bones that meet at a joint
are quite variable. For example, in some joints, the shapes of
the adjacent surfaces fit together congruently like adjacent
puzzle pieces, while in other joints, the surfaces that meet are
quite dissimilar, or incongruent (Fig. 7 . 9 ). Joints with more-
congruent articulations tend to restrain motion and are more
stable, while those having less-congruent surfaces typically
allow more mobility.

The amount of curvature of the surfaces of the articulating
surfaces also affects a joints mobility and stability. The radius

Figure 7.8: Factors that influence joint function. Joint function is influenced by the structure of the joint, the externally applied forces
such as limb weight and external loads, and the internal forces applied by the muscles and ligaments of the joint.

Chapter 7 I BIOMECHANICS OF JOINTS

111

Figure 7.9: Congruent and incongruent joint surfaces. A. Some
joint surfaces consist of similarly shaped surfaces such as the hip
joint and are described as congruent. B. Others consist of dissimi¬
lar surfaces (incongruent) such as the knee.

of curvature describes the curvature of an articular surface.
The radius of curvature of an articular surface equals the
radius of a circle that possesses the same curved surface as
the articular surface (Fig. 7.10). The more curved the surface,
the smaller the radius of curvature. Articulating surfaces that
possess similar radii of curvatures are congruent. The
amount of curvature of the articulating surfaces and their
congruence affect the combination of translation and rotation
that occurs at the joint.

Clinical Relevance

THE KNEE JOINT: The knee includes four articulating sur¬
faces between the femur and the tibia , each with a different
radius of curvature. These differences help produce the com¬
bination of rotation and translation that accompanies knee
flexion and extension. Clinicians need a clear understanding
of these complex motions to restore normal joint motion.

Metacarpal bone

Figure 7.10: Radius of curvature. The radius of curvature
describes the amount of curvature of a joint surface. It is the
length of the radius of a circle of the same curvature.

Joints with relatively flat surfaces allow translation, while the
more-curved surfaces allow rotation. Curved surfaces vary in
their shapes, defining still further the kinds of motions that
occur at the joint. For example, the pulley-shaped, or
trochlear, surfaces found at the articulation between the ulna
and the humerus or between the middle and distal phalanges
of the fingers constrain the available motion to rotation about
a single axis, much like a train rolling along a track. Other
articular surfaces exhibit shapes that appear to be parts of
spheres, with surfaces that are either concave in two direc¬
tions (typically anterior-posterior and medial-lateral) or con¬
vex in two directions. These surfaces are called biconcave
and biconvex, respectively. Articulations between biconvex
and biconcave surfaces allow freer motion, with rotations
about two or three axes, compared with the trochlear sur¬
faces. An example of a joint with these types of articular sur¬
faces is the radiocarpal joint. In this articulation, the carpus is
the biconvex surface, which articulates with the biconcave
distal end of the radius.

Even joints composed of convex on concave surfaces with
similar curvatures exhibit a wide spectrum of motions. Both
the glenohumeral and hip joints consist of convex bony sur¬
faces fitting onto concave surfaces with articular surfaces that
are relatively congruent. Yet these two joints allow very dif¬
ferent amounts of mobility. The glenoid fossa covers less than
half of the articular surface of the humeral head [13,15], but
approximately three quarters of the femoral head is covered
by the acetabulum at the hip [11,14]. As a result the gleno¬
humeral joint is more mobile than the hip, and the hip is
more stable than the glenohumeral joint. These two joints
demonstrate how the shapes of the articular surfaces influ¬
ence both the mobility and stability of a joint.

LIGAMENTOUS SUPPORT

The ligamentous support of a joint also influences its mobility
and stability. Ligaments exhibit unique designs to provide sta¬
bilization without limiting too much motion. Many synovial
joint capsules have folds that unfold as the capsule is
stretched to allow more joint movement. For example, the
inferior portion of the glenohumeral joint capsule lies in folds
when the shoulder is in the neutral position (next to the

112

Part I I BIOMECHANICAL PRINCIPLES

body), but unfolds during shoulder flexion and abduction.
This allows considerable mobility, although the consequence
is that the inferior joint capsule adds little to the stability of
the glenohumeral joint.

Clinical Relevance

GLENOHUMERAL JOINT STABILITY: Inferior subluxa¬
tions of the glenohumeral joint appear most frequently in
individuals with severe muscle weakness and are observed
with the shoulder in neutral. Muscles contribute important
stabilizing forces because the folded inferior glenohumeral
joint capsule is unable to stabilize the joint

Another common design characteristic in ligaments is seen in
collateral ligaments that are found on the medial and lateral
sides of most hinge and biaxial joints. Collateral ligaments
provide stability, preventing or limiting side-to-side move¬
ment. Many collateral ligaments radiate from a small, rather
localized proximal attachment to a broader, more extensive
distal attachment (Fig. 7.11). This arrangement allows some
part of the ligament to remain taut throughout the ROM of
the joint, providing a side-to-side stabilizing force in any joint
position. The medial collateral ligaments of the elbow and
knee are large triangular ligaments that maintain some stabi¬
lizing ability regardless of where in flexion or extension the
joints lie [3,8,16,22].

External Forces on a Joint

Chapter 1 describes the interaction between forces applied to
a joint from the environment and the internal forces pro¬
duced by the muscles and ligaments of a joint. The weight of
the limb and the forces of additional loads, such as the man¬
ual resistance applied by a therapist or the resistance from a
weight-training machine, all apply moments or torques to the
joint, producing rotation at the joint. These are counteracted
by the moments, or torques produced by the muscles and lig¬
aments (Fig. 7.12). If the external forces and moments bal¬
ance the internal forces and moments, static equilibrium
exists, and the joint remains at rest or in uniform motion.
Identification and characterization of the external forces
applied to a joint during activity helps clinicians deter¬
mine which muscles and ligaments are needed to
move or stabilize the joint.

Distal Middle Proximal Metacarpal

pharynx pharynx pharynx

Figure 7.11: Typical collateral ligaments. Typical collateral liga¬
ments attach to a small location proximal to the joint and radi¬
ate distally to a broader attachment across the joint.

Figure 7.12: The external and internal moments on a joint. A
joint is controlled by the externally applied loads including the
weight (1/1/) of the limb and any additional force such as the
manual resistance ( T) from a therapist and by the internally
applied loads (H) of the muscles and ligaments.

Clinical Relevance

MUSCLES USED TO DESCEND AND ASCEND STAIRS:

An individual who is descending a step gradually moves
from hip and knee extension to hip and knee flexion as the
opposite foot is lowered onto the step below (Fig. 7.13). Does
the individual need hip and knee flexor or extensor muscles
to lower the body onto the step? To answer this question ,
the clinician must recognize that the weight of the head '
arms , trunk , and opposite lower extremity produces a
flexion moment on the weight-bearing hip and knee joints
(Fig. 7.14). Consequently , the individual must use hip and
knee extensor muscles to lower the body and control the
hip and knee flexion produced by body weight. When climb¬
ing the stair ; the individual also uses the hip and knee
extensor muscles to lift the body onto the next step. In this
case, the hip and knee joints are extending , but the body
weight continues to produce a flexion moment at each joint.
The hip and knee extensor muscles must produce torques
that exceed the flexion torque produced by body weight. In
both ascending and descending a step , the extensor mus¬
cles of the weight-bearing leg are used. The difference is in
the type of contraction produced by the extensor muscles in
each case. When descending the step , the extensor muscles
contract eccentrically. While ascending stairs , the extensor
muscles contract concentrically. In both cases, identification
of the external loads and their effect on joint movement
allows the practitioner to determine what muscles must con¬
tract and the kind of contraction needed.

Many joints in the body sustain very large joint reaction
forces during normal daily activities. The surface area over

Chapter 7 I BIOMECHANICS OF JOINTS

113

Figure 7.13: Stair descent. As an individual descends a step, the
weight-bearing hip and knee move from a position of extension
to a position of flexion.

which the joint reaction force is applied determines the
stress (force/area) that a joint sustains during the activity.
Changes in joint surface or alignment can alter the stresses
applied to a joint. Joint reaction forces and the resulting
stresses appear to play a role in the production of joint pain
and degeneration [1,2]. The joint reaction forces that occur
during exercise and functional activities are influenced by
various factors, including joint position and the method of
application of external loads. Clinicians must design exercises
and functional activities that minimize joint reaction forces,
to protect the joint surfaces.

Clinical Relevance

JOINT PROTECTION TECHNIQUES: Joint protection
techniques are methods of performing common daily activi¬
ties in a way that minimizes joint reaction forces. These
techniques are important for patients with arthritis or other
conditions in which joints are at risk for degeneration. For
example ’ joint reaction forces in the carpometacarpal (basal)
joint of the thumb are extremely high during lateral pinch ,
commonly used to turn a key in the ignition of a car or to
open a jar lid. To reduce the joint reaction forces during
these activities, devices can be used so that two hands can
be substituted for the lateral pinch of one hand.

Figure 7.14: Joint moments on the knee during stair descent. In
stair descent the weight (1/1/) of the head, arms, trunk, and oppo¬
site lower extremity produce a flexion moment on the weight¬
bearing knee. The quadriceps muscle must apply an extension
moment to control the descent onto the lower step.

Interactions between Joints
and the External Environment

The human body consists of well over 150 joints. Movement
of one joint may affect several nearby joints. This is particu¬
larly true when the moving limb is fixed to a relatively immov¬
able object. For example, when one is standing upright with
the arm at the side of the body and the hand free, elbow flex¬
ion can occur without movement in adjacent joints. In con¬
trast, when an individual performs a push-up, elbow flexion
requires simultaneous wrist and shoulder movements. In the
first case, when the elbow moves in isolation, it is functioning
as part of an open chain. Other terms used to describe the
same condition include open kinetic chain and open kinematic
chain. When the elbow flexes during performance of a push¬
up, it is functioning as part of a closed chain ( closed kinetic
chain, or closed kinematic chain). This is described as a closed

114

Part I I BIOMECHANICAL PRINCIPLES

chain condition since the moving joints of the upper extrem¬
ity lie between the relatively immovable loads of the body and
the floor. Movement at any of the joints in the closed chain
will influence movement at adjacent joints. The lower
extremity also functions under both open- and closed-chain
conditions. For example, when kicking a ball, the end of the
kicking leg is not fixed, and joint movement in the hip, knee,
and ankle are independent of one another. However, in
the stance leg with the foot fixed on the unmoving floor,
movement at one joint influences movement at adjacent
joints. For example, flexion of the stance knee requires ankle
dorsiflexion and hip flexion. Joints of the upper and lower
extremities can function under both open- and
closed-chain conditions, and both are required during
functional activities.

Clinical Relevance

LOCOMOTION: Normal human locomotion is divided into
a weight-bearing and a non-weight-bearing phase for each
lower extremity. During the non-weight-bearing (swing)
phase of gait the lower extremity functions in an open
chain, and the motions of the hip, knee, and foot are
mechanically independent of one another. However, during
the weight-bearing (stance) phase of gait, the limb functions
in a closed chain. Abnormal movements at a joint, perhaps
due to pain, may cause abnormal compensatory move¬
ments in adjacent joints. These abnormal compensatory
movements may place excessive stress on the soft tissue lim¬
iters of the compensating joints. As a result, abnormal
movements or positions at one joint in the lower extremity
may cause pain or other symptoms in an adjacent but com¬
pensating joint. Accurate diagnosis of painful conditions at
a weight-bearing joint requires a thorough assessment of
the function of all the surrounding joints, and successful
intervention at one joint often includes interventions at adja¬
cent joints. An example is an individual with excessive
pronation at the subtalar joint of the foot during running.
This individual may develop knee pain due to compensa¬
tory movements at the knee. Treatment directed only at the
knee will not be effective until the abnormal subtalar joint
movement is addressed.

SUMMARY

Articulating bones are attached to one another at joints. Joints
are composed of a variety of connective tissue structures
arranged to provide a combination of mobility and stability. The
structure of some joints favors stability, while other joints favor
mobility. Synarthroses permit very little movement and are
designed for extreme stability. Types of synarthroses include
synostoses, synchondroses, and syndesmoses. Diarthroses,
or synovial joints, are moveable joints. Diarthroses are

subclassified either anatomically according to the shapes of the
articular surfaces or biomechanically according to the number
of axes about which movement occurs. Movement within syn¬
ovial joints includes rotation and gliding or translation.
Although these movements may occur in isolation, more typi¬
cally they occur together during joint movement. Rotations
describe the osteokinematics of joint movement, while glides,
or accessory movements, describe the arthrokinematics of joint
movement. The amount and nature of the movement that
occurs at a synovial joint is influenced by the shapes of the artic¬
ular surfaces, the connective tissue structures present at the
joint (e.g., ligaments and menisci), and the external forces
applied to the joint. Forces that are applied to joints and pro¬
duce movement include forces from contraction of muscles
that cross the joint as well as forces from body weight or the
weight of other limb segments. Joint movement can occur in
open- or closed-chain conditions. In open-chain conditions, the
ends of the extremity (arm or leg) are free; thus the joints with¬
in the extremity can move independently of one another. In
closed-chain conditions, the ends of the extremity are fixed or
relatively immovable, thus, movement at one joint within the
extremity will affect movement at the other joints in the
extremity. A thorough understanding of joint structure and
function is essential to appreciate the mechanics of normal
joints and the pathomechanics of abnormal joint movements
associated with painful conditions.

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PART

Kinesiology of the Upper
Extremity

Latissimus dorsi

UNIT 1: SHOULDER UNIT: THE SHOULDER COMPLEX

Chapter 8: Structure and Function of the Bones and Joints of the Shoulder Complex
Chapter 9: Mechanics and Pathomechanics of Muscle Activity at the Shoulder Complex
Chapter 10: Analysis of the Forces on the Shoulder Complex during Activity

UNIT 2: ELBOW UNIT

Chapter 11: Structure and Function of the Bones and Noncontractile Elements of the Elbow
Chapter 12: Mechanics and Pathomechanics of Muscle Activity at the Elbow
Chapter 13: Analysis of the Forces at the Elbow during Activity

UNIT 3: WRIST AND HAND UNIT

Chapter 14: Structure and Function of the Bones and Joints of the Wrist and Hand
Chapter 15: Mechanics and Pathomechanics of the Muscles of the Forearm
Chapter 16: Analysis of the Forces at the Wrist during Activity

Chapter 17: Mechanics and Pathomechanics of the Special Connective Tissues in the Hand
Chapter 18: Mechanics and Pathomechanics of the Intrinsic Muscles of the Hand
Chapter 19: Mechanics and Pathomechanics of Pinch and Grasp

117

UNIT 1

SHOULDER UNIT: THE SHOULDER COMPLEX

T he shoulder complex is the functional unit that results in movement of the arm with respect to the trunk. This
unit consists of the clavicle, scapula, and humerus; the articulations linking them; and the muscles that move
them. These structures are so functionally interrelated to one another that studying their individual functions
is almost impossible. However, a careful study of the structures that compose the shoulder unit reveals
an elegantly simple system of bones, joints, and muscles that together allow the shoulder an almost infinite number of
movements (Figure). An important source of patients' complaints of pain and dysfunction at the shoulder complex is
an interruption of the normal coordination of these interdependent structures.

The primary function of the shoulder complex is to position the upper extremity in space to allow the hand to perform
its tasks. The wonder of the shoulder complex is the spectrum of positions that it can achieve; yet this very mobility is
the source of great risk to the shoulder complex as well. Joint instability is another important source of patients' com¬
plaints of shoulder dysfunction. Thus an understanding of the function and dysfunction of the shoulder complex
requires an understanding of the coordinated interplay among the individual components of the shoulder complex as
well as an appreciation of the structural compromises found in the shoulder that allow tremendous mobility yet pro¬
vide sufficient stability.

118

The shoulder complex. The shoulder complex consists of the humerus,
clavicle, and scapula and includes the sternoclavicular, acromioclavicular,
glenohumeral, and scapulothoracic joints.

UNIT 1

SHOULDER UNIT: THE SHOULDER COMPLEX

This three-chapter unit on the shoulder complex describes the structure of the shoulder complex and its implications
for function and dysfunction. The purposes of this unit are to

■ Provide the clinician with an understanding of the morphology of the individual components of the complex

■ Identify the functional relationships among the individual components

■ Discuss how the structures of the shoulder complex contribute to mobility and stability

■ Provide insight into the stresses that the shoulder complex sustains during daily activity

The unit is divided into three chapters. The first chapter presents the bony structures making up the shoulder complex
and the articulations that join them. The second chapter presents the muscles of the shoulders and their contributions
to function and dysfunction. The third chapter investigates the loads to which the shoulder complex and its individual
components are subjected during daily activity.

119

CHAPTER

Structure and Function
of the Bones and Joints
of the Shoulder Complex

CHAPTER CONTENTS

STRUCTURE OF THE BONES OF THE SHOULDER COMPLEX .121

Clavicle.121

Scapula.121

Proximal Humerus.125

Sternum and Thorax.126

STRUCTURE OF THE JOINTS AND SUPPORTING STRUCTURES OF THE SHOULDER COMPLEX .127

Sternoclavicular Joint .127

Acromioclavicular Joint.130

Scapulothoracic Joint .133

Glenohumeral Joint .135

TOTAL SHOULDER MOVEMENT .140

Movement of the Scapula and Humerus during Arm-Trunk Elevation .141

Sternoclavicular and Acromioclavicular Motion during Arm-Trunk Elevation.142

Impairments in Individual Joints and Their Effects on Shoulder Motion.143

SHOULDER RANGE OF MOTION .146

SUMMARY .146

T his chapter describes the structure of the bones and joints of the shoulder complex as it relates to the func¬
tion of the shoulder. The specific purposes of this chapter are to

■ Describe the structures of the individual bones that constitute the shoulder complex
■ Describe the articulations joining the bony elements
■ Discuss the factors contributing to stability and instability at each joint

■ Discuss the relative contributions of each articulation to the overall motion of the shoulder complex
■ Review the literature's description of normal range of motion (ROM) of the shoulder
■ Discuss the implications of abnormal motion at an individual articulation to the overall motion of the
shoulder complex

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Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

121

STRUCTURE OF THE BONES
OF THE SHOULDER COMPLEX

The shoulder complex consists of three individual bones: the
clavicle, the scapula, and the humerus. Each of these bones is
discussed in detail below. However, the complex itself is con¬
nected to the axioskeleton via the sternum and rests on the
thorax, whose shape exerts some influence on the function of
the entire complex. Therefore, a brief discussion of the ster¬
num and the shape of the thorax as it relates to the shoulder
complex is also presented.

Clavicle

The clavicle functions like a strut to hold the shoulder complex
and, indeed, the entire upper extremity suspended on the
axioskeleton [84]. Other functions attributed to the clavicle are
to provide a site for muscle attachment, to protect underlying
nerves and blood vessels, to contribute to increased ROM of
the shoulder, and to help transmit muscle force to the scapula
[52,69]. This section describes the details of the clavicle that
contribute to its ability to perform each of these functions.
How these characteristics contribute to the functions of the
clavicle and how they are implicated in injuries to the clavicle
are discussed in later sections of this chapter.

The clavicle lies with its long axis close to the transverse
plane. It is a crank-shaped bone when viewed from above,
with its medial two thirds convex anteriorly, approximately
conforming to the anterior thorax, and its lateral one third
convex posteriorly (Fig. 8.1). The functional significance of
this unusual shape becomes apparent in the discussion of
overall shoulder motion.

Figure 8.1: Clavicle. A. View of the superior surface. B. View of
the inferior surface.

The superior surface of the clavicle is smooth and readily
palpated under the skin. Anteriorly, the surface is roughened
by the attachments of the pectoralis major medially and the
deltoid laterally. The posterior surface is roughened on the
lateral one third by the attachment of the upper trapezius.
Inferiorly, the surface is roughened medially by attachments
of the costoclavicular ligament and the subclavius muscle and
laterally by the coracoclavicular ligament. The latter produces
two prominent markings on the inferior surface of the lateral
aspect of the clavicle, the conoid tubercle and, lateral to it, the
trapezoid line.

The medial and lateral ends of the clavicle provide articu¬
lar surfaces for the sternum and acromion, respectively. The
medial aspect of the clavicle expands to form the head of the
clavicle. The medial surface of this expansion articulates with
the sternum and intervening articular disc, or meniscus, as
well as with the first costal cartilage. The articular surface of
the clavicular head is concave in the anterior posterior direc¬
tion and slightly convex in the superior inferior direction

[93.101] . Unlike most synovial joints, the articular surface of
the mature clavicle is covered by thick fibrocartilage. The lat¬
eral one third of the clavicle is flattened with respect to the
other two thirds and ends in a broad flat expansion that artic¬
ulates with the acromion at the acromioclavicular joint. The
actual articular surface is a small facet typically facing inferi¬
orly and laterally. It too is covered by fibrocartilage rather
than hyaline cartilage. The medial and lateral aspects of the
clavicle are easily palpated.

Scapula

The scapula is a flat bone whose primary function is to pro¬
vide a site for muscle attachment for the shoulder. A total of
15 major muscles acting at the shoulder attach to the scapula

[58.101] . In quadrupedal animals, the scapula is long and thin
and rests on the lateral aspect of the thorax. In primates, there
is a gradual mediolateral expansion of the bone along with a
gradual migration from a position lateral on the thorax to a
more posterior location (Fig. 8.2). The mediolateral expan¬
sion is largely the result of an increased infraspinous fossa and
costal surface that provide attachment for three of the four
rotator cuff muscles as well as several other muscles of the
shoulder [40,83]. These changes in structure and location of
the scapula reflect the gradual change in the function of the
upper extremity from its weight-bearing function to one of
reaching and grasping. These alterations in function require a
change in the role of muscles that now must position and sup¬
port a scapula and glenohumeral joint that are no longer pri¬
marily weight bearing and instead are free to move through a
much larger excursion.

The scapula has two surfaces, its costal, or anterior, surface
and the dorsal, or posterior surface (Fig. 8.3). The costal sur¬
face is generally smooth and provides proximal attachment
for the subscapularis muscle. Along the medial border of the
anterior surface, a smooth narrow surface gives rise to the ser-
ratus anterior muscle.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 8.2: Location of the scapula. A. In humans the scapula is located more posteriorly. B. The scapula is located on the lateral aspect
of the thorax in quadrupedal animals.

Figure 8.3: Scapula. A. Anterior surface. B. Posterior surface.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

123

The dorsal surface of the scapula is divided into two
regions by the spine of the scapula, a small superior space
called the supraspinous fossa and a large inferior space
known as the infraspinous fossa. The spine is a large dorsally
protruding ridge of bone that runs from the medial border of
the scapula laterally and superiorly across the width of the
scapula. The spine ends in a large, flat surface that projects
laterally, anteriorly, and somewhat superiorly. This process is
known as the acromion process. The acromion provides a
roof over the head of the humerus. The acromion has an
articular facet for the clavicle on the anterior aspect of its
medial surface. Like the clavicular surface with which it
articulates, this articular surface is covered by fibrocartilage
rather than hyaline cartilage. This facet faces medially and
somewhat superiorly. The acromion is generally described as
flat. However Bigliani et al. describe various shapes of the
acromion including flat, rounded, and hooked processes [4].
These authors suggest that the hooked variety of acromion
process may contribute to shoulder impingement syn¬
dromes. Additional factors contributing to impingement syn¬
dromes are discussed throughout this chapter.

The scapula has three borders: the medial or vertebral
border, the lateral or axillary border, and the superior bor¬
der. The medial border is easily palpated along its length
from inferior to superior. The medial border bends anteri¬
orly from the root of the spine to the superior angle, thus
conforming to the contours of the underlying thorax. It joins
the superior border at the superior angle of the scapula that
can be palpated only in individuals with small, or atrophied,
muscles covering the superior angle, particularly the trapez¬
ius and levator scapulae.

Projecting from the anterior surface of the superior bor¬
der of the scapula is the coracoid process, a fingerlike pro¬
jection protruding superiorly then anteriorly and laterally
from the scapula. It is located approximately two thirds of the
width of the scapula from its medial border. The coracoid
process is readily palpated inferior to the lateral one third of
the clavicle on the anterior aspect of the trunk. Just medial to
the base of the coracoid process on the superior border is the
supraspinous notch through which travels the suprascapular
nerve.

The medial border of the scapula joins the lateral border
at the inferior angle, an important and easily identified land¬
mark. The lateral border of the scapula is palpable along its
inferior portion until it is covered by the teres major, teres
minor, and latissimus dorsi muscles. The lateral border
continues superiorly and joins the superior border at the ante¬
rior angle or head and neck of the scapula. The head gives rise
to the glenoid fossa that provides the scapulas articular sur¬
face for the glenohumeral joint. The fossa is somewhat nar¬
row superiorly and widens inferiorly resulting in a “pear-
shaped” appearance. The depth of the fossa is increased by
the surrounding fibrocartilaginous labrum. Superior and infe¬
rior to the fossa are the supraglenoid and infraglenoid tuber¬
cles, respectively

The orientation of the glenoid fossa itself is somewhat con¬
troversial. Its orientation is described as

• Lateral [2]

• Superior [2]

• Inferior [80]

• Anterior [2,84]

• Retroverted [85]

Only the lateral orientation of the glenoid fossa appears
uncontested. Although the differences in the literature
may reflect real differences in measurement or in the pop¬
ulations studied, at least some of the variation is due to dif¬
ferences in reference frames used by the various investiga¬
tors to describe the scapulas position. The reference
frames used include one imbedded in the scapula itself
and one imbedded in the whole body. The scapula-fixed
reference frame allows comparison of the position of one
bony landmark of the scapula to another landmark on the
scapula. The latter body-fixed reference frame allows com¬
parison of the position of a scapular landmark to other
regions of the body.

To understand the controversies regarding the orientation
of the glenoid fossa, it is useful to first consider the orienta¬
tion of the scapula as a whole. Using a body-fixed reference
frame, the normal resting position of the scapula can be
described in relationship to the sagittal, frontal, and transverse
planes. In a transverse plane view, the scapula is rotated
inwardly about a vertical axis. The plane of the scapula is
oriented approximately 30-45° from the frontal plane
(Fig. 8.4) [46,86]. This position directs the glenoid anteriorly
with respect to the body However, a scapula-fixed reference
frame reveals that the glenoid fossa is retroverted, or rotated
posteriorly, with respect to the neck of the scapula [14,85].

Figure 8.4: Plane of the scapula. A transverse view of the scapula
reveals that the plane of the scapula forms an angle of approxi¬
mately 40° with the frontal plane.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Thus the glenoid fossa is directed anteriorly (with respect to
the body) and at the same time is retroverted (with respect to
the scapula).

Rotation of the scapula in the frontal plane about a
body-fixed anterior-posterior (AP) axis is also described
(Fig. 8.5). This frontal plane rotation of the scapula is
described by either the upward or downward orientation
of the glenoid fossa or by the medial or lateral location of
the scapulas inferior angle [2,25,80]. A rotation about this
AP axis that tips the glenoid fossa inferiorly, moving the
inferior angle of the scapula medially (i.e., closer to the
vertebral column), is described as downward or medial
rotation of the scapula. A rotation that tilts the glenoid
fossa upward, moving the inferior angle laterally away from
the vertebral column, is upward or lateral rotation. Two
investigations report that the glenoid fossa is upwardly
inclined in quiet standing [2,61]. Two other studies report
a downward inclination of approximately 5° [25,80]. The
posture of the studies’ subjects may help to explain these
reported differences. Perhaps subjects who demonstrate
an upward inclination are instructed to pull their shoulders
back into an “erect” posture while those who have a down¬
ward inclination of the glenoid fossa have slightly drooping

Figure 8.5: Scapular rotation. Rotation of the scapula about an
anterior-posterior (AP) axis causes the glenoid fossa to face
upward (2) or downward (3).

shoulders (Fig. 8.6). A final determination of the normal
orientation of the scapulae in the frontal plane requires
an accepted definition of normal postural alignment of
the shoulder. That definition unfortunately is presently

Figure 8.6: Postural changes of the scapula. A. This individual is
standing with drooping, or rounded, shoulders, and the scapulae
are rotated so that the glenoid fossa tilts downward. B. This indi¬
vidual stands with the shoulders pulled back and the scapulae
tilted upward.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

125

Figure 8.7: Scapular rotation. Rotation of the scapula about a ML
axis tilts the scapula anteriorly and posteriorly.

lacking. Therefore, the controversy regarding the orienta¬
tion of the scapula and its glenoid fossa in the frontal plane
continues.

Viewed sagittally, the scapula tilts forward from the frontal
plane approximately 10° about a medial lateral axis (Fig. 8.7)
[17]. This forward tilting is partly the result of the scapulas
position on the superior thorax, which tapers toward its apex.
Additional forward tilt of the scapula causes the inferior angle
of the scapula to protrude from the thorax.

Clinical Relevance

SCAPULAR POSITION IN SHOULDER DYSFUNCTION:

Abnormal scapular positions have been implicated in several
forms of shoulder dysfunction. Abnormal orientation of the
glenoid fossa has been associated with instability of the
glenohumeral joint [2,85,9!]. In addition , excessive anterior
tilting is found in individuals with shoulder impingement
syndromes during active shoulder abduction [61]. Careful
evaluation of scapular position is an essential component
of a thorough examination of patients with shoulder
dysfunction.

Proximal Humerus

The humerus is a long bone composed of a head, neck, and
body, or shaft. The body ends distally in the capitulum and
trochlea. This chapter presents only those portions of the
humerus that are relevant to a discussion of the mechanics
and pathomechanics of the shoulder complex. The rest of the

humerus is discussed in Chapter 11 with the elbow. The artic¬
ular surface of the head of the humerus is most often
described as approximately half of an almost perfect sphere
(Fig. 8.8) [39,89,99,101]. The humeral head projects medially,
superiorly, and posteriorly with respect to the plane formed
by the medial and lateral condyles (Fig. 8.9) [40]. The humer¬
al head ends in the anatomical neck marking the end of the
articular surface.

On the lateral aspect of the proximal humerus is the
greater tubercle, a large bony prominence that is easily pal¬
pated on the lateral aspect of the shoulder complex. The
greater tubercle is marked by three distinct facets on its supe¬
rior and posterior surfaces. These facets give rise from supe¬
rior to posterior to the supraspinatus, infraspinatus, and teres
minor muscles, respectively On the anterior aspect of the
proximal humerus is a smaller but still prominent bony pro¬
jection, the lesser tubercle. It too has a facet that provides
attachment for the remaining rotator cuff muscle, the sub-
scapularis. Separating the tubercles is the intertubercular, or
bicipital, groove containing the tendon of the long head of the
biceps brachii. The greater and lesser tubercles continue onto
the body of the humerus as the medial and lateral lips of the
groove. The surgical neck is a slight narrowing of the shaft of
the humerus just distal to the tubercles.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Humeral head

Figure 8.9: Orientation of the head of the humerus. A. In the
transverse plane, the humeral head is rotated posteriorly with
respect to the condyles of the distal humerus. B. In the frontal
plane, the head of the humerus is angled medially and superiorly
with respect to the shaft of the humerus.

Clinical Relevance

THE DEPTH OF THE BICIPITAL GROOVE: The depth of
the bicipital groove varies. A shallow groove appears to be
a contributing factor in dislocations of the biceps tendon
[56,58].

Approximately midway distally on the body of the humerus is
the deltoid tuberosity on the anterolateral surface. It provides
the distal attachment for the deltoid muscle. The spiral
groove is another important landmark on the body of the
humerus. It is found on the proximal half of the humerus, spi¬
raling from proximal to distal and medial to lateral on the pos¬
terior surface. The radial nerve travels in the spiral groove
along with the profunda brachii vessels. The radial nerve is
particularly susceptible to injury as it lies in the spiral groove.

Sternum and Thorax

Although the sternum and thorax are not part of the shoulder
complex, both are intimately related to the shoulder; there¬
fore, a brief description of their structure as it relates to the
shoulder complex is required. Both the sternum and thorax
are covered in greater detail in Chapter 29. The superior por¬
tion of the sternum, the manubrium, provides an articular
surface for the proximal end of each clavicle (Fig. 8.10). The
articular surface is a shallow depression called the clavicular
notch covered with fibrocartilage like the clavicular head with
which it articulates. Each notch provides considerably less
articular surface than the clavicular head that articulates with
it. The two clavicular notches are separated by the sternal or
jugular notch on the superior aspect of the manubrium. This
notch is very prominent and is a useful landmark for identify¬
ing the sternoclavicular joints. Another reliable and useful
landmark is the angle formed by the junction of the manubrium
with the body of the sternum, known as the sternal angle, or
angle of Louis. This is also the site of the attachment of the
second costal cartilage to the manubrium and body of the
sternum.

The bony thorax forms the substrate on which the two
scapulae slide. Consequently, the shape of the thorax serves as
a constraint to the movements of the scapulae [97]. Each
scapula rides on the superior portion of the thorax, positioned
in the upright posture approximately from the first through
the eighth ribs and from the vertebral bodies of about T2 to
T7 or T8. The medial aspect of the spine of the scapula is

Figure 8.10: The sternum's articular surface. The sternum pro¬
vides a shallow articular surface for the head of the clavicle.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

127

Figure 8.11: Shape of the thorax. The elliptical shape of the
thorax influences the motion of the scapula.

typically described as in line with the spinous process of T2.
The inferior angle is usually reported to be in line with the
spinous process of T7. It is important to recognize, however,
that postural alignment of the shoulder and vertebral column
can alter these relationships significantly.

The dorsal surface of the thorax in the region of the scapu¬
lae is characterized by its convex shape, known as a thoracic
kyphosis. The superior ribs are smaller than the inferior ones,
so the overall shape of the thorax can be described as ellipsoid
(Fig. 8.11) [99]. Thus as the scapula glides superiorly on the
thorax it also tilts anteriorly. An awareness of the shape of the
thorax on which the scapula glides helps to explain the resting
position of the scapula and the motions of the scapula caused
by contractions of certain muscles such as the rhomboids and
pectoralis minor [17,49].

In conclusion, as stated at the beginning of this chapter,
the shoulder complex is an intricate arrangement of three
specific bones, each of which is unique. These three bones
are also functionally and structurally related to parts of the
axioskeleton (i.e., to the sternum and the thorax). A clear
image of each bone and its position relative to the others is
essential to a complete and accurate physical exami¬
nation. The palpable bony landmarks relevant to the
shoulder complex are listed below:

• Sternal notch

• Sternal angle

• Second rib

• Head of the clavicle

• Sternoclavicular joint

• Superior surface of the clavicle

• Anterior surface of the clavicle

• Acromion

• Acromioclavicular joint

• Coracoid process

• Vertebral border of the scapula

• Spine of the scapula

• Inferior angle of the scapula

• Axillary border of the scapula

• Greater tubercle of the humerus

• Lesser tubercle of the humerus

• Intertubercular groove of the humerus

The following section describes the structure and mechanics
of the joints of the shoulder complex formed by these bony
components.

STRUCTURE OF THE JOINTS
AND SUPPORTING STRUCTURES
OF THE SHOULDER COMPLEX

The shoulder complex is composed of four joints:

• Sternoclavicular

• Acromioclavicular

• Scapulothoracic

• Glenohumeral

All but the scapulothoracic joint are synovial joints. The
scapulothoracic joint falls outside any traditional category of
joint because the moving components, the scapula and the
thorax, are not directly attached or articulated to one another
and because muscles rather than cartilage or fibrous material
separate the moving components. However, it is the site of
systematic and repeated motion between bones and thus jus¬
tifiably can be designated a joint. This section presents the
structure and mechanics of each of the four joints of the
shoulder complex.

Sternoclavicular Joint

The sternoclavicular joint is described by some as a ball-and-
socket joint [84] and by others as a saddle joint [93,101].
Since both types of joints are triaxial, there is little functional
significance to the distinction. The sternoclavicular joint
actually includes the clavicle, sternum, and superior aspect of
the first costal cartilage (Fig. 8.12). It is enclosed by a syn¬
ovial capsule that attaches to the sternum and clavicle just
beyond the articular surfaces. The capsule is relatively weak
inferiorly but is reinforced anteriorly, posteriorly, and superi¬
orly by accessory ligaments that are thickenings of the cap¬
sule itself. The anterior and posterior ligaments are known as
the anterior and posterior sternoclavicular ligaments. These
ligaments serve to limit anterior and posterior glide of the
sternoclavicular joint. They also provide some limits to the
joints normal transverse plane movement, known as pro¬
traction and retraction.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Anterior sternoclavicular

Figure 8.12: The sternoclavicular joint. The supporting structures of the sternoclavicular joint include the capsule, the intraarticular disc,
the anterior and posterior sternoclavicular ligaments, the interclavicular ligament, and the costoclavicular ligament.

The superior thickening of the joint capsule comes from
the interclavicular ligament, a thick fibrous band extending
from one sternoclavicular joint to the other and covering the
floor of the sternal notch. This ligament helps prevent supe¬
rior and lateral displacements of the clavicle on the sternum.
The capsule with its ligamentous thickenings is described as
the strongest limiter of excessive motion at the sternoclavicu¬
lar joint [3].

The capsule and ligaments described so far are the pri¬
mary limiters of anterior, posterior, and lateral movements.
However, other structures provide additional limits to medial
translation and elevation of the clavicle. As noted in the
descriptions of the bones, the articular surface of the clavicle
is considerably larger than the respective surface on the ster¬
num. Consequently, the superior aspect of the clavicular head
projects superiorly over the sternum and is easily palpated.
This disparity between the articular surfaces results in an
inherent joint instability that allows the clavicle to slide medi¬
ally over the sternum. Such migration can be precipitated by
a medially directed force on the clavicle, such as those that
arise from a blow to, or a fall on, the shoulder (Fig. 8.13). An
intraarticular disc interposed between the clavicle and ster¬
num increases the articular surface on which the clavicle
moves and also serves to block any medial movement of the
clavicle. The disc is attached inferiorly to the superior aspect
of the first costal cartilage and superiorly to the superior bor¬
der of the clavicle s articular surface dividing the joint into two
separate synovial cavities. The specific attachments of the disc
help it prevent medial migration of the clavicle over the ster¬
num. A blow to the lateral aspect of the shoulder applies a

medial force on the clavicle, tending to push it medially on
the sternum. The clavicle is anchored to the underlying first
costal cartilage by the intraarticular disc resisting any medial
movement of the clavicle. However a cadaver study suggests
that the disc can be torn easily from its attachment on the
costal cartilage [3]. Therefore, the magnitude of its role as a

Figure 8.13: Forces that tend to move the clavicle medially. A fall
on the lateral aspect of the shoulder produces a force on the
clavicle, tending to push it medially.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

129

stabilizer of the sternoclavicular joint, particularly in limiting
medial translation of the clavicle on the sternum, remains
unclear. The disc may also serve as a shock absorber between
the clavicle and sternum [50].

Another important stabilizing structure of the sternoclav¬
icular joint is the costoclavicular ligament, an extracapsular
ligament lying lateral to the joint itself. It runs from the lateral
aspect of the first costal cartilage superiorly to the inferior
aspect of the medial clavicle. Its anterior fibers run superiorly
and laterally, while the posterior fibers run superiorly and
medially. Consequently, this ligament provides significant
limits to medial, lateral, anterior, and posterior movements of
the clavicle as well as to elevation.

A review of the supporting structures of the sternoclavicu¬
lar joint reveals that despite an inherently unstable joint sur¬
face, these supporting structures together limit medial, lateral,
posterior, anterior, and superior displacements of the clavicle
on the sternum. Inferior movement of the clavicle is limited by
the interclavicular ligament and by the costal cartilage itself.
Thus it is clear that the sternoclavicular joint is so reinforced
that it is quite a stable joint [72,96].

Clinical Relevance

FRACTURE OF THE CLAVICLE: The sternoclavicular joint
is so well stabilized that fractures of the clavicle are consider¬
ably more common than dislocations of the sternoclavicular
joint. In fact the clavicle is the bone most commonly frac¬
tured in humans [32]. Trauma to the sternoclavicular joint
and clavicle most commonly occurs from forces applied to
the upper extremity. Although clavicular fractures are com¬
monly believed to occur from falls on an outstretched hand,
a review of 122 cases of clavicular fractures reports that 94%
of the clavicular fracture cases (115 patients) occurred by a
direct blow to the shoulder [92]. Falls on the shoulder are a
common culprit. As an individual falls from a bicycle, for
example, turning slightly to protect the face and head, the
shoulder takes the brunt of the fall. The ground exerts a
force on the lateral and superior aspect of the acromion and
clavicle. This force pushes the clavicle medially and inferiorly
[96]. However, the sternoclavicular joint is firmly supported
against such movements, so the ground reaction force tends
to deform the clavicle. The first costal cartilage inferior to the
clavicle is a barrier to deformation of the clavicle, and as a
result, the clavicle is likely to fracture (Fig. 8.14). Usually the
fracture occurs in the middle or lateral one third of the clavi¬
cle, the former more frequently than the latter [32]. The exact
mechanism of fracture is unclear. Some suggest that it is a
fracture resulting from bending, while others suggest it is a
direct compression fracture [32,92]. Regardless of the mecha¬
nism, it is clear that fractures of the clavicle are more com¬
mon than sternoclavicular joint dislocations, partially
because of the firm stabilization provided by the disc and
ligaments of the sternoclavicular joint [15,96].

Figure 8.14: A typical way to fracture the clavicle. A fall on the
top of the shoulder produces a downward force on the clavicle,
pushing it down onto the first rib. The first rib prevents depres¬
sion of the medial aspect of the clavicle, but the force of the fall
continues to depress the lateral portion of the clavicle, resulting
in a fracture in the middle third of the clavicle.

Whether regarded as a saddle or ball-and-socket joint,
motion at the sternoclavicular joint occurs about three axes,
an anterior-posterior (AP), a vertical superior-inferior (SI),
and a longitudinal (ML) axis through the length of the clavi¬
cle (Fig 8.15). Although these axes are described as slightly

Figure 8.15: Axes of motion of the sternoclavicular joint. A.
Elevation and depression of the sternoclavicular joint occur
about an anterior-posterior axis. B. Protraction and retraction of
the sternoclavicular joint occur about a vertical axis. C. Upward
and downward rotation of the sternoclavicular joint occur about
a medial-lateral axis.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

oblique to the cardinal planes of the body [93], the motions of
the clavicle take place very close to these planes. Movement
about the AP axis yields elevation and depression, which
occur approximately in the frontal plane. Movements about
the SI axis are known as protraction and retraction and
occur in the transverse plane. Rotations around the longitudi¬
nal axis are upward (posterior) and downward (anterior)
rotation, defined by whether the anterior surface of the clav¬
icle turns up (upward rotation) or down (downward rotation).

Although movement at the sternoclavicular joint is
rotational, the prominence of the clavicular head and the
location of the joint s axes allow easy palpation of the head of
the clavicle during most of these motions. This palpation
frequently results in confusion for the novice clinician.
Note that retraction of the clavicle causes the head of the
clavicle to move anteriorly on the sternum as the body of the
clavicle rotates posteriorly (Fig. 8.16). Similarly in protrac¬
tion the clavicular head rolls posteriorly as the body moves
anteriorly. Likewise in elevation the body of the clavicle and
the acromion rise, but the head of the clavicle descends on
the sternum; depression of the sternoclavicular joint is the
reverse. These movements of the proximal and distal clavic¬
ular surfaces in opposite directions are consistent with rota¬
tions of the sternoclavicular joint and are the result of the
location of the axes within the clavicle itself. The exact loca¬
tion of the axes about which the movements of the stern¬
oclavicular joint occur are debated, but probably the axes lie
somewhat lateral to the head of the clavicle [3,82]. This loca¬
tion explains the movement of the lateral and medial ends of
the clavicle in apparently opposite directions. With the axes
of motion located between the two ends of the clavicle, pure
rotation results in opposite movements of the two ends, just

Figure 8.16: Movement of the head of the clavicle. Rotation of
the sternoclavicular joint about an axis causes the head of the
clavicle to move in a direction opposite the motion of the rest
of the clavicle just as the two ends of a seesaw move in opposite
directions about a central pivot point.

as the two ends of a seesaw move in opposite directions dur¬
ing pure rotation about the pivot point.

Few studies are available that investigate the available
ROM of the sternoclavicular joint. The total excursion of
elevation and depression is reportedly 50 to 60°, with
depression being less than 10° of the total [69,93]. Elevation
is limited by the costoclavicular ligament, and depression by
the superior portion of the capsule and the interclavicular
ligament [3,93]. Some suggest that contact between the
clavicle and the first rib also limits depression of the stern¬
oclavicular joint [82]. Facets found in some cadaver speci¬
mens between the clavicle and first costal cartilage provide
strong evidence for contact between these structures in at
least some individuals [3,82].

Protraction and retraction appear to be more equal in
excursion, with a reported total excursion ranging from 30 to
60° [82,93]. Protraction is limited by the posterior sternoclav¬
icular ligament limiting the backward movement of the clav¬
icular head and by the costoclavicular ligament limiting the
forward movement of the body of the clavicle. Retraction is
limited similarly by the anterior sternoclavicular ligament and
by the costoclavicular ligament. The interclavicular ligament
assists in limiting both motions [3].

Upward and downward rotations appear to be more limited
than the other motions, with estimates of upward rotation
ROM that vary from 25 to 55° [3,40,82]. Although there are
no known studies of downward rotation ROM, it appears to
be much less than upward rotation, probably less than 10°.
Regardless of the exact amount of excursion available at the
sternoclavicular joint, it is well understood that motion at
the sternoclavicular joint is intimately related to motions of
the other joints of the shoulder complex. How these motions
are related is discussed after each joint is presented.

Acromioclavicular Joint

The acromioclavicular joint is generally regarded as a gliding
joint with flat articular surfaces, although the surfaces are
sometimes described as reciprocally concave and convex
[93,101] (Fig. 8.17). Roth articular surfaces are covered by
fibrocartilage rather than hyaline cartilage. The joint is sup¬
ported by a capsule that is reinforced superiorly and inferiorly
by acromioclavicular ligaments (Fig. 8.18). Although the cap¬
sule is frequently described as weak, the acromioclavicular
ligaments may provide the primary support to the joint in
instances of small displacements and low loads [26,55]. In
addition, the acromioclavicular ligaments appear to provide
important limitations to posterior glide of the acromioclavic¬
ular joint regardless of the magnitude of displacement or load
[26]. The inferior acromioclavicular ligament also may pro¬
vide substantial resistance to excessive anterior displacement
of the clavicle on the scapula [55]. The joint also possesses an
intraarticular meniscus that is usually less than a whole disc
and provides no known additional support.

The other major support to the acromioclavicular ligament
is the extracapsular coracoclavicular ligament that runs from
the base of the coracoid process to the inferior surface of the

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

131

are relatively flattened and beveled with respect to one another.

clavicle. This ligament provides critical support to the
acromioclavicular joint, particularly against large excursions
and medial displacements [26,55]. It is regarded by many as
the primary suspensory ligament of the shoulder complex.
Mechanical tests reveal that it is substantially stiffer than the

Figure 8.18: Acromioclavicular joint. The acromioclavicular joint
is supported by the capsule, acromioclavicular ligaments, and
the coracoclavicular ligament.

acromioclavicular, coracoacromial, and superior glenohumeral
ligaments [13].

It is curious that a ligament that does not even cross the
joint directly can be so important in providing stability. An
understanding of the precise orientation of the ligament helps
explain its role in stabilizing the joint. The ligament is com¬
posed of two parts, the conoid ligament that runs vertically
from the coracoid process to the conoid tubercle on the clav¬
icle and the trapezoid ligament that runs vertically and later¬
ally to the trapezoid line. The vertically aligned portion, the
conoid ligament, reportedly limits excessive superior glides at
the acromioclavicular joint. The acromioclavicular ligaments
purportedly limit smaller superior displacements [26,55].

The more obliquely aligned trapezoid ligament protects
against the shearing forces that can drive the acromion inferi-
orly and medially under the clavicle. Such forces can arise
from a fall on the shoulder or a blow to the shoulder. The
shape of the articular surfaces of the acromioclavicular joint
causes it to be particularly prone to such displacements. As
stated earlier, the articular facet of the clavicle faces laterally
and inferiorly, while that of the acromion faces medially and
superiorly These surfaces give the acromioclavicular joint a
beveled appearance that allows medial displacement of the
acromion underneath the clavicle. Medial displacement of
the acromion results in simultaneous displacement of the
coracoid process, since it is part of the same scapula.
Examination of the trapezoid ligament shows that it is aligned
to block the medial translation of the coracoid process, thus
helping to keep the clavicle with the scapula and preventing
dislocation (Fig. 8.19) [82]. Dislocation of the acromioclavicular

Figure 8.19: Trapezoid ligament. The trapezoid ligament helps
prevent medial displacement of the acromion under the clavicle
during a medial blow to the shoulder.

132

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

joint can be accompanied by disruption of the coracoclavicular
ligament and by fractures of the coracoid process.

Clinical Relevance

DISLOCATION OF THE ACROMIOCLAVICULAR JOINT:

Dislocation of the acromioclavicular (AC) joint is a common
sports injury , especially in contact sports such as football and
rugby. The mechanism is similar to that of clavicular fractures,
a blow to or fall on the shoulder. Because of the strength of
the coracoclavicular ligament , dislocation of the AC joint often
occurs with a fracture of the coracoid process (Type III disloca¬
tion) instead of a disruption of the ligament itself.

The coracoacromial ligament is another unusual ligament
associated with the acromioclavicular joint. It is unusual
because it crosses no joint. Instead it forms a roof over the
glenohumeral joint by attaching from one landmark to another
landmark on the scapula (Fig. 8.20). This ligament provides
protection for the underlying bursa and supraspinatus ten¬
don. It also provides a limit to the superior gliding of the
humerus in a very unstable glenohumeral joint [58]. The
coracoacromial ligament also is implicated as a factor in
impingement of the structures underlying it and is thicker in
some shoulders with rotator cuff tears. The question remains
whether the thickening is a response to contact with the

Coracoacromial

Figure 8.20: Coracoacromial ligament. The coracoacromial liga¬
ment forms a roof over the humeral head and helps create the
subacromial space.

unstable humerus resulting from the disrupted rotator cuff or
whether the thickening is itself a predisposing factor for rota¬
tor cuff tears [88]. Additional research is needed to clarify the
relationship between the morphology of the coracoacromial
ligament and the integrity of the rotator cuff muscles.

Few studies report objective measurements of the excur¬
sions of the acromioclavicular joint. Sahara et al. report total
translations of approximately 4 mm in the anterior and poste¬
rior directions and approximately 2 mm in inferior/superior
directions during shoulder movement [87].

Although gliding joints allow only translational movements,
many authors describe rotational movement about specific
axes of motion at the acromioclavicular joint [17,82,101]. The
axes commonly described are vertical, AP, and medial/lateral
(ML) (Fig. 8.21). The vertical axis allows motion of the scapula
that brings the scapula closer to, or farther from, the clavicle
in the transverse plane. Motion about the AP axis results in
enlarging or shrinking the angle formed by the clavicle and
spine of the scapula in the frontal plane. Motion about the ML
axis tips the superior border of the scapula toward the clavicle
or away from it. Direct measurements of angular excursions
vary and range from less than 10° to 20° about individual axes
[40,82]. Using a screw axis (a single axis that describes the total
rotation and translation), Sahara et al. report a total of 35° of
rotation with full shoulder abduction [87]. These studies sug¬
gest that the acromioclavicular joint allows significant motion
between the scapula and clavicle.

Vertical

Figure 8.21: Axes of motion of the acromioclavicular joint.
Motion about a vertical axis of the acromioclavicular joint moves
the scapula in the transverse plane. Motion about an anterior-
posterior (AP) axis turns the glenoid fossa upward and downward.
Motion about a medial-lateral (ML) axis tilts the scapula
anteriorly and posteriorly.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

133

Viewed in the context of the shoulder complex, the acromio¬
clavicular joint is responsible for maintaining articulation of the
clavicle with the scapula, even as these two bones move in sep¬
arate patterns. Whether this results in systematic rotational
motions or in a gliding reorientation of the bones is not critical
to the clinician, since in either case the motions cannot be read¬
ily measured. What is essential is the recognition that although
the clavicle and scapula move together, their contributions to
whole shoulder motion require that they also move somewhat
independently of one another. This independent movement
requires motion at the acromioclavicular joint.

Clinical Relevance

OSTEOARTHRITIS OF THE ACROMIOCLAVICULAR
JOINT: The acromioclavicular joint is a common site of
osteoarthritis particularly in individuals who have a history
of heavy labor or athletic activities. The normal mobility of
the joint helps explain why pain and lost mobility in it from
arthritic changes can produce significant loss of shoulder
mobility and function.

Scapulothoracic Joint

The scapulothoracic joint, as stated earlier, is an atypical joint
that lacks all of the traditional characteristics of a joint except
one, motion. The primary role of this joint is to amplify the
motion of the glenohumeral joint, thus increasing the range and
diversity of movements between the arm and trunk. In addition,
the scapulothoracic joint with its surrounding musculature is
described as an important shock absorber protecting the shoul¬
der, particularly during falls on an outstretched arm [50].

Primary motions of the scapulothoracic joint include two
translations and three rotations (Fig. 8.22). Those motions are

• Elevation and depression

• Abduction and adduction

• Downward (medial) and upward (lateral) rotations

• Internal and external rotations

• Scapular tilt

Elevation is defined as the movement of the entire scapula
superiorly on the thorax. Depression is the opposite.
Abduction is defined as the entire medial border of the
scapula moving away from the vertebrae, and adduction as
movement toward the vertebrae. Abduction and adduction of
the scapulothoracic joint are occasionally referred to as pro¬
traction and retraction. However, protraction also is used by
some to refer to the combination of abduction and upward
rotation of the scapula. Others use the term protraction to
refer to a rounded shoulder posture that may include abduc¬
tion and downward rotation of the scapula. Therefore to avoid
confusion, this text describes scapular movements discretely
as elevation and depression, abduction and adduction, and
upward and downward rotation. Protraction and retraction

refer solely to the motions of the sternoclavicular joint in the
transverse plane.

Downward (medial) rotation of the scapula is defined
as a rotation about an AP axis resulting in downward turn of
the glenoid fossa as the inferior angle moves toward the
vertebrae. Upward (lateral) rotation is the opposite. The
location of the axis of downward and upward rotation is con¬
troversial but appears to be slightly inferior to the scapular
spine, approximately equidistant from the vertebral and axil¬
lary borders [97]. It is likely that the exact location of the axis
varies with ROM of the shoulder.

Internal and external rotations of the scapula occur about a
vertical axis. Internal rotation turns the axillary border of the
scapula more anteriorly, and external rotation turns the border
more posteriorly. The shape of the thorax can enhance this
motion. As the scapula translates laterally on the thorax in
scapular abduction, the scapula rotates internally. Conversely,
as the scapula adducts, it tends to rotate externally.

Anterior and posterior tilt of the scapula occur about a ML
axis. Anterior tilt moves the superior portion of the scapula
anteriorly while moving the inferior angle of the scapula pos¬
teriorly. Posterior tilt reverses the motion. Again, the shape of
the thorax can enhance these motions. As the scapula elevates
it tends to tilt anteriorly, and as it depresses it tends to tilt pos¬
teriorly (Fig. 8.23).

The motions of the scapulothoracic joint depend upon the
motions of the sternoclavicular and acromioclavicular joints
and under normal conditions occur through movements at
both of these joints. For example, elevation of the scapu¬
lothoracic joint occurs with elevation of the sternoclavicular
joint. Therefore, an important limiting factor for scapulothor¬
acic elevation excursion is sternoclavicular ROM. Similarly,
limits to scapulothoracic abduction and adduction as well as
rotation include the available motions at the sternoclavicular
and acromioclavicular joints. Tightness of the muscles of the
scapulothoracic joint—particularly the trapezius, serratus
anterior, and rhomboid muscles—may limit excursion of the
scapula. The specific effects of individual muscle tightness are
discussed in Chapter 9.

Although excursion of the scapulothoracic joint is not typ¬
ically measured in the clinic and few studies exist that have
investigated the normal movement available at this joint, it is
useful to have an idea of the magnitude of excursion possible
at the scapulothoracic joint. Excursions of 2-10 cm of scapu¬
lar elevation and no more than 2 cm of depression are found
in the literature [46,50]. Ranges of up to 10 cm are reported
for abduction and 4-5 cm for adduction [46,50].

Upward rotation of the scapula is more thoroughly investi¬
gated than other motions of the scapulothoracic joint. The
joint allows at least 60° of upward rotation of the scapula, but
the full excursion depends upon the sternoclavicular joint
elevation and acromioclavicular joint excursion available
[40,63,80]. Tightness of the muscles that downwardly rotate
the scapula may prevent or limit normal excursion of
the scapula as well. Downward rotation on the scapula, on the
other hand, is poorly studied. There are no known studies that

134

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 8.22: Primary motions of the scapulothoracic joint. A. Translations. B. Rotations.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

135

Figure 8.23: Scapular motion on the elliptical thorax. The shape
of the thorax causes the scapula to tilt anteriorly as it elevates
on the thorax.

Figure 8.24: Articular surfaces of the glenohumeral joint. The
humeral head and the glenoid fossa possess similar curvatures.

describe its excursion. However, downward rotation is greatly
reduced compared with upward rotation. Although full
potential excursions are not reported, the scapula reportedly
tilts posteriorly and rotates externally approximately 30° and
25°, respectively, during shoulder elevation.

Glenohumeral Joint

Although the glenohumeral joint is frequently referred to as
the shoulder joint, it must be emphasized that the “shoulder”
is a composite of four joints, of which the glenohumeral joint
is only a part, albeit a very important part. The glenohumeral
joint is a classic ball-and-socket joint that is the most mobile
in the human body [18]. Yet its very mobility presents serious
challenges to the joints inherent stability. The interplay
between stability and mobility of this joint is a major theme
that must be kept in mind to understand the mechanics and
pathomechanics of the glenohumeral joint.

The two articular surfaces, the head of the humerus and
the glenoid fossa, are both spherical (Fig. 8.24). The curve of
their surfaces is described as their radius of curvature. As
detailed in Chapter 7, the radius of curvature quantifies the
amount of curve in a surface by describing the radius of the
circle from which the surface is derived. Although the bony

surfaces of the humeral head and glenoid fossa may have
slightly different curvatures, their cartilaginous articular sur¬
faces have approximately the same radius of curvature
[39,89,99]. Because these surfaces have similar curvatures,
they fit well together; that is, there is a high degree of congru¬
ence. Increased congruence spreads the loads applied to the
joint across a larger surface area and thus reduces the stress
(force/area) applied to the articular surface. However, the
amount of congruence is variable, even in healthy gleno¬
humeral joints [4]. In cadavers, decreased congruence leads to
an increase in the gliding motions between the humeral head
and the glenoid fossa [4,48]. Thus decreased congruence may
be a contributing factor in glenohumeral joint instability.

Although the articular surfaces of the glenohumeral joint
are similarly curved, the actual areas of the articular surfaces
are quite different from one another. While the head of the
humerus is approximately one half of a sphere, the surface
area of the glenoid fossa is less than one half that of
the humeral head [45,52]. This disparity in articular surface
sizes has dramatic effects on both the stability and mobility
of the glenohumeral joint. First, the difference in the size
of the articular surfaces allows a large degree of mobility since
there is no bony limitation to the excursion. The size of
the articular surfaces is an important factor in making the

136

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

glenohumeral joint the most mobile in the body. However, by
allowing tremendous mobility, the articular surfaces provide lit¬
tle or no stability for the glenohumeral joint [58]. The stability
of the glenohumeral joint depends upon nonbony structures.

SUPPORTING STRUCTURES OF
THE GLENOHUMERAL JOINT

The supporting structures of the glenohumeral joint consist
of the

• Labrum

• Capsule

• Three glenohumeral ligaments

• Coracohumeral ligament

• Surrounding musculature

The noncontractile supporting structures of the glenohumeral
joint are discussed in this section. The role of muscles in sup¬
porting the joint is discussed in Chapter 9.

The shallow glenoid fossa has already been identified as a
contributing factor in glenohumeral joint instability. The sta¬
bility is improved by deepening the fossa with the labrum
(Fig. 8.25). The labrum is a ring of fibrous tissue and fibro-
cartilage surrounding the periphery of the fossa, approxi¬
mately doubling the depth of the articular surface of the fossa
[38,70]. Besides increasing the depth of the articular surface,

Figure 8.25: Glenoid labrum. The glenoid labrum deepens the
glenoid fossa.

the ring increases the articular contact area, which also
decreases the stress (force/area) on the glenoid fossa. The
labrum provides these benefits while being deformable,
thereby adding little or no restriction to glenohumeral move¬
ment. Magnetic resonance imaging (MRI) shows consider¬
able variation in the shape of the labrum in asymptomatic
shoulders, including notches and separations, particularly in
the anterior aspect of the ring. A small percentage of individ¬
uals lack portions of the labrum [77].

Labral tears are well described in the clinical literature
[16,76]. Mechanical tests of the ring demonstrate that it is
weakest anteriorly and inferiorly, which is consistent with the
clinical finding that anterior tears are the most common [31].
However, the functional significance of a torn labrum in the
absence of other pathology remains controversial [17,76,79].
The amount of dysfunction that results from a labral tear
probably depends upon the severity of the lesion. Small tears
may have little or no effect, while large tears that extend to
other parts of the joint capsule produce significant instability.
The normal variability of the labrum in asymptomatic shoul¬
ders lends strength to the concept that small isolated labral
tears do not result in significant dysfunction. However, addi¬
tional studies are needed to clarify the role of labral tears in
glenohumeral dysfunction.

The remaining connective tissue supporting structures of
the glenohumeral joint are known collectively as the capsu-
loligamentous complex. It consists of the joint capsule and
reinforcing ligaments. It encircles the entire joint and provides
protection against excessive rotation and translation in all direc¬
tions. It is important to recognize that the integrity of the com¬
plex depends on the integrity of each of its components.

The fibrous capsule of the glenohumeral joint is intimately
related to the labrum. The capsule attaches distally to the
anatomical neck of the humerus and proximally to the periph¬
ery of the glenoid fossa and/or to the labrum itself. Inferiorly,
it is quite loose, forming folds (Fig. 8.26). These folds must
open, or unfold, as the glenohumeral joint elevates in abduc¬
tion or flexion.

Clinical Relevance

ADHESIVE CAPSULITIS: In adhesive capsulitis, fibrous
adhesions form in the glenohumeral joint capsule. The cap¬
sule then is unable to unfold to allow full flexion or abduc¬
tion, resulting in decreased joint excursion. Onset is fre¬
quently insidious, and the etiology is unknown. However,
the classic physical findings are severe and painful limita¬
tions in joint ROM [30,73].

The normal capsule is quite lax and, by itself, contributes
little to the stability of the glenohumeral joint. However, it is
reinforced anteriorly by the three glenohumeral ligaments
and superiorly by the coracohumeral ligament. It also is sup¬
ported anteriorly, superiorly, and posteriorly by the rotator
cuff muscles that attach to it. Only the most inferior portion
of the capsule is without additional support.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

137

Figure 8.26: Glenohumeral joint capsule. A. When the shoulder is in neutral, the inferior portion of the capsule is lax and appears
folded. B. In abduction the folds of the inferior capsule are unfolded, and the capsule is pulled more taut.

The three glenohumeral ligaments are thickenings of the
capsule itself (Fig. 8.27). The superior glenohumeral ligament
runs from the superior portion of the labrum and base of the
coracoid process to the superior aspect of the humeral neck.
The middle glenohumeral ligament has a broad attachment
on the anterior aspect of the labrum inferior to the superior
glenohumeral ligament and passes inferiorly and laterally,
expanding as it crosses the anterior aspect of the gleno¬
humeral joint. It attaches to the lesser tubercle deep to the
tendon of the subscapularis. The superior glenohumeral liga¬
ment along with the coracohumeral ligament and the tendon
of the long head of the biceps lies in the space between the
tendons of the supraspinatus and subscapularis muscles. This
space is known as the rotator interval.

The inferior glenohumeral ligament is a thick band that
attaches to the anterior, posterior, and middle portions of the
glenoid labrum and to the inferior and medial aspects of the
neck of the humerus. The coracohumeral ligament attaches
to the lateral aspect of the base of the coracoid process and to
the greater tubercle of the humerus. It blends with the
supraspinatus tendon and with the capsule.

These reinforcing ligaments support the glenohumeral joint
by limiting excessive translation of the head of the humerus on
the glenoid fossa. Tightness of these ligaments actually con¬
tributes to increased translation of the humeral head in the
opposite direction [34]. The coracohumeral ligament provides
protection against excessive posterior glides of the humerus on
the glenoid fossa [7]. All three of the glenohumeral ligaments
help to prevent anterior displacement of the humeral head on
the glenoid fossa, especially when they are pulled taut by lateral

Figure 8.27: Glenohumeral joint. The glenohumeral joint capsule
is reinforced by the superior, middle, and inferior glenohumeral lig¬
aments. The joint is also supported by the coracohumeral ligament.

138

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

rotation of the glenohumeral joint [68]. The position of the
glenohumeral joint in the frontal plane influences what parts of
these ligaments are pulled taut [18]. In neutral and in moderate
abduction, the superior and middle glenohumeral ligaments
are pulled tight. However in more abduction, the inferior
glenohumeral ligament provides most of the resistance to ante¬
rior displacement [5,37,78,81]. The three glenohumeral liga¬
ments also limit excessive lateral rotation of the glenohumeral
joint [54,75]. As with anterior displacement, increasing
abduction increases the role the inferior glenohumeral
ligament plays in limiting lateral rotation [43].

Although the posterior capsule is reinforced passively only by
a portion of the inferior glenohumeral ligament, it too provides
resistance to excessive glide of the glenohumeral joint. The
posterior capsule functions as a barrier to excessive posterior
glide of the humeral head. It also limits excessive medial rota¬
tion of the joint. In certain positions of the glenohumeral joint,
the anterior and posterior portions of the glenohumeral joint
capsule are under tension simultaneously, demonstrating how
the function of the capsule and its reinforcing ligaments is com¬
plex and interdependent [10,95].

There are opposing views regarding the support of the
glenohumeral joint against inferior glide. The weight of the
upper extremity in upright posture promotes inferior glide of
the humeral head on the glenoid fossa. Some authors suggest
that inferior glide of the humeral head is resisted by the pull of
the coracohumeral ligament and to a lesser degree by the
superior glenohumeral ligament, particularly when the joint is
laterally rotated [18,34,42]. However, another cadaver study
reports little support from the superior glenohumeral ligament
against inferior subluxation [90]. This study, which suggests
that the inferior glenohumeral ligament provides more support
in the inferior direction, with additional support from the cora¬
cohumeral ligament, examines smaller displacements than the
preceding studies. The individual contributions from these
supporting structures may depend on the position of the gleno¬
humeral joint and the magnitude of the humeral displace¬
ments. Additional research is needed to elucidate the roles the
glenohumeral joint capsule and ligaments play in supporting
the glenohumeral joint. Subtle changes in joint position also
appear to alter the stresses applied to the capsuloligamentous
complex.

Clinical Relevance

EXAMINING OR STRETCHING THE GLENOHUMERAL
JOINT LIGAMENTS: Altering the position of the gleno¬
humeral joint allows the clinician to selectively assess specific
portions of the glenohumeral capsuloligamentous complex . For
example\ lateral rotation of the glenohumeral joint reduces the
amount of anterior translation of the humeral head by several
millimeters. If the clinician assesses anterior glide of the humer¬
al head with the joint laterally rotated and does not observe a
reduction in the anterior glide excursion , the clinician may
suspect injury to the anterior capsuloligamentous complex.

Similarly , by altering the position of the glenohumeral
joint, the clinician can direct treatment toward a particular
portion of the complex. Anterior glide with the glenohumeral
joint abducted applies a greater stretch to the inferior
glenohumeral ligament than to the superior and middle
glenohumeral ligaments. The clinician can also use such
knowledge to reduce the loads on an injured or repaired
structure.

One of the factors coupling the support of the gleno¬
humeral ligaments and capsule to each other is the intraar-
ticular pressure that also helps to support the glenohumeral
joint [41,42]. Puncturing, or venting, the rotator interval in
cadavers results in a reduction of the inferior stability of the
humeral head, even in the presence of an otherwise intact
capsule [42,102]. Isolated closure of rotator interval defects
appears to restore stability in young subjects who have no
additional glenohumeral joint damage [23]. This supports the
notion that tears in this part of the capsule can destabilize the
joint not only by a structural weakening of the capsule itself
but also by a disruption of the normal intraarticular pressure.

Thus the capsule with its reinforcing ligaments acts as a
barrier to excessive translation of the humeral head and lim¬
its motion of the glenohumeral joint, particularly at the ends
of glenohumeral ROM. It also contributes to the normal glide
of the humerus on the glenoid fossa during shoulder motion.
However, this complex of ligaments still is insufficient to sta¬
bilize the glenohumeral joint, particularly when external loads
are applied to the upper extremity or as the shoulder moves
through the middle of its full ROM. The role of the muscles
in stabilization of the glenohumeral joint is discussed in
Chapter 9.

MOTIONS OF THE GLENOHUMERAL JOINT

As a ball-and-socket joint, the glenohumeral joint has three
axes of motion that lie in the cardinal planes of the body.
Therefore the motions available at the glenohumeral joint are

• Flexion/extension

• Abduction/adduction

• Medial/lateral (internal/external) rotation

Abduction and flexion sometimes are each referred to as
elevation. Authors also distinguish between elevation of the
glenohumeral joint in the plane of the scapula and that in the
sagittal and frontal planes.

Flexion and abduction in the sagittal and frontal planes of
the body, respectively, occur with simultaneous rotation of the
glenohumeral joint about its long axis. Rotation of the
humerus during shoulder elevation is necessary to maximize
the space between the acromion and proximal humerus. This
space, known as the subacromial space, contains the sub¬
acromial bursa, the muscle and tendon of the supraspinatus,
the superior portion of the glenohumeral joint capsule, and the
intraarticular tendon of the long head of the biceps brachii

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

139

Subacromial space

Figure 8.28: Subacromial space during abduction. A. The subacromial space is large when the shoulder is in neutral. B. During shoulder
abduction the greater tubercle moves closer to the acromion, narrowing the subacromial space.

(Fig. 8.28). Each of these structures could sustain injury with
the repeated or sustained compression that would occur with¬
out humeral rotation during shoulder elevation.

Determining the exact direction and pattern of humeral
rotation during shoulder elevation has proven challenging.
The traditional clinical view is that lateral rotation of the
humerus accompanies shoulder abduction, and medial rota¬
tion occurs with shoulder flexion [6,86,93]. Consistent with
this view is that little or no axial rotation occurs with shoulder
elevation in the plane of the scapula [86]. Data to support
these concepts come from cadaver and two-dimensional
analysis of humeral motion in vivo.

More recently three-dimensional studies of arm-trunk
motion call these data into question. Most of these reports
agree that the humerus undergoes lateral rotation during
shoulder abduction [63,94]. However, these studies also sug¬
gest that lateral rotation may occur in shoulder flexion as well.
In order to interpret these differing views regarding axial rota¬
tion and shoulder flexion, it is essential to note that these more
recent studies use three-dimensional analysis and employ euler
angles to describe these motions. Euler angles are extremely
sensitive to the order in which they are determined and are not
comparable to two-dimensional anatomical measurements.

Despite the confusion regarding the exact anatomical rota¬
tions that occur with shoulder elevation, it remains clear that
axial rotation of the humerus is an essential ingredient of
shoulder elevation. Large compressive forces are reported on
the coracohumeral ligament in healthy individuals who actively
medially rotate the shoulder through the full ROM while
maintaining 90° of shoulder abduction [103]. Such forces
would also compress the contents of the subacromial space,
thereby creating the potential for an impingement syndrome.

Clinical Relevance

SHOULDER IMPINGEMENT SYNDROME IN
COMPETITIVE SWIMMERS: Impingement syndrome is
the cluster of signs and symptoms that result from chronic
irritation of any or all of the structures in the subacromial
space. Such irritation can come from repeated or sustained
compression resulting from an intermittent or prolonged
narrowing of the subacromial space. Symptoms of impinge¬
ment are common in competitive swimmers and include
pain in the superior aspect of the shoulder beginning in the
midranges of shoulder elevation and worsening with
increasing excursion of flexion or abduction.

Most competitive swimming strokes require the shoulder to
actively and repeatedly assume a position of shoulder abduc¬
tion with medial rotation. This position narrows the subacro¬
mial space and consequently increases the risk of impinge¬
ment Some clinicians and coaches suggest that to prevent
impingement swimmers must strengthen their scapular mus¬
cles so that scapular position can enhance the subacromial
space even as the humeral position tends to narrow it

Although flexion, abduction, and rotation of the gleno¬
humeral joint imply pure rotational movements, the asym¬
metrical articular areas of the humeral head and glenoid
fossa, the pull of the capsuloligamentous complex, and the
forces from the surrounding muscles result in a complex com¬
bination of rotation and gliding motions at the glenohumeral
joint. If the motion of the glenohumeral joint consisted
entirely of pure rotation, the motion could be described as a

140

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

rotation about a fixed axis. When rotation is accompanied by
gliding, the rotation can be described as occurring about a
moving axis. As described in Chapter 7, the degree of mobil¬
ity of the axis of rotation in the two-dimensional case is
described by the instant center of rotation (ICR). The ICR
is the location of the axis of motion at a given joint position.
The more stable the axis of motion, the more constant is the
ICR. The ICR of the glenohumeral joint moves only slightly
during flexion or abduction of the shoulder, indicating only
minimal translation [100].

The amount of humeral head translation during shoulder
motion has received considerable attention among clinicians
and researchers [28,33,34,52,100]. Glenohumeral translation
is less during active shoulder motions when muscle contrac¬
tions help to stabilize the humeral head than during passive
motions [28]. In active elevation of the glenohumeral joint in
the plane of the scapula, the humeral head undergoes mini¬
mal superior glide (<3 mm) and then remains fixed or glides
inferiorly no more than 1 mm [11,21,28,50,80,89]. Individuals
with muscle fatigue or glenohumeral instability, however,
consistently exhibit excessive superior glide during active
shoulder elevation [10,18,21,46].

The humeral head glides posteriorly in shoulder extension
and in lateral rotation; it translates anteriorly during abduc¬
tion and medial rotation [28,33,68,70,75,89]. These data con¬
tradict the so-called concave-convex rule, which states that
the convex humeral head glides on the concave glenoid fossa
in directions opposite the humeral roll. For example, the con¬
cave-convex rule predicts that inferior glide of the humerus
accompanies its superior roll in flexion or abduction, and
lateral rotation occurs with anterior glide [86,93]. Direct
measurements reveal otherwise, showing repeatedly that the
concave-convex rule does not apply to the glenohumeral joint.

Although slight, joint glides appear to accompany gleno¬
humeral motions. This recognition supports the standard clin¬
ical practice of restoring translational movement to restore full
ROM at the glenohumeral joint. The concept of joint glide at
the glenohumeral joint also forms the theoretical basis for
many mobilization techniques used in the clinic. Reporting
the amount of available passive humeral head glide as a per¬
centage of the glenoid diameter in the direction of the glide, a
study of anesthetized subjects without shoulder pathology
reports that the humeral head can glide 17, 26, and 29% in the
anterior, posterior, and inferior directions, respectively, with
the glenohumeral joint in neutral [35]. Passive glides of almost
1.5 cm are reported in subjects without shoulder impairments
[9,65]. Patients with anterior instabilities demonstrate signifi¬
cant increases in both anterior and inferior directions. Patients
with multidirectional instabilities exhibit significantly
increased excursions in all three directions [11,21]. It is essen¬
tial for the clinician to understand that slight translation occurs
in normal glenohumeral joint motion. Yet excessive translation
may contribute to significant dysfunction.

Total glenohumeral joint elevation is most frequently
described as a percentage of shoulder complex motion.
Glenohumeral flexion and abduction are reported to be
100-120° [40,80,98]; however, shoulder rotation comes solely

from the glenohumeral joint. Although protraction of the
sternoclavicular joint and abduction and internal rotation of
the scapulothoracic joint cause the humerus to face medially,
these are substitutions for medial rotation of the shoulder
rather than contributions to true medial rotation. Similarly,
retraction of the sternoclavicular joint and adduction, posterior
tilting, and external rotation of the scapulothoracic joint can
substitute for lateral rotation of the shoulder. True shoulder
rotation ROM values range from approximately 70 to 90° for
both medial and lateral rotation. There are no known studies
that identify the contribution of the glenohumeral joint to
shoulder extension, but the glenohumeral joint is the likely
source of most extension excursion, with only a minor contri¬
bution from adduction, downward rotation and anterior tilt of
the scapulothoracic joint.

In summary, this section reviews the individual joints that
constitute the shoulder complex. Each joint has a unique struc¬
ture that results in a unique pattern of mobility and stability.
The overall function of the shoulder complex depends
on the individual contributions of each joint. A patients
complaints to the clinician usually are focused on the
function of the shoulder as a whole, such as an inability to reach
overhead or the presence of pain in throwing a ball. The clini¬
cian must then determine where the impairment is within the
shoulder complex. A full understanding of the role of each joint
in the overall function of the shoulder complex is essential to
the successful evaluation of the shoulder complex. The follow¬
ing section presents the role of each joint in the production of
normal motion of the shoulder complex.

TOTAL SHOULDER MOVEMENT

The term shoulder means different things to different people
(i.e., the shoulder complex or the glenohumeral joint).
Therefore, motion in this region is perhaps more clearly pre¬
sented as arm-trunk motion, since motion of the shoulder
complex generally is described by the angle formed between
the arm and the trunk (Fig. 8.29). However, the literature and
clinical vocabulary commonly use shoulder motion to mean
arm-trunk motion. Therefore, both terms, arm-trunk motion
and shoulder motion, are used interchangeably in the rest of
this chapter. For the purposes of clarity, the terms
arm-trunk elevation and shoulder elevation are used to
mean abduction or flexion of the shoulder complex. These
can occur in the cardinal planes of the body or in the plane of
the scapula. When the distinction is important, the plane of
the motion is identified. It is essential to recognize the dis¬
tinction between shoulder elevation, which involves all of the
joints of the shoulder complex, and scapular elevation, which
is motion of the scapulothoracic joint and indirectly produces
elevation at the sternoclavicular joint but does not include
glenohumeral joint motion. The following section describes
the individual contributions of the four joints of the shoulder
complex to the total arm-trunk motion. In addition, the
timing of these contributions and the rhythmic interplay of
the joints are discussed.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

141

Figure 8.29: Arm-trunk motion. Shoulder motion is described by
the orientation of the mechanical axis of the arm with respect to
the trunk.

Movement of the Scapula and Humerus
during Arm-Trunk Elevation

During arm-trunk elevation the scapula rotates upward as
the glenohumeral joint flexes or abducts. In addition, the
scapula tilts posteriorly about a medial-lateral axis and rotates
externally about a vertical axis during shoulder elevation
[29,47,60,63]. Upward rotation is the largest scapulothoracic
motion in shoulder elevation. It has long been recognized that
the upward rotation of the scapula and the flexion or abduc¬
tion of the humerus occur synchronously throughout
arm-trunk elevation in healthy individuals [66]. In the last 50
years, several systematic studies have been undertaken to
quantify this apparent rhythm, known as scapulohumeral
rhythm. The vast majority of these studies have examined
the relationship of movement at the joints of the shoulder
complex during voluntary, active shoulder movement. In
addition, some of these investigations examine arm-trunk
movement in the cardinal planes of the body, while others
report motions in the plane of the scapula. Some of the dif¬
ferences in the results of the studies discussed below may be
attributable to these methodological differences.

The classic study of the motion of the shoulder is by Inman
et al. in 1944 [40]. Although some of the data reported in this
study have been refuted, the study continues to form the basis
for understanding the contributions made by the individual
joints to the total movement of the shoulder complex. These
investigators report on the active, voluntary motion of
the shoulder complex in the sagittal and frontal planes of the
body in individuals without shoulder pathology. They state that
for every 2° of glenohumeral joint abduction or flexion there is
1° of upward rotation at the scapulothoracic joint, resulting in

Figure 8.30: Contribution of the glenohumeral and scapulo¬
humeral joints to arm-trunk motion. There is approximately 2°
of glenohumeral motion to every 1° of scapulothoracic motion
during shoulder flexion or abduction.

a 2:1 ratio of glenohumeral to scapulothoracic joint movement
in both flexion and abduction (Fig. 8.30). Thus these authors
suggest that the glenohumeral joint contributes approximately
120° of flexion or abduction and the scapulothoracic joint con¬
tributes approximately 60° of upward rotation of the scapula,
yielding a total of about 180° of arm-trunk elevation. The
authors state that the ratio of glenohumeral to scapulothoracic
motion becomes apparent and remains constant after approx¬
imately 30° of abduction and approximately 60° of flexion.
McClure et al. also found a 2:1 ratio for scapulohumeral
rhythm during active shoulder flexion [63]. In contrast these
authors and others report mostly smaller ratios for shoulder
elevation in the scapular plane [1,25,29,63,80]. In other words,
these authors report more scapular (or less glenohumeral)
contribution to the total movement.

These results are presented in Table 8.1. McQuade and
Smidt do not report average ratios [66]. However, in contrast
to the data reported in Table 8.1, their data suggest even more
contribution to the total movement by the glenohumeral joint
than is suggested by Inman et al., with ratios varying from
approximately 3:1 to 4:1 through the range. In addition, sev¬
eral authors report a variable ratio rather than the constant
ratio reported by Inman et al. [1,29,66,80]. Although there is
little agreement in the actual change in the ratios, most

TABLE 8.1: Reported Average Ratios of
Glenohumeral to Scapulothoracic Motion during
Active Arm-Trunk Elevation in the Plane of the
Scapula

Authors Ratio

Freedman and Munro [25]

1.58:1

Poppen and Walker [80]

1.25:1

Bagg and Forrest [1]

1.25:1 to 1.33:1

Graichen et al. [29]

1.5:1 to 2.4:1

McClure [63]

1.7:1

142

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

authors report a greater contribution to arm-trunk motion
from the scapulothoracic joint late in the ROM rather than in
the early or midrange.

Some authors have also investigated the effect of muscle
activity on the scapulohumeral rhythm. Passive motion is
reported to have a higher glenohumeral contribution to the
movement early in the range, with a greater scapulothoracic
joint contribution at the end of the motion as well as a higher
overall glenohumeral contribution to the total motion [29,66].
Resistance and muscle fatigue during active movement report-
edly decrease the scapulohumeral rhythm, resulting in
an increased scapulothoracic contribution to the
motion [64,66].

In addition to upward rotation, the scapula also exhibits
slight external rotation until at least 90° of shoulder elevation
[19,22,63]. The scapula also exhibits a few degrees of poster¬
ior tilt through at least the first 90° of shoulder elevation.

Despite the differences reported in the literature, some
very important similarities exist. Conclusions to be drawn
from these studies of healthy shoulders are

• The scapulothoracic and glenohumeral joints move simul¬
taneously through most of the full range of shoulder ele¬
vation.

• Roth the glenohumeral and scapulothoracic joints con¬
tribute significantly to the overall motion of flexion and
abduction of the shoulder.

• The scapula and humerus move in a systematic and coor¬
dinated rhythm.

• The exact ratio of glenohumeral to scapulothoracic motion
may vary according to the plane of motion and the location
within the ROM.

• The exact ratio of glenohumeral to scapulothoracic motion
during active ROM is likely to depend on muscle activity.

• There is likely to be significant variability among individuals.

The clinician can use these observations to help identify
abnormal movement patterns and to help understand the
mechanisms relating shoulder impairments to dysfunction.

Clinical Relevance

ANOTHER POSSIBLE MECHANISM PRODUCING
SHOULDER IMPINGEMENT SYNDROME: Shoulder ; or
subacromial impingement syndrome results from a persist¬
ent or repeated compression of the structures within the
subacromial space, the space between the acromion process
and humeral head. As noted earlier in the chapter, abnor¬
mal humeral axial rotation may contribute to the compres¬
sive forces leading to impingement. Another possible source
of impingement is abnormal scapulothoracic motion during
shoulder elevation. Either excessive scapular internal rota¬
tion or anterior tilt could narrow the subacromial space and
produce compression of the subacromial contents. Repeated
or prolonged compression could cause an inflammatory
response resulting in pain.

Sternoclavicular and Acromioclavicular
Motion during Arm-Trunk Elevation

With the upward rotation of the scapula during arm-trunk
elevation, there must be concomitant elevation of the clavicle
to which the scapula is attached. The sternoclavicular joint
elevates 15—40° during arm-trunk elevation [1,40,59,98]. The
joint also retracts and upwardly rotates during arm-trunk ele¬
vation [40,63,59].

Note that the total scapular upward rotation is 60° and the
total clavicular elevation is approximately 40°. This disparity of
motion suggests that the scapula moves away from the clavicle,
causing motion at the acromioclavicular joint (Fig. 8.31).
Although the motion at the acromioclavicular joint is inade¬
quately studied, its motion during arm-trunk flexion and
abduction appears undeniable [82,87,98]. A possible mecha¬
nism to control the acromoclavicular motion is proposed by
Inman et al. [40]. As the scapula is pulled away from the clav¬
icle by upward rotation, the conoid ligament (the vertical por¬
tion of the coracoclavicular ligament) is pulled tight and pulls
on the conoid tubercle situated on the inferior surface of
the crank-shaped clavicle. The tubercle is drawn toward the
coracoid process, causing the clavicle to be pulled into upward
rotation (Fig. 8.31). The crank shape of the clavicle allows the
clavicle to remain close to the scapula as it completes its lateral
rotation, without using additional elevation ROM at the
sternoclavicular joint. The sternoclavicular joint thus elevates

Figure 8.31: Upward rotation of the clavicle during arm-trunk
motion. As the scapula moves away from the clavicle during
arm-trunk elevation, the conoid ligament is pulled taut and
causes the clavicle to rotate upwardly.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

143

Hand drill

Figure 8.32: Crank shape of clavicle. The shape of the clavicle
allows the conoid tubercle to rotate toward the scapula while
the medial and lateral ends of the clavicle stay relatively fixed.

less than its full available ROM, which is approximately 60°.
Therefore, full shoulder flexion or abduction can still be aug¬
mented by additional sternoclavicular elevation in activities
that require an extra-long reach, such as reaching to the very
top shelf. This sequence of events demonstrates the signifi¬
cance of the crank shape of the clavicle and the mobility of the
acromioclavicular joint to the overall motion of the shoulder
complex (Fig. 8.32). The coordinated pattern of movement at
the sternoclavicular and scapulothoracic joints during normal
shoulder flexion and abduction also reveals the role of the
conoid ligament in producing movement, unlike most liga¬
ments that only limit movement.

This description of sternoclavicular and acromioclavicular
motion reveals the remarkable synergy of movement among
all four joints of the shoulder complex necessary to complete
full arm-trunk flexion and abduction. The scapulothoracic
joint must rotate upward to allow full glenohumeral flexion or
abduction. The clavicle must elevate and upwardly rotate to
allow scapular rotation. This extraordinary coordination
occurs in activities as diverse and demanding as lifting a 20-lb
child overhead and throwing a 95-mph fastball. However, the
rhythm certainly is interrupted in some individuals. Any of
the four joints can be impaired. The following section consid¬
ers the effects of impairments of the individual joints on over¬
all shoulder movement.

Impairments in Individual Joints
and Their Effects on Shoulder Motion

The preceding section discusses the intricately interwoven
rhythms of the four joints of the shoulder complex during
arm-trunk motion. This section focuses on the effects of

alterations in the mechanics of any of these joints on shoulder
motion. Common pathologies involving the glenohumeral joint
include capsular tears, rheumatoid arthritis, and inferior sub¬
luxations secondary to stroke. The sternoclavicular joint can be
affected by rheumatoid arthritis or by ankylosing spondylitis.
The acromioclavicular joint is frequently dislocated and also is
susceptible to osteoarthritis. Scapulothoracic joint function can
be compromised by trauma such as a gunshot wound or by
scarring resulting from such injuries as bums. These are just
examples to emphasize that each joint of the shoulder complex
is susceptible to pathologies that impair its function. Each joint
is capable of losing mobility and thus affecting the mobility of
the entire shoulder complex. It is not possible to consider all
conceivable pathologies and consequences. The purpose of this
section is to consider the altered mechanics and potential sub¬
stitutions resulting from abnormal motion at each of the joints
of the shoulder complex. Such consideration illustrates a
framework from which to evaluate the function of the shoulder
complex and the integrity of its components.

LOSS OF GLENOHUMERAL OR SCAPULOTHORACIC
JOINT MOTION

As discussed earlier in this chapter, the data from studies of
scapulohumeral rhythm suggest that the glenohumeral joint
provides more than 50% of the total shoulder flexion or
abduction. Therefore, the loss of glenohumeral motion has a
profound effect on shoulder motion. However, it must be
emphasized that shoulder motion is not lost completely, even
with complete glenohumeral joint immobility. The scapu¬
lothoracic and sternoclavicular joints with the acromioclavic¬
ular joint combine to provide the remaining one third or
more motion. In the absence of glenohumeral movement
these joints, if healthy, may become even more mobile. Thus
without glenohumeral joint motion and in the presence of
intact scapulothoracic, sternoclavicular, and acromioclavicular
joints, an individual should still have at least one third the nor¬
mal shoulder flexion or abduction ROM.

Complete loss of glenohumeral joint motion, however,
results in total loss of shoulder rotation. Yet even under these
conditions scapulothoracic motion can provide some substi¬
tution. Forward tipping of the scapula about a medial-
lateral axis is a common substitution for decreased
medial rotation of the shoulder.

Clinical Relevance

MEASUREMENT OF MEDIAL ROTATION ROM OF THE
SHOULDER: Goniometry manuals describe measurement of
medial rotation of the shoulder with the subject lying supine
and the shoulder abducted to 90° [74]. In this position the
shoulder is palpated to identify anterior tilting of the scapula
as the shoulder is medially rotated. Firm manual stabilization
is usually necessary to prevent the scapula from tilting anteri¬
orly to substitute for medial rotation (Fig. 8.33).

144

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 8.33: Scapular substitutions in shoulder ROM. A. Standard goniometric measurement of medial rotation ROM of shoulder
requires adequate stabilization of the scapula. B. Inadequate stabilization allows anterior tilting of the scapula with an apparent
increase in medial rotation ROM of the shoulder.

Conversely, the loss of scapulothoracic motion results in a
loss of at least one third of full shoulder elevation ROM.
Although this appears to be roughly true in passive ROM,
Inman et al. report that in the absence of scapulothoracic joint
motion, active shoulder abduction is closer to 90° of abduction
rather than the expected 120° [40]. These authors hypothesize
that upward rotation of the scapula is essential to maintaining

an adequate contractile length of the deltoid muscle. Scapular
upward rotation lengthens the deltoid even as the muscle
contracts across the glenohumeral joint during abduction
(Fig. 8.34). In the absence of upward scapular rotation, the del¬
toid contracts and reaches its maximal shortening, approxi¬
mately 60% of its resting length, by the time the glenohumeral
joint reaches about 90° of abduction. (See Chapter 4 for details

Figure 8.34: Scapular motion and deltoid muscle function. A. During normal active shoulder abduction, the upward rotation of the
scapula lengthens the deltoid, maintaining an adequate contractile length. B. During shoulder abduction without scapular rotation,
the deltoid reaches its maximal shortening and is unable to pull the glenohumeral joint through its full abduction ROM.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

145

on muscle mechanics.) Thus without the contributions of the
scapulothoracic joint motion, passive ROM of shoulder flexion
and abduction are reduced by at least one third. However,
active ranges in these two directions appear to be even more
severely affected.

In addition to the overall loss of passive and active excursion,
decreased scapulothoracic joint motion impairs the synergistic
rhythm between the scapulothoracic and glenohumeral joints.
This may contribute directly to abnormal glenohumeral joint
motion and result in an impingement syndrome.

Clinical Relevance

NO WONDER SHOULDER IMPINGEMENT IS SO
COMMON!: Shoulder impingement syndrome is the most
common source of shoulder complaints, and the complicated
and finely coordinated mechanics of the shoulder complex
help explain the frequency of complaints [67]. Earlier clinical
relevance boxes demonstrate the possible contributions to
impingement syndromes from dysfunction within individual
components of the shoulder complex , such as abnormal
axial rotation of the humerus or abnormal scapular positions
[50,53,61]. Abnormal scapulothoracic rhythm during shoul¬
der flexion or abduction is also associated with impingement
syndromes, although it is unclear whether the abnormal
rhythm is a cause or an effect of the impingement [12,60].

The multiple mechanical dysfunctions that can lead to
symptoms of impingement demonstrate the importance of
understanding the normal mechanical behavior of each indi¬
vidual component of the shoulder complex as well as the
behavior of the complex as a whole. With such an understand¬
ing the clinician will be able to thoroughly and accurately eval¬
uate the movements and alignments of the individual parts of
the shoulder as well as the coordinated function of the entire
complex in order to develop a sound strategy for intervention.

LOSS OF STERNOCLAVICULAR OR
ACROMIOCLAVICULAR JOINT MOTION

For the scapulothoracic joint to rotate upwardly 60°, the ster¬
noclavicular joint must elevate, and the acromioclavicular joint
must glide or rotate slightly. If the clavicle is unable to elevate
but the acromioclavicular joint can still move, the scapulotho¬
racic joint may still be able to contribute slightly to total shoul¬
der motion but is likely to have a significant reduction in
movement. The effects of lost or diminished scapulothoracic
joint motion noted in the preceding section would then follow.
If acromioclavicular joint motion is lost, disruption of scapu¬
lothoracic joint motion again occurs, although perhaps to a
lesser degree than with sternoclavicular joint restriction.

It is important to recognize the potential plasticity of the
shoulder complex. Decreased motion at the acromioclavicu¬
lar joint appears to result in increased sternoclavicular
motions, and decreased motion at the sternoclavicular joint
results in increased motion at the acromioclavicular joint [82].

Inman et al. report that one subject with the acromioclavicu¬
lar joint pinned had only 60° of shoulder elevation remaining
[40]. However, others report far less dysfunction with loss of
motion at the acromioclavicular joint [51]. Perhaps the effects
of the loss of acromioclavicular motion depend upon the via¬
bility of the remaining structures or the presence of pain.

Case reports suggest that total resection of the clavicle sec¬
ondary to neoplastic disease and chronic infection have no
negative effects on passive ROM of the shoulder [57].
However, scapulohumeral rhythms are not reported.
Similarly in another study, 71% of the individuals who under¬
went distal resection of the clavicle to decrease acromioclav¬
icular pain returned to recreational sports [24]. These data
suggest that while there is clear interplay among the four
joints of the shoulder complex, there also appears to be a
remarkable capacity to compensate for losses by altering the
performance of the remaining structures. However, an
important consequence of such alterations may be the over¬
use of remaining joints or the development of hypermobility
elsewhere in the system. Therefore, diagnosis of mechanical
impairments of the shoulder requires careful assessment of
overall shoulder function but also identification of each joints
contribution to the shoulders total motion.

Clinical Relevance

IDENTIFYING LINKS BETWEEN A PATIENT S
COMPLAINTS AND ABNORMAL JOINT MOBILITY: A

60-year-old male patient came to physical therapy with
complaints of shoulder pain. He reported a history of a
severe "shoulder" fracture from a motorcycle accident 30
years earlier. He noted that he had never regained normal
shoulder mobility. However, he reported that he had good
functional use of his shoulder. He owned a gas station and
was an auto mechanic and was able to function fully in
those capacities, but he reported increasing discomfort in his
shoulder during and after activity. He noted that the pain
was primarily on the "top" of his shoulder.

Active and passive ROM were equally limited in the symp¬
tomatic shoulder: 0-80° of flexion, 0-70° of abduction, 0°
medial and lateral rotation. Palpation during ROM revealed a
1:1 ratio of scapular to arm-trunk motion, revealing that all
of the arm-trunk motion was coming from the scapulotho¬
racic joint. Palpation revealed tenderness and crepitus at the
acromioclavicular joint during shoulder movement.

These findings suggested that in the absence of gleno¬
humeral joint motion, the sternoclavicular and acromioclavic¬
ular joints developed hypermobility as the patient maximized
shoulder function, ultimately resulting in pain at the acromio¬
clavicular joint. This impression was later corroborated by
radiological findings of complete fusion of the glenohumeral
joint and osteoarthritis of the acromioclavicular joint. Since
there was no chance of increasing glenohumeral joint mobility,
treatment was directed toward decreasing the pain at the
acromioclavicular joint.

146

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

It should be clear that because shoulder motion originates
from several locations and normally occurs in a systematic and
coordinated manner, evaluation of total shoulder function
depends on the ability to assess the individual components
and then consider their contributions to the whole. The eval¬
uation requires consideration of the shoulder movement as a
whole as well. The following section presents a review of nor¬
mal arm-trunk ROM.

SHOULDER RANGE OF MOTION

Values of “normal” ROM reported in the literature are pre¬
sented in Table 8.2. Examination of this table reveals large dif¬
ferences among the published values of normal ROM,
particularly in extension, abduction, and lateral rotation of the
shoulder complex. Unfortunately, many authors offer no
information to explain how these normal values were deter¬
mined. Consequently, it is impossible to explain the disparity
displayed in the literature.

All but one of the references report that lateral rotation is
greater than medial rotation. The two studies that report
empirical data also suggest that abduction ROM may be slightly
greater than flexion ROM, although direct comparisons are
not reported [8,71]. In addition, these two studies indicate that
gender and age may have significant effects on these values.
Thus at the present time, values of “normal” ROM must be
used with caution to provide a perspective for the clinician
without serving as a precise indicator of the presence or
absence of pathology. The clinician must also consider the
contributions to the total motion made by the individual com¬
ponents as well as the sequencing of those contributions.

Clinical Relevance

SHOULDER MOTION IN ACTIVITIES OF DAILY LIVING:

Magermans et at. report the shoulder mobility required in
diverse activities of daily living (ADL) [62]. Activities such as
combing one's hair use an average of 90° of glenohumeral
flexion or abduction , 70° of lateral rotation of the shoulder ;
and approximately 35° of concomitant scapular upward
rotation. In contrast personal hygiene activities such as per¬
ineal care use glenohumeral hyperextension and essentially
full medial rotation ROM. As the clinician strives to help a
patient regain or maintain functional independence, the
clinician must work to ensure that the mobility needed for
function is available and that all four components of the
shoulder complex contribute to the mobility appropriately.

SUMMARY

This chapter examines the bones and joints of the shoulder
complex, which allow considerable mobility but possess
inherent challenges to stability. The bones provide little limi¬
tation to the motion of the shoulder under normal conditions.
The primary limits to normal shoulder motion are the capsu-
loligamentous complex and the surrounding muscles of the
shoulder. The normal function of the shoulder complex
depends on the integrity of four individual joint structures
and their coordinated contributions to arm-trunk motion.
The glenohumeral joint is the sole contributor to medial and
lateral rotation of the shoulder and contributes over 50% of

TABLE 8.2: Normal ROM Values from the Literature (in Degrees)

Flexion

Extension

Abduction

Medial

Rotation

Lateral

Rotation

Abduction
in Scapular Plane

Steindler [93]

180

30-40

150

US Army/Air Force [20]

180

Boone and Azin [8] a

165.0 ± 5.0

57.3 ± 8.1

182.7 ± 9.0

67.1 ± 4.1

99.6 ± 7.6

Hislop and Montgomery [36p

180

170

Murray, et al [71]

170 ± 2 C

57 ± 3 C

178 ± 1 c

49 ± 3 C

94 ± 2 C

172 ± 1 d

58 ± 3 d

180 ± 1 rf

53 ± 3 d

101 ± 2 d

165 ± 2 e

55 ± 2 e

178 ± 1 e

59 ± 2 e

82 ± 4 e

170 ± V

61 ± 2 f

178 ± V

56 ± 2 f

94 ± 2<

Gerhardt and Rippstein [27]

170

Bagg and Forrest [1]

168.1

Freedman and Munro [25]

167.17 ± 7.57

a Data from 56 adult males. These values are also used as "normal" values by the American Academy of Orthopedic Surgeons.

^Reported wide ranges from the literature.

c Data from 20 young adult males.

d Data from 20 young adult females.

e Data from 20 male elders.

'Data from 20 female elders.

Chapter 8 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE SHOULDER COMPLEX

147

the motion in arm-trunk elevation. The remaining arm-trunk
elevation comes from upward rotation of the scapula. The
scapula also undergoes posterior tilting and lateral rotation
about a vertical axis during arm-trunk elevation. In addition
to glenohumeral and scapulothoracic contributions to
arm-trunk elevation, the sternoclavicular and acromioclavic¬
ular joints contribute important motions to allow full, pain-
free arm-trunk elevation. Impairments in the individual
joints of the shoulder complex produce altered arm-trunk
movement and are likely contributors to complaints of pain in
the shoulder complex.

Throughout this chapter the importance of muscular sup¬
port to the shoulder is emphasized. The following chapter pres¬
ents the muscles of the shoulder complex and discusses their
contributions to the stability and mobility of the shoulder.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity at the
Shoulder Complex

CHAPTER CONTENTS

AXIOSCAPULAR AND AXIOCLAVICULAR MUSCLES.151

Trapezius .152

Serratus Anterior.157

Levator Scapulae, Rhomboid Major, and Rhomboid Minor.161

Pectoralis Minor .163

Subclavius .166

Sternocleidomastoid.166

Summary of Axioscapular and Axioclavicular Muscles.167

SCAPULOHUMERAL MUSCLES.167

Deltoid.167

Supraspinatus .170

Infraspinatus .172

Teres Minor .173

Subscapularis.174

Teres Major .177

Coracobrachialis .178

Summary of the Scapulohumeral Muscles .179

AXIOHUMERAL MUSCLES .179

Pectoralis Major.179

Latissimus Dorsi.182

Shoulder Depression.183

Summary of the Axiohumeral Muscles .183

MUSCLE STRENGTH COMPARISONS .184

SUMMARY .185

T he preceding chapter describes the bones and joints of the shoulder complex as well as the interaction among
these structures. The present chapter presents the muscles that support and move this complex (Fig. 9.1). The
purpose of this chapter is to

■ Describe the characteristics of the individual muscles

■ Discuss how these muscles work together to provide both mobility and stability to the shoulder complex
■ Discuss how impairments of these muscles contribute to the pathomechanics of the shoulder

150

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

151

Figure 9.1: Superficial muscles of the shoulder complex. The muscles of the shoulder are grouped by their functions to move the scapu-
lothoracic joint, move the glenohumeral joint, or add additional force to the whole complex. A. Anterior view. B. Posterior view.

It is important to recognize that many muscles of the shoulder have actions on the axioskeleton as well. These actions
are addressed in the appropriate chapters on the head, vertebral column, and trunk. The present chapter focuses on
the shoulder muscles' contribution to shoulder function.

The muscles of the shoulder can be divided into three groups according to their attachments and the joints they affect.
These groups are

■ Axioscapular and axioclavicular

■ Scapulohumeral

■ Axiohumeral

Each group is discussed separately so that the clinician can recognize the primary function of each group as well as the
functions of the individual muscles that compose each group.

AXIOSCAPULAR AND AXIOCLAVICULAR
MUSCLES

The muscles of the axioscapular and axioclavicular group all
possess an attachment on the axioskeleton as well as on the
shoulder girdle, that is, on the scapula or clavicle (Fig. 9.2).
The primary role of these muscles is to position the scapula
and clavicle by moving the sternoclavicular and scapulotho-
racic joints, with resulting motion at the acromioclavicular

joint. To understand fully the role of these muscles, it is
important to recall that the scapula s only bony attachment is
at the small acromioclavicular joint. Thus the scapula floats
freely on the thorax, supported primarily by muscles. The
muscles of the axioscapular group frequently work in teams to
hold the scapula stable as it moves on the thorax. The
axioscapular and axioclavicular group includes the following
muscles: trapezius, serratus anterior, levator scapulae, rhom¬
boid major and minor, pectoralis minor, subclavius, and

152

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Serratus

Figure 9.2: Axioscapular and axioclavicular muscles. The axioscapular and axioclavicular muscles (A) on the anterior surface of the trunk
are the subclavius, pectoralis minor, and sternocleidomastoid muscles. The posterior axioscapular muscles (B) are the trapezius, rhom¬
boid major and minor, levator scapulae, and serratus anterior.

sternocleidomastoid. Each is discussed separately below. The
muscles that function together as teams are also discussed.

Trapezius

The trapezius is composed of three distinct bellies: upper,
middle, and lower (Fig. 9.3) (Muscle Attachment Box 9.1).
Each of these has its unique function, and each contributes
significantly to the function of the trapezius as a whole. The
muscle s attachments on both the clavicle and scapula indi¬
cate that it acts at both the sternoclavicular and scapulotho-
racic joints. The actions and effects of weakness and tightness
of each belly of the muscle are discussed below. Then the role
of the whole muscle is presented.

ACTIONS OF THE UPPER TRAPEZIUS
MUSCLE ACTION: UPPER TRAPEZIUS

Action

Evidence

Elevation of sternoclavicular joint

Supporting

Scapular elevation

Supporting

Scapular adduction

Supporting

Scapular upward rotation

Supporting

Careful cadaver dissection of individual fascicles of the
trapezius reveals that the upper trapezius actually has much
smaller muscle bundles than the other two portions of the
trapezius [31]. This study also suggests that the fibers of the
upper trapezius attach only to the clavicle, with no direct
attachment on the scapula.

Electromyography (EMG) reveals considerable activity in
the upper trapezius during active elevation of the shoulder
girdle as in a shoulder shrug (Fig. 9.4 ) [8,11,12]. Gradual
descent of the scapula from the elevated position in the
upright position also is accompanied by contraction of
the upper trapezius, presumably as an eccentric control of the
action [11,12]. Similarly, scapular adduction in the erect
standing position elicits significant upper trapezius activity.
Thus EMG data support the notion that the upper trapezius
actively elevates and adducts the shoulder girdle. Although
there are no known studies that directly examine the upper
trapezius muscles contribution to upward rotation of the
scapulothoracic joint, its attachment on the lateral clavicle is
consistent with that function, since elevation of the clavicle
must accompany normal scapulothoracic joint upward rota¬
tion. EMG studies reporting activity of the upper trapezius
during shoulder abduction indirectly support its role as an

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

153

Figure 9.3: Trapezius muscle. The trapezius is divided into upper,
middle, and lower parts.

MUSCLE ATTACHMENT BOX 9.1

ATTACHMENTS AND INNERVATION
OF THE TRAPEZIUS

Proximal attachment: Medial third of the superior
nuchal line, external occipital protuberance, ligamen-
tum nuchae, the tips of the spinous processes, and
the supraspinous ligaments from C7 through T12.

Distal Attachment: Posterior aspect of the lateral
one third of the clavicle, medial aspect of the
acromion, superior lip of the crest of the scapular
spine, and the medial aspect and tubercle of the
scapular spine.

Innervation: Spinal accessory nerve (11th cranial
nerve, spinal portion). It also receives sensory fibers
from the ventral rami of C3 and C4.

Palpation: The trapezius is superficial, and all three
portions of the trapezius are easily palpated and
distinguished from one another.

Figure 9.4: Function of the upper trapezius. The upper trapezius is
active during a shoulder shrug (A) and during scapular adduction (B).

154

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

upward rotator of the scapula because upward rotation is an
essential ingredient of shoulder abduction (30,49).

The role of the upper trapezius in maintaining an erect
posture is less certain. While some studies do demonstrate
electrical activity in the upper trapezius during quiet upright
standing, other EMG studies of the upper trapezius reveal lit¬
tle or no activity in quiet erect posture, even though in the
upright posture the weight of the upper extremity tends
to depress the clavicle and scapula [3,27]. According to
Basmajian, even the addition of weights in the hand has no
apparent facilitating effect on the upper trapezius muscle [3].
Additional studies are needed to determine if these differ¬
ences in the literature represent methodological and popula¬
tion differences in the various investigations or the range of
responses seen in a normal population. The resting tension of
a normal upper trapezius probably does contribute passively
to the upward support of the shoulder complex through its
attachment on the clavicle [5,27]. Thus even without a direct
attachment on the scapula, there remains broad agreement
that the upper trapezius plays some role in supporting the
shoulder girdle in erect posture [5,27,31].

EFFECTS OF WEAKNESS OF THE UPPER TRAPEZIUS

Isolated upper trapezius weakness is unusual but is likely to
contribute to diminished strength in elevation of the shoulder
girdle. Steindler notes that standing posture in the presence of
trapezius weakness is characterized by depression, abduction,
and forward tilting of the scapula [75]. Bearn notes that
although the clavicle is depressed with trapezius paralysis, the
depression is not as great as expected, nor is the resulting
depression all that is available [5]. Even though some studies
cited in the previous section deny active upper trapezius con¬
tributions to upright posture, the postural abnormalities typi¬
cally associated with upper trapezius weakness may be the
result of the loss of sufficient resting tone in a weakened upper
trapezius. However, they may also be the result of weakness
throughout the whole trapezius muscle. Additional studies are
needed to demonstrate a clear relationship between strength
of the upper trapezius and postural alignment.

EFFECTS OF TIGHTNESS OF THE UPPER TRAPEZIUS

Tightness of the upper trapezius is associated with elevated
shoulders or asymmetrical head positions as well as restricted
head and neck ranges of motion (ROM). However because
other scapular elevators exist, it is difficult to identify pure
upper trapezius tightness. If the upper trapezius alone is tight,
scapular elevation is likely to be accompanied by upward rota¬
tion of the scapula. Therefore, careful assessment of scapular
position is essential to distinguish among the scapular elevators.

ACTIONS OF THE MIDDLE TRAPEZIUS
MUSCLE ACTION: MIDDLE TRAPEZIUS

Actions

Evidence

Scapular adduction

Supporting

Scapular elevation

Supporting

The middle trapezius is regarded as a pure scapular adduc¬
tor because of its horizontally aligned fibers. In cadaver
specimens, the middle trapezius has the largest cross-
sectional area of the three trapezius muscle segments [31].
Thus the middle trapezius provides considerable strength
in scapular adduction and plays an important role in stabi¬
lizing the scapula. EMG studies report activity during
shoulder shrugs, suggesting its upper fibers may assist the
smaller upper trapezius in scapular elevation [30].

WEAKNESS OF THE MIDDLE TRAPEZIUS

Weakness of the middle trapezius results in a significant
decrease in strength of scapular adduction. Isolated weakness
of the middle trapezius is unusual, although some authors
suggest that it can occur from prolonged stretch of the mus¬
cle as might occur in a posture characterized by scapular
abduction [35]. However, attempts to correlate scapular posi¬
tion with middle trapezius strength have been unsuccessful
thus far [14]. Loss of middle trapezius strength also presents
difficulties when contracting the scapulohumeral muscles.
For example, the lateral rotators of the shoulder, including
the infraspinatus and posterior deltoid muscles, require a sta¬
ble scapula to exert their force at the glenohumeral joint.
Decreased scapular adduction strength can allow these mus¬
cles to pull the scapula toward the humerus instead of pulling
the humerus toward the scapula.

TIGHTNESS OF THE MIDDLE TRAPEZIUS

Tightness of the middle trapezius alone is rare because the
weight of the entire upper extremity pulls the scapula toward
abduction.

ACTIONS OF THE LOWER TRAPEZIUS
MUSCLE ACTION: LOWER TRAPEZIUS

Actions

Evidence

Scapular depression

Supporting

Scapular adduction

Inadequate

Scapular upward rotation

Supporting

Careful inspection of the line of pull of the lower trapez¬
ius explains its potential contributions to all these actions.
Such inspection also suggests that the muscle is best suited
for depression and upward rotation. The line of pull of the
lower trapezius is ideal for depression of the scapula.
However, in the upright posture, the weight of the upper
extremity already pulls the scapula toward depression.
Additional depression by activity of the lower trapezius is
not required. In contrast, when the subject is prone, manu¬
al resistance against scapular depression does elicit electri¬
cal activity of the lower trapezius [8]. The importance of the
scapular depression force provided by the lower trapezius is
most apparent with simultaneous contraction of the upper

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

155

trapezius. This combined activity is discussed when the
trapezius is discussed as a whole.

Clinical Relevance

MANUAL MUSCLE TESTING OF THE LOWER
TRAPEZIUS: In the prone position with the shoulder flexed,
the weight of the upper extremity tends to pull the scapula
into elevation. Consequently, the lower trapezius is used to sta¬
bilize the scapula (Fig. 9.5). Thus the resistance in the "Fair"
test of the lower trapezius is the weight of the upper extremity

EMG activity of the lower trapezius during isometric shoulder
adduction from the abducted position may also support the
role of the lower trapezius as a scapular adductor [30].

To understand the lower trapezius muscle s contribution to
upward rotation of the scapula, it is essential to recall the loca¬
tion of the axis for upward and downward scapular rotation
(Chapter 8). Although the precise location of the axis remains
controversial, it is clear that the axis lies lateral to the scapu¬
lar attachment of the lower trapezius on the root of the spine
of the scapula. Therefore, as the lower trapezius pulls the
medial aspect of the scapular spine inferiorly, the scapula
rotates upwardly (Fig. 9.6). Like the upper trapezius, the
activity of the lower trapezius during shoulder elevation sup¬
ports its role as an upward rotator of the scapula.

WEAKNESS OF THE LOWER TRAPEZIUS

Isolated weakness of the lower trapezius has been suggested
to be the consequence of prolonged stretch resulting from an
elevated and downwardly rotated scapula [35]. However,
there are no known studies that verify such a relationship.
Weakness of the lower trapezius may lead to difficulty in

Figure 9.5: MMT position for the lower trapezius. The lower
trapezius stabilizes the scapula as the weight of the upper
extremity tends to elevate the scapula on the thorax when the
subject lies prone.

Figure 9.6: Upward rotation of the scapula by the lower trapezius.
The attachment of the lower trapezius on the medial aspect of
the spine of the scapula causes upward rotation of the scapula.

stabilizing the scapula during contraction of the other upward
rotators of the scapula.

TIGHTNESS OF THE LOWER TRAPEZIUS

Tightness of the lower trapezius theoretically results in
decreased elevation and downward rotation ROM of the
scapulothoracic joint and perhaps a depressed and posteriorly
tilted shoulder girdle in quiet standing. However, there are no
known reports of isolated lower trapezius tightness, although
a difference in shoulder height is often reported in healthy
adults. This difference reportedly is associated with hand
dominance [35,72]. The absence of identified lower trapezius
tightness despite the apparent scapular depression in some
individuals has several possible explanations. The depression
may be accompanied by concomitant scapular downward
rotation and/or abduction, which may balance or indeed
overcome the shortening effect of the depression (Fig. 9.7).
There may be no adaptive change in the lower trapezius

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.7: Length of the lower trapezius. The lower trapezius
may actually be stretched in a subject when the scapula is
abducted and downwardly rotated, producing a posture in
which the shoulder appears lowered.

Figure 9.8: Force couple composed of upper and lower trapezius.
The elevation and depression pulls by the upper and lower
trapezius, respectively, are balanced out while both rotate the
scapula upwardly.

despite the prolonged scapular depression because it is inter¬
rupted enough by scapular elevation during upper extremity
use. In addition, since there is no accepted standard of scapular
position in healthy individuals, what appears as scapular
depression actually may be contralateral elevation. Thus
impairments resulting from lower trapezius tightness are
unclear and may be nonexistent.

ACTIONS OF THE ENTIRE TRAPEZIUS

The actions of the whole trapezius can be considered the vec¬
tor sum of the forces from the upper, middle, and lower
trapezius muscles. As a whole the trapezius adducts and
upwardly rotates the scapula. The elevation and depression
motions of the upper and lower components, respectively,
balance each other out. In fact it is this balancing by two
opposing forces that is essential to the stability of the scapula.

These two muscles pulling in opposite directions and together
causing rotation of the scapula form what is known as an
anatomical force couple (Fig. 9.8). The combined action of
the upper and lower trapezius muscles allows the scapula to
rotate upwardly without being displaced superiorly or inferi-
orly on the thorax. An imbalance between these two muscles
either from tightness or weakness of one of them can lead to
difficulty in stabilizing the scapula during upward rotation of
the scapulothoracic joint (i.e., during shoulder flexion or
abduction).

The trapezius as a whole is an important contributor to the
scapular upward rotation that is a necessary ingredient of nor¬
mal arm-trunk flexion or abduction. It appears to play a larger
role in shoulder abduction than in shoulder flexion [27,49]. Its
greater role in shoulder abduction is consistent with the fact
that the muscle lies primarily in the frontal plane. Acting by
itself, the entire trapezius upwardly rotates the scapula and

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

157

adducts it. Yet normal arm-trunk elevation occurs without sig¬
nificant scapular adduction. Thus the whole trapezius requires
another muscle to balance its adduction component. This bal¬
ance is provided by the serratus anterior, described next.

Clinical Relevance

SPINAL ACCESSORY NERVE INJURY: Weakness of any
or all of the trapezius can result from an injury to the spinal
accessory nerve, which lies superficially in the posterior tri¬
angle of the neck, formed by the anterior border of the
upper trapezius muscle, the posterolateral border of the ster¬
nocleidomastoid muscle, and the middle third of the clavi¬
cle. The nerve can be injured during neck surgery, such as a
lymph node biopsy, or by a blow or laceration to the top of
the shoulder.

A patient with a spinal accessory nerve palsy typically
reports weakness in activities overhead. The individual's
posture may be characterized by a drooping shoulder and
the scapula may be pulled into abduction. The abducted
position of the scapula is accentuated during active shoul¬
der abduction and is sometimes known as lateral winging .
Assessment of shoulder strength reveals decreased strength
in shoulder elevation, particularly in shoulder abduction as
well as in weakness in the discrete movements of the scapula
attributable to the trapezius.

Serratus Anterior

The serratus anterior is a large muscle described by some as
a single muscle uniformly distributed along the costal
attachments (Muscle Attachment Box 9.2) (Fig. 9.9) [63].
Others describe the serratus anterior as consisting of a small
upper and a larger lower bundle, each having its separate
role [27,31,84].

Figure 9.9: Serratus anterior. The serratus anterior passes over
the anterior surface of the scapula to attach to its medial border.

MUSCLE ATTACHMENT BOX 9.2

ATTACHMENTS AND INNERVATION
OF THE SERRATUS ANTERIOR

Proximal attachment: Anterolateral surfaces and
superior borders of the first 8 to 10 ribs and the
intercostal muscles in between.

Distal attachment: Medial border of the ventral sur¬
face of the scapula from the superior to the inferior
angles.

Innervation: Long thoracic nerve, C5-7.

Palpation: The serratus anterior is most easily pal¬
pated at its attachment on the lateral thorax as it
interdigitates with the external oblique abdominal
muscle.

ACTIONS OF THE SERRATUS ANTERIOR
MUSCLE ACTION: SERRATUS ANTERIOR

Actions

Evidence

Scapular abduction

Supporting

Scapular upward rotation

Supporting

Scapular elevation

Supporting

Textbooks often name the action of the serratus anterior pro¬
traction. However, as discussed in Chapter 8, protraction can
mean abduction of the scapula with either upward or down¬
ward rotation. Therefore, to avoid ambiguity, this text lists the
actions specifically. When the serratus anterior is described as
two parts, the role of elevation is ascribed to the upper portion
and that of abduction and upward rotation to the larger lower
portion [12,27,84].

158

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

WEAKNESS OF THE SERRATUS ANTERIOR

Weakness of the serratus anterior usually results from injury
to its nerve supply, the long thoracic nerve. The long thoracic
nerve lies on the ventral surface of much of the muscle and
can be injured during surgical procedures such as mastec¬
tomies in which tumors close to the nerve must be excised.
Injuries also are reported following other surgical procedures
and during the administration of local anesthesia [32]. Direct
traction injuries to the nerve in young athletes are reported as
well [20]. In all reports of injury, the resulting impairments
are reported to be severe, although recovery is possible
[32,78,80].

Weakness of the serratus anterior results in weakness of
scapular abduction, upward rotation, and, to some extent,
scapular elevation. Scapular abduction is used to reach

Figure 9.10: Function of the serratus anterior. The serratus is
active as an individual pushes forward on a revolving door.

forward. So weakness of the serratus anterior is apparent
when pushing forward against a resistance, as in pushing a
revolving door forward (Fig. 9.10). In this situation the door
exerts a reaction force on the upper extremity (including the
shoulder girdle) that tends to adduct the scapula. In the
absence of sufficient serratus anterior strength, the scapula
slides medially on the thorax. Because the serratus anterior
attaches to the medial aspect of the ventral surface of the
scapula, the serratus anterior holds the scapula firmly onto
the thorax. Consequently, in the presence of serratus anterior
weakness, forces that adduct the scapula also tend to cause
the medial aspect of the scapula to protrude posteriorly from
the thorax. This is known as medial winging and is a sign of
weakness of the serratus anterior prominent during abduc¬
tion of the scapula against resistance (Fig. 9.11). Medial wing¬
ing due to weakness of the serratus anterior also is apparent
during active shoulder flexion and abduction.

To understand fully the mechanics of this winging during
active shoulder elevation, the role of the serratus anterior
with the trapezius muscle in shoulder elevation must be
understood. Recall that the trapezius adducts and upwardly
rotates the scapula while the serratus anterior abducts and
upwardly rotates the scapula. Recall also that the scapula is

Figure 9.11: Medial winging of the scapula due to serratus ante¬
rior weakness. The scapula is pushed into adduction and wings
medially as an individual pushes forward against a wall in the
presence of serratus anterior weakness.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

159

free to slide on the posterior thorax. Finally, recall that flexion
and abduction of the shoulder require approximately 60° of
upward rotation of the scapula. The trapezius and serratus
anterior form another force couple that rotates the scapula
upwardly while counteracting the adduction and abduction
components of the respective muscles (Fig. 9.12). This com¬
bined activity is essential to stabilize the scapula during
arm-trunk flexion and abduction [2]. In contrast to the
trapezius, the serratus anterior plays a greater role in shoul¬
der flexion in keeping with its orientation closer to the sagit¬
tal plane [27,49].

Since both the trapezius and serratus anterior muscles
contribute to scapular rotation during shoulder elevation,
distinguishing between weakness of one or the other is crit¬
ical to choosing an appropriate treatment. As noted above,
weakness of the serratus anterior is manifested by a classic
physical sign called medial winging of the scapula. This
winging is apparent during upper extremity activities
requiring serratus anterior contraction including active

Middle

trapezius

Lower

trapezius

Figure 9.12: Force couple formed by trapezius and serratus
anterior. The adduction and abduction pulls of the trapezius and
serratus anterior counteract each other while the two muscles
produce upward rotation of the scapula.

shoulder elevation, particularly shoulder flexion. Scapular
winging secondary to serratus anterior weakness is a
protrusion of the medial border of the scapula away from
the thorax, visible during active shoulder elevation, espe¬
cially flexion. It results from the residual imbalance of mus¬
cle pull on the scapula. The serratus anterior is attached to
the ventral surface of the scapula, while the trapezius is
attached to the dorsal surface. During normal cocontrac¬
tion of these muscles, the dorsal pull of the trapezius tends
to adduct the scapula and to pull it slightly dorsally.
However, the simultaneous ventral pull of the serratus
anterior muscle holds the vertebral border of the scapula
firmly to the thorax (Fig. 9.13). With weakness of the serra¬
tus anterior muscle, there is loss of this ventral pull, and the
scapulas medial border protrudes posteriorly from the
thorax; that is, it wings.

Figure 9.13: Transverse plane view of the scapula. A transverse
view of the scapula on the thorax reveals how the pull of the
serratus anterior on the medial border of the scapula stabilizes
the scapula against the pull of the trapezius on the lateral aspect
of the scapula.

160

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Clinical Relevance

SCAPULAR WINGING DUE TO SERRATUS ANTERIOR
WEAKNESS: Medial winging of the sapula during
arm-trunk elevation is a classic sign of weakness of the
serratus anterior muscle. Scapular winging due to serratus
anterior weakness is visible during activities requiring active
contraction of the serratus anterior. Such activities include
active shoulder flexion or abduction or resisted scapular
abduction (Fig. 9.14). In contrast winging of the scapula at
rest or during passive motion of the shoulder can be the
sign of restricted ROM at the glenohumeral joint or postural
abnormalities. For example, as described in the previous
chapter, anterior tilting of the scapula is an effective substi¬
tute for inadequate medial rotation of the glenohumeral
joint. The position of the scapula when it is tilted anteriorly
is similar to that seen with serratus weakness (Fig. 9.15).
However, in the case of glenohumeral joint restriction, the
prominence of the scapula occurs during activities such as
reaching into a back pocket, when no serratus anterior con¬
traction is required. Thus the scapular prominence in this
case cannot be the result of serratus anterior muscle weak¬
ness. This is a critical distinction for the clinician to make.

CONSEQUENCES OF WEAKNESS OF THE SERRATUS
ANTERIOR AND TRAPEZIUS MUSCLES

Chapter 8 describes the contribution the scapulothoracic
joint makes to shoulder motion. Passively, upward rotation of
the scapulothoracic joint contributes at least one third of the
shoulders total flexion and abduction ROM. However, loss of
scapulothoracic joint motion appears to have a more dramatic

Figure 9.14: Medial winging due to serratus anterior weakness
during resisted shoulder flexion. With serratus anterior weak¬
ness, the scapula wings medially during shoulder flexion when
upward rotation of the scapula is required.

Figure 9.15: Apparent medial winging due to decreased ROM.

An individual with decreased medial rotation ROM of the shoulder
can use anterior tilting of the scapula to reach behind the back.
The scapula appears to be winging; however, this position is not
the result of serratus anterior weakness, since the serratus anterior
is not required to perform the activity.

effect on active ROM of the shoulder. Weakness of the
trapezius and/or the serratus anterior hampers active shoul¬
der elevation in two ways. First, weakness in either or both of
these muscles limits, and perhaps eliminates, active upward
rotation of the scapulothoracic joint and thereby reduces the
active shoulder flexion or abduction ROM by at least one
third. Additionally, however, the normal upward rotation of
the scapula helps maintain adequate contractile length of the
deltoid and rotator cuff muscles to permit full movement of
the glenohumeral joint. Thus weakness of either or both of
the upward rotators of the scapula not only impairs the
motion of the scapula but also disrupts the actions of the mus¬
cles at the glenohumeral joint. Weakness of the serratus ante¬
rior and or trapezius muscles can result in profound disability
at the shoulder complex.

Clinical Relevance

WEAKNESS OF THE SERRATUS ANTERIOR OR
TRAPEZIUS MUSCLE: Weakness of either the serratus
anterior or the trapezius results in impaired function in both

( continued )

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

161

(Continued)

flexion and abduction of the shoulder [20,50]. Regardless of
whether the serratus anterior, the trapezius, or both are
weak, the weakness impairs the active motion of the scapu-
lothoracic joint. Abnormal scapulothoracic joint rotation
during shoulder elevation may contribute to impingement of
the contents of the subacromial space. Consequently, a
patient's complaints originating from weakness of the
scapular rotators usually consist of complaints of weakness
and difficulty reaching overhead but also may include com¬
plaints of pain when attempting overhead activities.

Similarly, an evaluation of an individual with a shoulder
impingement syndrome must include a careful assessment
of the muscles that rotate the scapula upward.

Su et al. [76] assessed 20 competitive swimmers with
complaints and signs consistent with subacromial impinge¬
ment syndrome and 20 matched swimmers without
complaints or signs of impingement. Scapular movements
during shoulder elevation were similar in both groups
prior to swim practice. However, after a hard practice,
those swimmers with complaints exhibited significantly
less upward rotation of the scapula. These data suggest
the importance of active scapular control during repetitive
shoulder elevation activities in protecting against
impingement syndromes.

TIGHTNESS OF THE SERRATUS ANTERIOR

Although tightness of the serratus anterior muscle is rarely
described, tightness of the upper portion can conceivably
occur with tightness of the upper trapezius. Such tightness
may result in a posture characterized by elevated shoulders
and upwardly rotated scapulae.

Levator Scapulae, Rhomboid Major,
and Rhomboid Minor

The levator scapulae, rhomboid major, and rhomboid minor
are almost parallel to each other, running superiorly and
medially from the vertebral border of the scapula to the ver¬
tebral column (Fig. 9.16) (Muscle Attachment Boxes 9.3 and
9.4). These three muscles produce essentially the same
motions at the scapulothoracic joint. However, their attach¬
ments on the vertebral column result in differing actions at
the spine. The rhomboid muscles cause contralateral rotation
of the cervical spine, and the levator scapulae produces ipsi-
lateral rotation of the cervical spine. These differences allow
the careful clinician to distinguish between the rhomboid
muscles and the levator scapulae. Actions of these muscles at
the spine are detailed in Chapter 27. The actions of these
three muscles on the scapula as well as the effects of their
weakness and tightness are very similar and are discussed
together.

Rhomboid:
Minor

Major

Figure 9.16: Levator scapulae, rhomboid major, and rhomboid
minor muscles. The line of pull is approximately the same for the
levator scapulae and the rhomboid major and minor.

MUSCLE ATTACHMENT BOX 9.3

ATTACHMENTS AND INNERVATION
OF THE LEVATOR SCAPULAE

Proximal attachment: Posterior tubercles of the
transverse processes of the first four cervical
vertebrae.

Distal attachment: Medial border of the scapula
between the superior angle and scapular spine.

Innervation: Spinal nerves C3-5. The contribution
from C5 is by way of the dorsal scapular nerve.

Palpation: The levator scapulae is palpated between
the upper trapezius and the sternocleidomastoid,
particularly with elevation and downward rotation
of the scapula.

162

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

MUSCLE ATTACHMENT BOX 9.4

ATTACHMENTS AND INNERVATION OF
THE RHOMBOID MAJOR AND MINOR

Proximal attachment: The rhomboid major is attached
proximally to the spines and supraspinous ligaments
of the second through fifth thoracic vertebrae. The
rhomboid minor muscle has a more superior attach¬
ment on the inferior portion of the ligamentum
nuchae and the spinous processes of C7 and T1.

Distal attachment: The rhomboid major attaches to
the medial border of the scapula from the root of
the spine to the inferior angle. The rhomboid minor
attaches to the medial aspect of the scapula at the
level of the scapular spine.

Innervation: Dorsal scapular nerve, C4 and 5.

Palpation: The fibers of the rhomboid major and
minor are approximately parallel to one another.
These two muscles lie deep to the trapezius but can
be palpated through the trapezius during motions
combining active downward rotation and adduction
of the scapula.

ACTIONS OF THE LEVATOR SCAPULAE, RHOMBOID
MAJOR, AND RHOMBOID MINOR

MUSCLE ACTION: LEVATOR SCAPULAE, RHOMBOID
MAJOR, AND RHOMBOID MINOR

Actions

Evidence

Scapular elevation

Supporting

Scapular adduction

Supporting

Scapular downward rotation

Supporting

Inman demonstrates active contraction of the levator scapu¬
lae along with the upper trapezius and upper portion of the
serratus anterior muscles in quiet standing, suggesting that
these muscles are providing upward support for the shoulder
girdle and upper extremity [27]. However, Johnson notes that
only the levator scapulae and the rhomboid major and minor
muscles can directly suspend the scapula [31]. EMG studies
show that in the presence of voluntary relaxation of the upper
trapezius in quiet standing, there is an increase in EMG
activity of the two rhomboid muscles but a decrease in activity
in the levator scapulae [56]. These data support the notion
that the rhomboid muscles can and do support the upright
position of the shoulder girdle, at least under certain circum¬
stances. Whether the levator scapulae contributes additional
support remains debatable.

Inspection of the lines of pull of these three muscles sug¬
gests that the rhomboid muscles are better aligned to adduct
the scapula than is the levator scapulae. EMG activity in the
rhomboids increases with resisted adduction of the scapulae,

supporting their role as adductors [68]. One study shows the
rhomboid major and minor muscles active with the middle
trapezius during shoulder flexion and abduction, with more
activity in the latter motion [27]. However, most authors
agree that the trapezius and serratus anterior muscles are the
primary scapulothoracic muscles needed for shoulder flexion
and abduction.

Downward rotation of the scapula may be used as an indi¬
vidual reaches into a back hip pocket or scratches the middle
of the back (Fig. 9.17). These activities elicit considerable
EMG activity in the rhomboid muscles (greater than 50% of
the EMG elicited during a manual muscle test) [74]. The
attachment of the levator scapulae, rhomboid major, and
rhomboid minor along the vertebral border of the scapula
allows them to rotate the scapula downwardly about its axis
located lateral to the root of the spine. As they contract to
downwardly rotate the scapula, they cause simultaneous ele¬
vation and adduction of the scapula. Therefore, to obtain
more-isolated downward rotation these muscles require the
contraction of another muscle to provide a balance. The mus¬
cle that forms an anatomical force couple with the levator
scapulae, rhomboid major, and rhomboid minor to produce
isolated downward rotation of the scapulothoracic joint is the
pectoralis minor. It is discussed later in this chapter.

WEAKNESS OF THE LEVATOR SCAPULAE,

RHOMBOID MAJOR, AND RHOMBOID MINOR

Pulling actions such as pulling open doors and rowing can be
impaired by weakness of the levator scapulae, rhomboid

Figure 9.17: Function of the levator scapulae and the rhomboid
major and minor. Reaching to a back pocket typically requires
the rhomboid major and minor and the levator scapulae.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

163

major, and rhomboid minor. Weakness of these muscles also
is cited as a cause of a posture characterized by rounded
shoulders. Some suggest that the muscles are needed for
erect posture and thus their weakness allows the scapulae to
abduct and depress [14]. However, no studies have success¬
fully identified a relationship between the strength of these
muscles and postural alignment. Nor has the incidence of
weakness of these muscles been described. Consequently,
while there is a widespread belief that weakness of these mus¬
cles may contribute to postural impairment of the shoulder
girdle, this relationship has yet to be established.

TIGHTNESS OF THE LEVATOR SCAPULAE,

RHOMBOID MAJOR, AND RHOMBOID MINOR

Like weakness of the levator scapulae, rhomboid major, and
rhomboid minor, tightness in these muscles has been described
as the basis for the rounded-shoulders posture [35]. Adaptive
shortening of these muscles results in elevation, adduction, and
downward rotation of the scapula. This position turns the gle¬
noid fossa downward and causes the scapula to tilt anteriorly,
thus lowering and rounding the shoulder. Consequently, tight¬
ness of these three muscles results theoretically in a complex
three-dimensional positional change of the scapula. However,
no direct link between tightness of these muscles and postural
abnormalities has been established. The difficulty in establish¬
ing the link between tightness of these muscles and postural
abnormalities lies in the difficulty in quantifying the exact posi¬
tion of the scapula and the muscles’ true length. Until results of
studies using more precise measurement tools are available, a
clear understanding of the postural impact of tightness or
weakness of the levator scapulae, rhomboid major, and rhom¬
boid minor muscles will remain elusive.

Clinical Relevance

PAIN IN THE LEVATOR SCAPULAE, RHOMBOID
MAJOR, AND RHOMBOID MINOR: Pain and tenderness
in the ievator scapulae, rhomboid major, and rhomboid
minor are common clinical findings [56,77]. Both weakness
and tightness have been described as the explanation for
pain along the medial aspect of the scapula and at its
superior angle. At the present time, although there are wide¬
spread beliefs regarding the contributions of weakness and
tightness to these complaints, there are no clear findings
supporting or refuting these beliefs. Therefore, it is essential
that more precise means of assessing strength and tightness
of these muscles become available.

Pectoralis Minor

The pectoralis minor muscle is an unusual axioscapular mus¬
cle because it lies entirely on the anterior surface of the tho¬
rax and attaches to the coracoid process, an anterior projec¬
tion of the scapula (Muscle Attachment Box 9.5) (Fig. 9.18).

MUSCLE ATTACHMENT BOX 9.5

ATTACHMENTS AND INNERVATION
OF THE PECTORALIS MINOR

Proximal attachment: The anterior surfaces and
superior borders of the third through fifth ribs close
to the costal cartilages but may include the second
or sixth rib as well. Attachment is also provided by
the fascia covering the external intercostal muscles
in the area.

Distal attachment: Medial border and superior
surface of the coracoid process of the scapula.

Innervation: Medial and lateral pectoral nerves,
C5-T1.

Palpation: This muscle lies deep to the pectoralis
major and consequently is difficult to palpate.
However, it may be palpated just inferior to the
coracoid process of the scapula during activities that
elicit active contractions of the muscles in the force
couple for downward rotation of the scapula.

Figure 9.18: Pectoralis minor, subclavius, and sternocleidomastoid
muscles. The pectoralis minor, subclavius, and sternocleidomas¬
toid muscles lie on the anterior aspect of the thorax.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Even the serratus anterior, which has an anterior attachment
on the thorax, attaches posteriorly on the scapula. The anterior
location of the pectoralis minor has a rather dramatic effect
on the actions resulting from contraction of the pectoralis
minor. Its location also produces confusing contradictions in
its reported actions.

ACTION

MUSCLE ACTION: PECTORALIS MINOR

Actions

Evidence

Scapular anterior tilt

Supporting

Scapular elevation

Inadequate

Scapular depression

Inadequate

Scapular adduction

Inadequate

Scapular abduction

Inadequate

Scapular upward rotation

Inadequate

Isolated contraction of the pectoralis minor pulls the coracoid
process anteriorly, tipping the scapula anteriorly (Fig. 9.19).
However, because the scapula lies on the posterior aspect of
the thorax, for it to tip forward, it also must elevate. Thus the
pectoralis minor elevates the scapula as it tips it anteriorly. Yet
inspection of the line of pull of the pectoralis minor reveals that
it is aligned to pull the coracoid process down. When other

Figure 9.19: Function of the pectoralis minor. Contraction of the
pectoralis minor pulls the coracoid process anteriorly, causing the
scapula to tilt anteriorly and to elevate.

muscles contract to prevent the anterior tilting of the scapula
caused by the pull on the coracoid process by the pectoralis
minor, the pectoralis minor with these other muscles con¬
tributes to scapular depression. Chapter 8 notes that there is
very little ROM of scapulothoracic joint depression available.
Thus active contraction of the pectoralis minor muscle along
with other scapular depressors is most important when the
upper extremity is exposed to an upwardly directed external
load such as the reaction force from a crutch that applies an
elevation force on the scapula through the upper extremity.
Under this circumstance the pectoralis minor and other shoul¬
der depressor muscles provide a depressive force to stabilize
the scapula and shoulder girdle against the force of elevation
(Fig. 9.20 ) [24]. Therefore, the pectoralis minor elevates the

Figure 9.20: Function of the pectoralis minor to depress the
shoulder. The pectoralis minor exerts a downward force on the
scapula to stabilize it against the reaction force of a crutch that
is directed upward and tends to elevate the shoulder.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

165

scapula when contracting alone and depresses the shoulder gir¬
dle when contracting with other shoulder depressors.

Similarly, inspection of the line of pull of the pectoralis
minor creates confusion regarding its role in abduction and
adduction of the scapula [24,89]. The muscle certainly pulls
medially on the coracoid process, which is interpreted by
some as scapular adduction. However, the muscles position
on the anterior aspect of the thorax means that its anterior
pull on the coracoid process causes the scapula to slide ante¬
riorly on the thorax, causing the scapula to abduct. Hence the
pectoralis minor abducts the scapula despite its medial pull
on the coracoid process.

The ability of the pectoralis minor to abduct the scapula
makes it a suitable partner with the levator scapulae, rhom¬
boid major, and rhomboid minor muscles in an anatomical
force couple for downward rotation of the scapula. The pec¬
toralis minors action of abduction balances the adduction
component of the levator scapulae, rhomboid major, and
rhomboid minor, while together they contribute to the scapu¬
la s downward rotation (Fig. 9.21) [63,84]. The action of these
muscles in scapular downward rotation creates a confusing
clinical picture in which the inferior angle of the scapula is
elevated but the acromion is depressed. Inspection of only
one of these landmarks can lead a clinician to conclude
that the scapula is elevated or depressed when it is merely

downwardly rotated. Clinicians must use caution when ana¬
lyzing the position of the scapula.

WEAKNESS OF THE PECTORALIS MINOR

Weakness may contribute to difficulty in controlling the
shoulder girdle, particularly during upper extremity weight¬
bearing activities such as crutch walking. It may also decrease
the stability of the scapula during activities requiring down¬
ward rotation of the scapulothoracic joint, since weakness of
the pectoralis minor disrupts the force couple for scapular
downward rotation.

TIGHTNESS OF THE PECTORALIS MINOR

Tightness of the pectoralis minor will pull the scapula into an
anterior tilt (Fig. 9.22). Additionally tightness of the pectoralis
minor may, with the other muscles of its force couple,
contribute to the “rounded shoulder” posture [6].
Individuals with shortened pectoralis minor muscles
exhibit less posterior tilting and more internal rotation of the
scapula during shoulder elevation [7]. The alterations in
scapular motions during shoulder elevations reported
in individuals with shortened pectoralis minor muscles
may increase the risk of impingement syndromes in these
individuals.

Figure 9.21: Force couple formed by the rhomboid muscles, the levator scapulae, and the pectoralis minor. The pectoralis minor abducts
the scapula and balances the adduction pull of the rhomboid major and minor and levator scapulae as all four muscles rotate the
scapula downward.

166

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.22: Tightness of the pectoralis minor. Tightness of the
pectoralis minor muscle pulls the scapula into an anterior tilt. In
supine the shoulder with tightness looks more forward than the
opposite side.

Clinical Relevance

TIGHTNESS OF THE PECTORALIS MINOR: The

brachial plexus and axillary blood vessels lie deep to the
pectoralis minor muscle. Therefore , a stretch of a tightened
pectoralis minor muscle can compress these sensitive struc¬
tures and cause symptoms radiating distally into the upper
extremity. Impingement of the brachial plexus or axillary
blood vessels by a tight pectoralis minor muscle is one form
of thoracic outlet syndrome (TOS) [44]. Neurological symp¬
toms typically include tingling and perhaps numbness in the
hand. Vascular symptoms may include blanching of the skin
and a diminished pulse. Exercises to stretch the pectoralis
minor muscle must proceed carefully to avoid exacerbating
the symptoms.

Subclavius

The subclavius is a small muscle binding the clavicle to the
first rib (Muscle Attachment Box 9.6). Because of its size and
location it is not well studied. It lies too deep to be palpated.

ACTIONS OF THE SUBCLAVIUS
MUSCLE ACTION: SUBCLAVIUS

Actions

Evidence

Sternoclavicular joint depression

Inadequate

The subclavius is believed to depress the clavicle. Since there
is actually little ROM of the sternoclavicular joint in the direc¬
tion of depression, this muscle, like the pectoralis minor, is
more likely a stabilizer of the clavicle against forces that tend
to elevate the sternoclavicular joint, such as weight bearing on

MUSCLE ATTACHMENT BOX 9.6

ATTACHMENTS AND INNERVATION
OF THE SUBCLAVIUS

Proximal attachment: Junction of the first rib and
first costal cartilage, anterior to the costoclavicular
ligament.

Distal attachment: Inferior surface of the middle
one third of the clavicle.

Innervation: Subclavian branch of the upper trunk
of the brachial plexus, C5 and C6.

Palpation: This muscle cannot be palpated directly.

the upper extremity. There are many other muscles that also
depress the shoulder. Their role in stabilizing the shoulder
during weight-bearing activities is discussed at the end of this
chapter. The one known EMG study of the subclavius sug¬
gests that the muscle contracts to stabilize the sternoclavicu¬
lar joint, reinforcing the ligamentous supports [62].

EFFECTS OF WEAKNESS AND TIGHTNESS
OF THE SUBCLAVIUS

Because the subclavius has not been studied in detail, effects
of weakness and tightness can only be theorized. Weakness is
unlikely to have significant effects on strength, since there
are many other larger muscles to depress the sternoclavicu¬
lar joint. However, this muscles contribution to dynamic
stabilization of the joint would be lost in the presence of sub¬
clavius weakness. Tightness is likely to bind the clavicle onto
the first rib, thus limiting elevation at the sternoclavicular
joint. The effects of diminished sternoclavicular joint eleva¬
tion are discussed in detail in Chapter 8. That discussion
reveals that inadequate sternoclavicular joint elevation is
likely to impair shoulder elevation ROM either by limiting
scapulothoracic joint upward rotation and thus restricting
total shoulder ROM or by causing excessive acromioclavicu¬
lar joint excursion and pain during shoulder elevation.

Sternocleidomastoid

The sternocleidomastoid is regarded generally as a muscle of
the head and neck (Muscle Attachment Box 9.7). However, its
attachment to the clavicle allows it to participate with other
axioscapular and axioclavicular muscles to position the shoul¬
der girdle.

MUSCLE ACTION: STERNOCLEIDOMASTOID

Actions

Evidence

Sternoclavicular joint elevation

Conflicting

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

167

MUSCLE ATTACHMENT BOX 9.7

ATTACHMENTS AND INNERVATION
OF THE STERNOCLEIDOMASTOID

Proximal attachment: Lateral surface of the mastoid
process and lateral half or one third of the superior
nuchal line of the occiput.

Distal attachment: By two heads to the manubrium
of the sternum and the superior surface of the
medial one third of the clavicle.

Innervation: Spinal accessory nerve and from the
ventral rami of C2-3.

Palpation: This muscle is easy to palpate at its distal
attachments. However, contraction of the sternoclav¬
icular is elicited best with side bending and con¬
tralateral rotation of the head. These movements
are discussed again in Chapter 27.

The sternocleidomastoid purportedly can assist in elevating
the clavicle. However, because the axes of the sternoclavicu¬
lar joint are lateral to the joint itself, the sternocleidomastoid’s
attachment on the clavicle is very close to the axes of motion
of the sternoclavicular joint. Therefore, the muscle has a very
short moment arm and a poor mechanical advantage for ster¬
noclavicular joint elevation [5]. Its actions and impairments
are more readily observed at the head and neck. It is dis¬
cussed in greater detail in Chapter 27 with other muscles of
the head and neck.

Summary of Axioscapular
and Axioclavicular Muscles

The muscles of the axioscapular and axioclavicular group
position and stabilize the shoulder girdle. Movement of the
scapulothoracic and sternoclavicular joints is essential to the
full and normal motion of the shoulder complex. For exam¬
ple, a complex three-dimensional model of the shoulder
apparatus suggests that the scapulothoracic muscles provide
up to 45% of the energy to flex the shoulder rapidly through
small excursions [22]. Therefore, weakness of these muscles
can disrupt the motion of the shoulder complex by prevent¬
ing or limiting the essential contribution from the shoulder
girdle and seriously altering the mechanics of the whole
shoulder complex.

The scapulothoracic muscles’ role in stabilizing the scapula
also is critical to the proper function of the shoulder. Because
the scapula is free to glide across the posterior thorax, mus¬
cles of the axioscapular and axioclavicular group frequently
contract in pairs, creating anatomical force couples. These
force couples are composed of muscles that exert opposing
forces to stabilize the scapula while providing similar forces to

produce a rotation. The ability of the scapulohumeral muscles
to move the glenohumeral joint depends upon their contrac¬
tion from a stable scapula. If the scapula is not fixed ade¬
quately, pull from any of the scapulohumeral muscles may
move the scapula instead of the humerus. Recognition of
these general principles dictating the function of the muscles
in this group provides the clinician with the tools to identify
the abnormal mechanics contributing to a patients com¬
plaints of shoulder pain or dysfunction.

SCAPULOHUMERAL MUSCLES

The scapulohumeral muscles provide motion and dynamic
stabilization to the glenohumeral joint (Fig. 9.23). The
glenohumeral joint provides over 50% of the ROM of
arm-trunk elevation. Therefore, these muscles are critical to
the active mobility of the shoulder as a whole. The muscles of
the scapulohumeral group are the deltoid, teres major, coraco-
brachialis, and the four muscles of the rotator cuff, which are
the supraspinatus, infraspinatus, teres minor, and subscapu-
laris. The functions of these muscles are intimately related to
each other, particularly those of the deltoid and rotator cuff
muscles. The deltoid and the rotator cuff muscles are first
presented individually; then their functional interplay is
presented. The remaining two scapulohumeral muscles are
then discussed.

Deltoid

The deltoid exhibits substantial change from the deltoid of
lower primates and other mammals (Muscle Attachment
Box 9.8) [27]. It has greatly increased in size in keeping with
the increased breadth of the scapula, described in Chapter 8.
The expansion of the acromion in humans increases the
mechanical advantage of the deltoid as the distal migration
of the deltoid tuberosity effectively increases the deltoids
contractile length [27]. These changes improve the deltoids
ability to move the glenohumeral joint through its large
available ROM.

The deltoid is divided into three parts: anterior, middle,
and posterior (Fig. 9.24). Like some of the axioscapular mus¬
cles, the deltoids individual components have unique actions
that are presented first, followed by the effects of impair¬
ments of each component. The muscle’s actions and impair¬
ments as a whole are then discussed.

ACTIONS OF THE ANTERIOR DELTOID
MUSCLE ACTION: ANTERIOR DELTOID

Actions

Evidence

Shoulder flexion

Supporting

Shoulder medial rotation

Conflicting

Shoulder abduction

Conflicting

Shoulder horizontal adduction

Inadequate

168

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.23: Scapulohumeral muscles.
The scapulohumeral muscles consist of
the deltoid, supraspinatus, infraspina¬
tus, teres minor, subscapularis, teres
major, and coracobrachialis.

There is general agreement regarding the anterior deltoid
muscle s contribution to flexion of the shoulder [3,24,35,39,
63,82,84]. However, there is less agreement about its role in
the other actions. Some EMG studies report activity of the
anterior deltoid muscle in medial rotation, while others deny
activity [3,39]. Analysis of its moment arm supports its role as
a medial rotator in most positions [41].

MUSCLE ATTACHMENT BOX 9.8

ATTACHMENTS AND INNERVATION
OF THE DELTOID

Proximal attachment: The anterior and superior sur¬
faces of the lateral one third of the clavicle, the lat¬
eral border and superior surface of the acromion,
and the lower lip of the crest of the scapular spine.
It is a multipennate muscle.

Distal attachment: Deltoid tuberosity.

Innervation: Axillary nerve, C5-6.

Palpation: Each part of the deltoid muscle is easily
identified over the superior aspect of the shoulder.

Brandell and Wilkinson demonstrate selective contrac¬
tion of the anterior deltoid during isometric contraction
with resistance in the combined direction of abduction and
flexion with the shoulder slightly rotated laterally (Fig. 9.25)
[8]. This position represents a standard position to test the
strength of the anterior deltoid muscle in manual muscle
testing, and these data support the view that the position
selectively recruits the anterior deltoid without the other
two portions of the deltoid [35]. Although there are no
known studies denying the anterior deltoids activity in
horizontal adduction, only a few authors mention the
action at all [3,24,82].

Most of the EMG studies in the literature report data col¬
lected from only a few subjects and assess a relatively small
group of muscles [8,39]. Therefore, the normal variation in
recruitment patterns exhibited in a healthy population is
probably underrepresented. In addition, clearly certain
actions have multiple muscles contributing to the motion.
The recruitment order may be individual. For example,
Jackson et al. report that during shoulder flexion, the anterior
deltoid is generally recruited before the clavicular portion of
the pectoralis major muscle, but some individuals reverse that
order [28]. Thus continued investigation is required to clarify
the role of the anterior deltoid in actions of the shoulder. It
undoubtedly contributes to shoulder flexion, but its contribu¬
tion to other actions remains unclear.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

169

Figure 9.24: Deltoid muscle. The deltoid muscle consists of an
anterior, middle, and posterior portion.

EFFECTS OF WEAKNESS OF THE ANTERIOR DELTOID

The effects of weakness of the anterior deltoid muscle depend,
of course, upon the role it plays in the actions listed above.

Figure 9.25: The manual muscle test (MMT) position for the anter¬
ior deltoid muscle. EMG studies show that the MMT position of
flexion, abduction, and slight lateral rotation of the shoulder iso¬
lates the anterior deltoid better than other suggested positions.

Weakness of the anterior deltoid muscle is likely to produce
weakness in shoulder flexion. However, weakness may also
result in diminished strength of shoulder medial rotation,
shoulder abduction, and horizontal adduction.

EFFECTS OF TIGHTNESS OF THE ANTERIOR DELTOID

Like the effects of weakness, the effects of tightness of the
anterior deltoid depend on its actions. However, it is commonly
accepted that tightness of the anterior deltoid can contribute to
diminished shoulder extension and lateral rotation ROM.

ACTIONS OF THE POSTERIOR DELTOID
MUSCLE ACTION: POSTERIOR DELTOID

Actions

Evidence

Shoulder extension

Supporting

Shoulder lateral rotation

Conflicting

Shoulder abduction

Conflicting

Shoulder adduction

Conflicting

Shoulder horizontal abduction

Supporting

As with the anterior deltoid, there is confusion and disagree¬
ment regarding the actions of the posterior deltoid. There is
widespread agreement that the posterior deltoid muscle con¬
tributes to shoulder extension [3,8,82]. In one study, shoulder
hyperextension isolates posterior deltoid activity from the rest
of the deltoid muscle better than lateral rotation, abduction,
or combined movements [8]. Lateral rotation also is reported
by some to elicit activity of the posterior deltoid muscle, but
others deny its contribution [3,39]. Its moment arm for rota¬
tion is small but could produce lateral rotation with the shoul¬
der in neutral [41].

Several studies demonstrate that horizontal abduction acti¬
vates the posterior deltoid [2,3,8,71,82]. Finally, some authors
report activity in the posterior deltoid during abduction, while
others find it active during adduction [3,39]. Analysis of the
moment arm of the posterior deltoid supports its role as an
adductor of the shoulder in both the plane of the scapula and
in the frontal plane, especially with the shoulder laterally
rotated [4,40,55]. It is clear from this discussion that the full
role of the posterior deltoid in shoulder motion remains to be
elucidated. It is also likely that the position of the shoulder can
alter the line of pull of the posterior deltoid with respect to the
axes of motion of the shoulder. Such alteration may allow the
posterior deltoid to produce an action in one shoulder position
and the opposite action in another shoulder position where the
line of pull of the muscle has crossed the axis of motion.
Additional EMG studies combined with thorough analyses of
the muscle s line of pull are required to define clearly the role
of the posterior deltoid muscle.

EFFECTS OF WEAKNESS OF THE POSTERIOR
DELTOID

The effects of weakness of the posterior deltoid depend on its
roles listed above but surely include decreased shoulder

170

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

extension strength. Identification of additional effects requires
further study.

EFFECTS OF TIGHTNESS OF THE POSTERIOR DELTOID

As in weakness, the most likely effects of tightness of the pos¬
terior deltoid muscle are restricted shoulder flexion and hori-
zonal adduction ROM. Additional effects may include
reduced medial rotation ROM, but additional study is need¬
ed to determine this conclusively.

ACTIONS OF THE MIDDLE DELTOID

The middle deltoid is the only head of the deltoid that is mul-
tipennate [63,84]. In addition, it has a larger proximal attach¬
ment and a larger cross-sectional area than the other two
parts of the deltoid muscle [43,45]. These findings suggest
that the middle portion of the deltoid muscle is specialized for
force production.

MUSCLE ACTION: MIDDLE DELTOID

Actions

Evidence

Shoulder abduction

Supporting

Shoulder flexion

Supporting

Shoulder extension

Inadequate

There is little doubt that the middle deltoid is an
abductor of the shoulder. Although some authors
suggest that the anterior and posterior deltoid mus¬
cles also contribute to shoulder abduction [39], Brandell
and Wilkinson state that maximally resisted abduction with
neutral shoulder rotation or with slight medial rotation
yielded consistent isolated middle deltoid muscle activity
compared to the other parts of the deltoid muscle in three
individuals without shoulder pathology [8]. Regardless of
the contributions of the anterior and posterior segments,
the middle deltoid muscle is a major contributor to shoul¬
der abduction. It contracts throughout active abduction
but is most active in the middle of the ROM [39].
However, the role of the deltoid muscle as an abductor is
intimately related to the roles of the rotator cuff muscles.
Therefore, the action of the deltoid muscle (particularly
the middle deltoid) in abduction is revisited following the
presentation of the rotator cuff muscles.

EMG studies reveal activity of the middle deltoid during
shoulder flexion [2,3]. Analysis of the muscles moment arm
also supports its capacity to assist in shoulder flexion [40]. In
contrast, there is less evidence supporting a role in shoulder
extension [39].

EFFECTS OF WEAKNESS OF THE MIDDLE DELTOID

Loss of the middle deltoid weakens, but does not eliminate,
active abduction of the shoulder [25,82]. Case reports suggest
that deltoid paralysis results in only a moderate decrease in
abduction strength [82]. Effects of weakness in the abductor
component of the deltoid muscle is discussed again following
the discussion of the rotator cuff muscles. Weakness of the

middle deltoid muscle probably also contributes to decreased
strength in shoulder flexion.

EFFECTS OF TIGHTNESS OF THE MIDDLE DELTOID

It is unlikely that tightness of the middle deltoid muscle actu¬
ally can restrict shoulder adduction ROM. However, the posi¬
tion of shoulder adduction applies tension to the middle
deltoid and may cause pain or additional disruption to the
tendon of the deltoid or the bursa lying deep to it.

Supraspinatus

The supraspinatus is part of the rotator cuff, which also
includes the infraspinatus, teres minor, and subscapularis. All
of these muscles play an essential role in stabilizing the gleno¬
humeral joint. EMG data demonstrate activity in these mus¬
cles throughout most active shoulder elevation [27,39,65].
Some of this activity reflects the muscles’ function as prime
movers and some likely reflects their roles as dynamic stabi¬
lizers. This section presents the individual muscles and their
specific roles as prime movers. Following the discussion of
the individual muscles, their group function as dynamic stabi¬
lizers during shoulder motion is presented.

The supraspinatus muscle is the most superior muscle
of the rotator cuff group (Fig. 9.26) (Muscle Attachment
Box 9.9). It lies deep to the subacromial (subdeltoid) bursa,
the coracoacromial ligament, and the deltoid muscle and
acromion process [47].

ACTIONS OF THE SUPRASPINATUS
MUSCLE ACTION: SUPRASPINATUS

Actions

Evidence

Shoulder abduction

Supporting

Shoulder lateral rotation

Supporting

Shoulder medial rotation

Supporting

Shoulder stabilization

Supporting

There is general consensus that the supraspinatus is an
abductor of the shoulder [3,63,84]. Analysis of the muscles
abduction moment arm supports this view [55]. Maximum
activity of the supraspinatus with minimal activity in sur¬
rounding muscles is seen during shoulder abduction in the
plane of the scapula accompanied by lateral rotation [34].
However, a classic test of the integrity of the supraspinatus is
resisted shoulder abduction in the plane of the scapula with
medial rotation of the shoulder [44,85]. These positions sug¬
gest that the supraspinatus may contribute to either medial or
lateral rotation of the shoulder. Analysis of the moment arms
of the supraspinatus suggests that the posterior portion of the
muscle is capable of lateral rotation; the anterior portion has
a slight medial rotation moment arm when the shoulder is in
neutral or in flexion but can cause a lateral rotation moment
when the shoulder is moderately abducted [41,55]. EMG
activity supports the role of the supraspinatus in shoulder
lateral rotation [61].

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

171

Figure 9.26: Supraspinatus muscle. The supraspinatus lies deep
to the acromion, the deltoid, and the subacromial bursa.

Some references state that the supraspinatus “initiates”
shoulder abduction [63]. However EMG data clearly reveal
activity in all of the rotator cuff muscles and the deltoid mus¬
cle throughout the range of active abduction [27,39]. A study
in which the supraspinatus muscle was temporarily paralyzed
by a neural block demonstrates full active shoulder abduction
range even in the absence of any supraspinatus force [79].
These studies demonstrate that the supraspinatus muscle is
not solely responsible for the initiation of shoulder abduction.

The supraspinatus muscle also is reported to participate
specifically in stabilizing the glenohumeral joint in the infer¬
ior direction [3,73]. Although the deltoid muscle is well
aligned to prevent descent of the humeral head in the glenoid
fossa in quiet standing [21], some individuals show no activity
in the deltoid in upright posture [3]. In contrast, the
supraspinatus in some individuals exhibits EMG activity dur¬
ing erect standing, particularly as the upper extremity is
pulled inferiorly by a weight in the hand. The supraspinatus
helps stabilize the glenohumeral joint by exerting a horizontal

MUSCLE ATTACHMENT BOX 9.9

ATTACHMENTS AND INNERVATION
OF THE SUPRASPINATUS

Proximal attachment: Medial two thirds of the supra¬
spinous fossa and the overlying supraspinous fascia.

Distal attachment: Superior facet of the greater
tubercle of the humerus and the glenohumeral joint
capsule.

Innervation: Suprascapular nerve, C5-6.

Palpation: The superficial portion of the supraspinatus
muscle belly can be palpated in the supraspinous
fossa through a relaxed trapezius. The tendon of the
supraspinatus muscle also can be palpated at its inser¬
tion through a relaxed deltoid muscle with the shoul¬
der in extension and adduction. Data from cadavers
reveal that a position combining maximal adduction,
80-90° of medial rotation, and 30-40° of hyperexten¬
sion gives best exposure of the tendon [46].

pull to hold the humeral head against the glenoid process [3].
This role may be enhanced by the upward tilt of the glenoid
fossa in upright posture, because descent of the humeral
head on the upwardly turned glenoid fossa requires simulta¬
neous lateral movement of the humeral head (Fig. 9.27).

Figure 9.27: Pull of the supraspinatus. The medial pull of the
supraspinatus helps prevent inferior displacement of the humeral
head, since in an upwardly turned glenoid fossa, the humeral
head must move laterally as it slides inferiorly on the fossa.

172

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

A force that prevents lateral movement, such as the one
applied by the supraspinatus, also prevents descent.
However, since some authors suggest that the glenoid fossa is
actually turned downward in normal alignment, the role of
the supraspinatus in preventing inferior instability of the
glenohumeral joint remains unresolved.

Clinical Relevance

INFERIOR SUBLUXATION OF THE GLENOHUMERAL
JOINT: The proposed function of the supraspinatus in pre¬
venting the inferior subluxation of the glenohumeral joint is
facilitated by the upward tilt of the glenoid fossa. Thus
weakness of the muscles that suspend the scapula may con¬
tribute to inferior subluxations of the joint. Such inferior sub¬
luxations of the shoulder are frequently found in patients
with diffuse upper extremity weakness following stroke
(Fig. 9.28). Weakness of the trapezius is reportedly charac¬
terized by depression of the acromion process demonstrat¬
ing downward rotation of the scapula. Thus the inferior sub¬
luxation of the glenohumeral joint may be the result of the
combined effects of weakness of the supraspinatus and
trapezius muscles. Splints that provide an upward force on
the humerus to stabilize an inferiorly subluxed glenohumeral
joint are generally unsuccessful in reducing the subluxation.
Current treatment approaches include exercises to restore an
upward tilt of the glenoid fossa while facilitating the activity
of the rotator cuff muscles [13,64]. Additional study is
required to clarify the mechanism of inferior subluxation
and to optimize treatment. However, the position of the
scapula should be considered when developing strategies to
stabilize the glenohumeral joint.

Figure 9.28: Inferior subluxation of the glenohumeral joint.
Inferior subluxation of the glenohumeral joint is seen frequently
in individuals following a stroke. Downward rotation of the
scapula may decrease the stabilizing forces of the horizontally
aligned muscles and ligaments.

EFFECTS OF WEAKNESS OF THE SUPRASPINATUS

Weakness of the supraspinatus muscle is rather common. It
can result from denervation secondary to an entrapment of
the suprascapular nerve [36,51]. However, it more com¬
monly results from mechanical disruption of the muscle s
tendon or its insertion into the glenohumeral joint capsule.
Weakness may also result from inhibition of muscle con¬
traction caused by pain secondary to such disorders as ten¬
dinitis. Degeneration of the tendons of the rotator cuff with
age is well documented and is particularly evident in the
supraspinatus [9,66,67]. Inherent in this process of degen¬
eration is a decrease in the vascularity of the supraspinatus
tendon, predisposing the tendon to further damage.
Degeneration of the supraspinatus tendon is correlated
with a decrease in the material strength of the tendon.
Consequently, degeneration of the supraspinatus tendon
may be a causative factor in rotator cuff tears, particularly
since rotator cuff tears most frequently involve the
supraspinatus [84]. Thus there are many factors to consider
to explain the presence of supraspinatus weakness.

Weakness of the supraspinatus is manifested by a signifi¬
cant decrease in the strength and endurance of shoulder
abduction [79]. However, it must be emphasized that active
shoulder abduction is still possible, albeit significantly weak¬
ened, even in the presence of complete supraspinatus paraly¬
sis or disruption.

EFFECTS OF TIGHTNESS OF THE SUPRASPINATUS

Although spontaneous tightness of the supraspinatus tendon
is unlikely, it can be present following surgical repair of a rota¬
tor cuff tear. Consideration should be given to positions that
could stretch the supraspinatus since they should be avoided
in the presence of a rotator cuff tear or following repair of the
supraspinatus tendon. Adduction or medial rotation, particu¬
larly with shoulder hyperextension, stretches the supraspina¬
tus [53]. Shoulder adduction across the plane of the body also
may stretch the supraspinatus. Kelley advises care when exer¬
cising in the position of shoulder medial rotation and adduc¬
tion in the presence of supraspinatus pathology [33].

Infraspinatus

The infraspinatus is described in most anatomy textbooks as a
single muscle belly (Muscle Attachment Box 9.10) [63,84].
However, in biomechanical literature the muscle is described
with two or three separate portions (Fig. 9.29) [31,55].

ACTIONS OF THE INFRASPINATUS

MUSCLE ACTION:

Actions

Evidence

Shoulder lateral rotation

Supporting

Shoulder horizontal abduction

Supporting

Shoulder abduction

Supporting

Shoulder stabilization

Supporting

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

173

MUSCLE ATTACHMENT BOX 9.10

ATTACHMENTS AND INNERVATION
OF THE INFRASPINATUS

Proximal attachment: Medial two thirds of the
infraspinous fossa and overlying infraspinous fascia.

Distal attachment: Middle facet on the greater
tubercle of the humerus and the glenohumeral joint
capsule.

Innervation: Suprascapular nerve, C5-6.

Palpation: The infraspinatus muscle belly is palpable
in the infraspinous fossa lateral to the trapezius and
inferior to the deltoid muscle. The tendon also can
be palpated with the shoulder flexed, adducted,
and medially rotated [46].

The infraspinatus muscle is regarded by most authors as an
important and powerful lateral rotator muscle [24,35,63,84].
This is consistent with the muscle s large attachment on the
scapula and its large lateral rotation moment arm. Kuechle

describes it as the most efficient lateral rotator of the shoul¬
der [41]. EMG data and analysis of its moment arms also sup¬
port the role of the infraspinatus in horizontal abduction
[2,3,40]. Although not typically described as an abductor of
the shoulder, careful analysis of the moment arms of the indi¬
vidual parts of the infraspinatus suggests that the infraspina¬
tus also is positioned to contribute to the total abductor
moment [26,40,55]. Selective ablation of the infraspinatus
decreases the abduction moment produced by simulated
activity of the remaining muscles in cadaver specimens [52].

EFFECTS OF WEAKNESS OF THE INFRASPINATUS

Isolated weakness of the infraspinatus is unusual but has been
reported [36,42]. It is manifested clinically by a significant
reduction in the strength of lateral rotation of the shoulder.
More frequently, the infraspinatus is weakened together with
other muscles of the rotator cuff through a mechanical dis¬
ruption of the cuff itself.

EFFECTS OF TIGHTNESS OF THE INFRASPINATUS

Tightness of the infraspinatus contributes to decreased ROM
of shoulder medial rotation and may also contribute to
decreased horizontal adduction ROM. However, Muraki
et al. suggest that the posterior deltoid and posterior gleno¬
humeral joint capsule are more likely limiters [53].

Teres Minor

Some describe the teres minor as a distal belly of the deltoid
muscle, noting both their common innervation and the
attachment of the teres minor muscle distal to the gleno¬
humeral joint capsule (Muscle Attachment Box 9.11) [27,31].
It also has the smallest physiological cross-sectional area of
the rotator cuff muscles, although it is substantially larger
than in other mammals.

MUSCLE ATTACHMENT BOX 9.11

ATTACHMENTS AND INNERVATION
OF THE TERES MINOR

Proximal attachment: Superior two thirds of the
lateral aspect of the dorsal surface of the scapula,
lateral to the infraspinatus.

Distal attachment: Inferior facet of the greater
tubercle of the humerus and distally onto the shaft
of the humerus. It also attaches to the capsule of
the glenohumeral joint.

Innervation: Axillary nerve, C5-6.

Palpation: The teres minor muscle can be palpated
with the infraspinatus muscle.

174

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

ACTIONS OF THE TERES MINOR

MUSCLE ACTION: TERES MINOR

Actions

Evidence

Shoulder lateral rotation

Supporting

Shoulder adduction

Supporting

Shoulder stabilization

Supporting

The role of the teres minor as a lateral rotator of the shoulder
is well established [24,35,63,84]. However, its physiological
cross-sectional area is approximately one third that of the
infraspinatus [31]. Therefore, the teres minor can contribute
only a small additional amount of force to lateral rotation.
Although adduction is not mentioned as an action of the teres
minor in most anatomy texts, analysis of its moment arm sup¬
ports its ability to produce an adduction moment [55].

EFFECTS OF WEAKNESS OF THE TERES MINOR

Weakness of the teres minor can contribute to a decrease in
the strength of shoulder lateral rotation. However, since the
physiological cross-sectional area of the teres minor is so much
smaller than that of the other lateral rotators, the decrease in
lateral rotation strength is unlikely to be significant.

EFFECTS OF TIGHTNESS OF THE TERES MINOR

Isolated tightness of the teres minor is unlikely. The size of
the teres minor also suggests that tightness of the teres minor
by itself has little functional significance. However, tightness
of the teres minor is likely to be accompanied by tightness of
the infraspinatus. Together they limit medial rotation ROM.

Subscapularis

The subscapularis muscle is the largest of the rotator cuff
muscles (Muscle Attachment Box 9.12) (Fig. 9.30) [27,31,43].

ACTIONS OF THE SUBSCAPULARIS

MUSCLE ACTION:

Actions

Evidence

Shoulder medial rotation

Supporting

Shoulder flexion

Inadequate

Shoulder extension

Inadequate

Shoulder abduction

Supporting

Shoulder adduction

Supporting

Shoulder horizontal adduction

Inadequate

Shoulder stabilization

Supporting

There is broad agreement regarding the role of the sub¬
scapularis in medial rotation of the shoulder [24,35,63,84].
The remaining actions are reported infrequently. The role
of the subscapularis in abduction and adduction may depend
on the position of the glenohumeral joint [71]. Analysis of the
moment arms of the subscapularis muscle suggest that it may
adduct when the shoulder is medially rotated but may abduct

MUSCLE ATTACHMENT BOX 9.12

ATTACHMENTS AND INNERVATION
OF THE SUBSCAPULARIS

Proximal attachment: Subscapularis fossa and the
lateral border of the ventral surface of the scapula.

It also attaches to tendinous intramuscular septa and
the aponeurosis that covers the muscle ventrally.

Distal attachment: Lesser tubercle of the humerus
and the anterior aspect of the glenohumeral joint
capsule.

Innervation: Upper and lower subscapular nerves,
C5-6 and perhaps C7.

Palpation: This muscle is difficult to palpate but can
be felt in the axilla by palpating the ventral surface
of the scapula when the scapula is abducted. Cadaver
data also suggest that the tendon is palpable in the
deltopectoral triangle with the upper extremity
against the thorax and the shoulder in neutral [46].

Figure 9.30: Subscapularis muscle. The subscapularis is the only
rotator cuff muscle on the anterior aspect of the glenohumeral
joint.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

175

when the shoulder is in neutral or is laterally rotated
[26,40,55].

One explanation for the confusion about the actions of the
subscapularis may be the fact that the subscapularis contracts
during most active motions of the glenohumeral joint [39].
However, that recorded activity may be the activity of
the subscapularis to stabilize the glenohumeral joint.
Distinguishing its role as a dynamic stabilizer of the shoulder
from prime mover of the shoulder is difficult and requires
concerted effort combining EMG data and careful analysis
of the muscle s line of pull.

EFFECTS OF WEAKNESS OF THE SUBSCAPULARIS

Weakness of the subscapularis results in a significant decrease
in strength of shoulder medial rotation [19]. Weakness of the
subscapularis may also contribute to anterior instability of the
glenohumeral joint.

Clinical Relevance

SUBSCAPULARIS WEAKNESS: Decreased activation of
the subscapularis is reported in some individuals who can
sublux their glenohumeral joints spontaneously using lateral
rotation [10,29,33]. Muscle re-education to facilitate the sub¬
scapularis and other medial rotators is an important compo¬
nent of the rehabilitation program to increase stability [48].

EFFECTS OF TIGHTNESS OF THE SUBSCAPULARIS

Tightness of the subscapularis causes decreased lateral rota¬
tion ROM at the shoulder. Tightness of the subscapularis
muscle sometimes is induced deliberately to improve joint
stability surgically in individuals with chronic anterior disloca¬
tions of the glenohumeral joints.

DYNAMIC STABILIZATION BY THE ROTATOR CUFF

Chapter 8 presents the role of the noncontractile supporting
structures of the glenohumeral joint in stabilizing the joint.
While these structures provide some stability, they are insuf¬
ficient to stabilize the joint against large forces and in all joint
positions. EMG and cadaver studies as well as mathematical
models consistently indicate the importance of the rotator
cuff in stabilizing the glenohumeral joint [2,22]. The rotator
cuff muscles provide critical additional support to the joint.
One study demonstrates that contraction of the rotator cuff
prevents visible instability of the glenohumeral joint during
shoulder movement, even in the presence of large anterior
disruptions of the joint capsule [1]. This same study suggests
that contraction of the rotator cuff muscles even prevents dis¬
location after complete anterior-posterior disruption of the
capsule. Conversely, decreased contraction force of the rota¬
tor cuff results in increased anterior and posterior gliding of
the glenohumeral joint during abduction in the plane of the

scapula [86]. Weakness may also allow increased superior
glide of the humeral head during shoulder elevation.

Clinical Relevance

ROTATOR CUFF MUSCLES AND REHABILITATION
OF THE UNSTABLE GLENOHUMERAL JOINT: Studies
demonstrating the importance of rotator cuff activity in sta¬
bilizing the glenohumeral joint suggest that these muscles
should be evaluated carefully in the presence of gleno¬
humeral joint instability. Additionally, exercises to strengthen
the rotator cuff muscles are an important element of the
treatment of the unstable shoulder [17,38].

COORDINATED ACTIVITY OF DELTOID
AND ROTATOR CUFF MUSCLES DURING
SHOULDER ELEVATION

The roles of the deltoid and rotator cuff muscles in producing
shoulder flexion and abduction are well studied. These stud¬
ies grew out of the clinical observation that individuals with
rotator cuff weakness, particularly of the supraspinatus, had
severe difficulty elevating the shoulder. These observations
led to the myth that the supraspinatus is responsible for initiat¬
ing shoulder abduction, which has subsequently been refuted
although not completely abandoned. Clinical evidence sup¬
ported, but did not explain, the integral role of the rotator cuff
in arm-trunk elevation. Careful anatomical and biomechani¬
cal studies have now provided firm evidence for, and a clear
explanation of, the integrated function of the deltoid and the
rotator cuff during these motions.

When the glenohumeral joint is in the neutral position, the
deltoid muscle has a small angle of application, or moment
arm, for abduction, while the supraspinatus has a larger
abduction moment arm (1.42 vs. 2.6 cm) (Fig. 9.31) [55]. In
this position the line of pull of the middle deltoid, the primary
abductor of the shoulder, is directed mostly superiorly, so that
contraction of the middle deltoid muscle tends to produce
superior translation of the humeral head on the glenoid fossa
rather than an abduction rotation. The supraspinatus has a
mechanical advantage by virtue of its larger moment arm, so
contraction by the supraspinatus tends to produce abduction
while simultaneously compressing the glenohumeral joint.
Thus the deltoid and supraspinatus muscles form another
anatomical force couple to produce abduction. However, the
physiological cross-sectional area of the supraspinatus muscle
is considerably smaller that that of the deltoid muscle, and
consequently, the supraspinatus is incapable of generating
large abduction moments. Thus powerful abduction requires
the simultaneous activity of both the deltoid and supraspina¬
tus muscles.

Unrestricted superior glide of the humeral head results in
compression of the contents of the subacromial space. However,
as the deltoid contracts at the beginning of elevation, all of the

176

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.31: Moment arms of the deltoid and supraspinatus
for abduction. The abduction moment arm of the supraspinatus
muscle is slightly greater than that of the deltoid muscle with
the shoulder in neutral. However, the deltoid's moment arm
improves in the midrange of abduction.

rotator cuff muscles also contract and exert a compressive
force on the proximal humerus holding the head of the
humerus firmly against the glenoid fossa. Simultaneously,
the teres minor, the lower portion of the infraspinatus, and
the subscapularis apply an inferior force on the humeral
head, providing additional protection against superior glide
of the humeral head (Fig. 9.32 ) [27,58]. The contraction of
the teres minor, infraspinatus, and subscapularis with the del¬
toid is another example of an anatomical force couple in
which the upward and downward pulls of the muscles are bal¬
anced, and the forces contribute to abduction.

By midrange of abduction the mechanical advantage of the
deltoid improves, its abduction moment arm exceeding that
of the supraspinatus [55]. Thus as the moments required to
abduct the upper extremity increase, the large deltoid muscle
is better able to perform the task. However, the entire rotator
cuff continues contracting throughout the full range of
abduction, providing continued stabilizing forces to the
glenohumeral joint [27,39]. Details of the forces involved in
this function are presented in Chapter 10.

Thus abduction and flexion of the glenohumeral joint
depend on three factors: the deltoid, the supraspinatus, and the
depressors of the humeral head, including the infraspinatus,
teres minor, and subscapularis muscles. The deltoid provides

Figure 9.32: Force couple formed by the deltoid and the infra¬
spinatus, teres minor, and subscapularis. The upward pull of the
deltoid is balanced by the downward pull of the infraspinatus,
teres minor, and subscapularis muscles.

strength to the movement; the supraspinatus provides
mechanical advantage early in the ROM and, with the rest of
the rotator cuff, joint compression throughout the movement;
and the infraspinatus, teres minor, and subscapularis muscles
stabilize the humeral head inferiorly. Loss of any of these
elements results in significant impairment in the ability to
elevate the shoulder.

Clinical Relevance

ROTATOR CUFF WEAKNESS: ANOTHER POSSIBLE
CAUSE OF IMPINGEMENT SYNDROME: The rotator cuff
muscles seem to be particularly susceptible to fatigue and
overuse ’ especially in middle-aged adults. Thus it is not sur¬
prising to see middle-aged patients who report a history of
acute onset of shoulder pain following unusual and pro¬
longed overhead activity such as three sets of tennis at the
beginning of the tennis season or an afternoon of window
washing. A likely scenario to explain the complaints is (a) pro¬
longed overhead activity; (b) fatigue of the rotator cuff mus¬
cles; (c) inadequate stabilization of the humeral head; and
(d) superior glide of the humerus causing compression of the
contents of the subacromial space ’ including the subacromial

(continued)

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

177

(Continued)

bursa and supraspinatus tendon; with (e) resuiting bursitis or
tendinitis. Successful treatment of the patient's complaints
must include interventions to reduce inflammation of the
bursa or tendon. These interventions include medication , rest
and ice. However ; treatment should also address the underly¬
ing pathomechanics, with particular focus on strength and
endurance training for the rotator cuff muscles. Patient educa¬
tion explaining the relationship between fatigue and patho¬
mechanics also may help the patient avoid a recurrence.

Teres Major

ACTIONS OF THE TERES MAJOR

MUSCLE ACTION:

Actions

Evidence

Shoulder medial rotation

Supporting

Shoulder extension

Supporting

Shoulder adduction

Supporting

The teres major is not well studied (Muscle Attachment Box
9.13) (Fig. 9.33). However, available EMG data reveal activi¬
ty of the teres major muscle during all three of these actions
in the presence of resistance but no activity without resistance
unless the shoulder is hyperextended [3]. The teres major
appears to be recruited without resistance during shoulder
hyperextension and during adduction in the hyperextended
position. Moment arm analysis supports its potential as a
medial rotator in most shoulder positions [41].

The teres major also exhibits EMG activity with the shoulder
held in static positions of flexion or abduction [27]. This contra¬
dicts the classic view of the actions of the teres major. However,
the teres major is also able to pull on the scapula when the
humerus is held fixed. Perhaps the reported activity of the teres
major during isometric shoulder flexion or abduction is to assist
in stabilizing the scapulothoracic joint rather than to move or
hold the glenohumeral joint (Fig. 9.34). Additional study is
needed to clarify its role in shoulder movements.

MUSCLE ATTACHMENT BOX 9.13

ATTACHMENTS AND INNERVATION
OF THE TERES MAJOR

Proximal attachment: Dorsal surface of the inferior
angle of the scapula and surrounding fascia.

Distal attachment: Medial lip of the intertubercular
groove of the humerus.

Innervation: Lower subscapular nerve, C6-7 and per¬
haps C5.

Palpation: This muscle is easily identified at the infe¬
rior angle of the scapula.

Humerus

Teres

major

Latissimus dorsi

Figure 9.33: Teres major and latissimus dorsi. The teres major and
latissimus dorsi have similar directions of pull on the shoulder.

EFFECTS OF WEAKNESS OF THE TERES MAJOR

Because few studies exist investigating the role of the teres
major muscle, the effects of weakness can only be hypothe¬
sized. It is logical to expect weakness in shoulder medial
rotation, extension and hyperextension, and adduction with
weakness of the teres major. However data are needed to
substantiate these expectations.

EFFECTS OF TIGHTNESS OF THE TERES MAJOR

Again, in the absence of detailed data, tightness of the teres
major can be expected to result in restricted ROM in shoul¬
der lateral rotation, flexion, and abduction. The tightness
could also influence the resting position and mobility of the
scapulothoracic joint. Specifically, tightness of the teres major

178

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.34: Role of the teres major during overhead lifts. A
heavy overhead load tends to adduct the scapula, and the con¬
tracting teres major may help to stabilize the scapula.

could pull the scapula into a position of abduction and
upward rotation, contributing to another variant of the
rounded-shoulders posture (Fig 9.35).

Coracobrachialis

ACTIONS OF THE CORACOBRACHIALIS

MUSCLE ACTION:

Actions

Evidence

Shoulder flexion

Inadequate

Shoulder adduction

Inadequate

Figure 9.35: Tightness of the teres major. Tightness of the teres
major can pull the scapula into upward rotation if the humerus
is fixed.

The coracobrachialis is even more poorly studied than the
teres major (Muscle Attachment Box 9.14) (Fig. 9.36). An
examination of the moment arm of the coracobrachialis in
cadaver shoulders positioned in 90° of abduction and lateral
rotation supports its role as a shoulder flexor [4]. In this posi¬
tion the muscle has almost negligible moment arms for shoul¬
der adduction and lateral rotation.

MUSCLE ATTACHMENT BOX 9.14

ATTACHMENTS AND INNERVATION
OF THE CORACOBRACHIALIS

Proximal attachment: Tip of the coracoid process of
the scapula.

Distal attachment: Middle of the medial aspect of
the shaft of the humerus between the attachments
of the triceps brachii and the brachialis muscles.

Innervation: Musculocutaneous nerve, C6-7 and per¬
haps C5.

Palpation: The coracobrachialis is palpable in the
proximal arm just distal to the attachment of
the pectoralis major and medial to the tendon
of the short head of the biceps muscle.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

179

Figure 9.36: Coracobrachialis and pectoralis major. The coraco-
brachialis and pectoralis major contribute to shoulder flexion.

EFFECTS OF WEAKNESS OR TIGHTNESS
OF THE CORACOBRACHIALIS

Effects of weakness must be hypothesized and include dimin¬
ished strength in flexion and adduction of the shoulder.
Effects of tightness also must be hypothesized and presum¬
ably include decreased ROM in abduction and extension of
the shoulder. Isolated tightness of the coracobrachialis is
unlikely.

Summary of the Scapulohumeral Muscles

The scapulohumeral muscles are responsible for positioning
the glenohumeral joint as well as providing dynamic sta¬
bility to the joint. Weakness of these muscles can seriously
decrease the strength of shoulder motions. Additionally,
weakness can impair the stability of the glenohumeral joint,
contributing to a variety of movement dysfunctions ranging
from glenohumeral suhluxations to shoulder impingement
disorders. Careful analysis of each of these muscles is essen¬
tial to identify the basis of glenohumeral joint dysfunction.

AXIOHUMERAL MUSCLES

The axiohumeral muscles include the pectoralis major and the
latissimus dorsi. These muscles attach to the thorax and to the
humerus. Thus their fibers cross and consequently affect all
four joints of the shoulder complex. The muscles described in
the previous two sections, axioscapular, axioclavicular, and
scapulohumeral, are capable of moving all four joints of the

shoulder complex through their full ROM. The two muscles of
the axiohumeral group are redundant for the purposes of pro¬
viding full ROM of the shoulder complex. Rather, these two
muscles are characterized by their massive attachment sites
and large physiological cross-sectional areas. These character¬
istics suggest that the roles of the pectoralis major and the
latissimus dorsi are to provide additional strength to the move¬
ments of the shoulder. Between them, the pectoralis major
and the latissimus dorsi assist in all motions of the shoulder
except lateral rotation. Their combined role in shoulder
depression is discussed specifically at the end of this section.

Pectoralis Major

The pectoralis major has two distinct bellies, a smaller clavic¬
ular portion and a much larger sternal portion, each named
for its proximal attachment (Muscle Attachment Box
9.15). These two portions can function together or
independently of one another. The following discusses
the muscle as a whole and then presents the individual
components of the pectoralis major.

ACTIONS OF THE PECTORALIS MAJOR

MUSCLE ACTION:

Actions

Evidence

Shoulder medial rotation

Supporting

Shoulder adduction

Inadequate

Shoulder horizontal adduction

Inadequate

Shoulder depression

Supporting

MUSCLE ATTACHMENT BOX 9.15

ATTACHMENTS AND INNERVATION
OF THE PECTORALIS MAJOR

Proximal attachment: Anterior surface of medial
one half or two thirds of the clavicle, one half of
the anterior surface of the sternum from the sternal
notch to the level of about the sixth or seventh
costal cartilage, the first through sixth or seventh
costal cartilages, and the aponeurosis of the exter¬
nal oblique abdominal muscle.

Distal attachment: Lateral lip of the intertubercular
groove of the humerus.

Innervation: Medial and lateral pectoral nerves.

The clavicular portion receives innervation from C5-6
and perhaps C7. The sternal portion receives inner¬
vation from C8 through T1 and perhaps also from
C6 and C7.

Palpation: The clavicular and sternal portions of the
pectoralis major are palpated individually anterior
to the axilla.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Although there is general agreement that the pectoralis major
medially rotates the shoulder, there is less agreement about
the conditions under which this occurs. Moment arm analysis
supports its potential as a medial rotator with its greatest
potential with the shoulder in the neutral position [41]. Some
investigators note that it participates in medial rotation only
against resistance, while others report activity particularly in
the clavicular portion regardless of resistance [3]. These dif¬
ferences may represent individual variability in recruitment
patterns, as has been reported with the anterior deltoid [24].
The subscapularis and pectoralis major appear to have the
greatest medial rotation potential of all the medial rotators
regardless of shoulder position [41]. However, additional
studies are needed to clarify the pectoralis major muscle s role
in medial rotation as well as in the other purported actions.
EMG studies verify that the pectoralis major, including both
its bellies, plays an important role in shoulder depression
along with the latissimus dorsi [18,59]. This role is described
in more detail with the latissimus dorsi. EMG data also
demonstrate activity of the pectoralis major muscle during
forced inspiration [3]. Its role as a muscle of respiration is dis¬
cussed again in Chapter 30.

EFFECTS OF WEAKNESS OF THE PECTORALIS MAJOR

Weakness of the pectoralis major affects the combined
actions of the whole muscle and the actions of each part in
isolation. Weakness of the whole pectoralis major may result
in decreased strength in medial rotation, adduction, horizon¬
tal adduction of the shoulder, and shoulder depression.

greater detail in the discussion of the individual components
of the pectoralis major.

ACTIONS OF THE PECTORALIS MAJOR-
CLAVICULAR PORTION

MUSCLE ACTION:

Actions

Evidence

Shoulder flexion

Supporting

Shoulder medial rotation

Supporting

Shoulder depression

Supporting

These actions are widely accepted and substantiated by EMG
studies [3,27]. The clavicular head of the pectoralis major and
the anterior deltoid muscles are the prime flexors of the
shoulder (Fig. 9.37 ) [27,28]. The anterior deltoid muscle was
recruited first, followed by the clavicular head of the pec¬
toralis major in seven of eight healthy male subjects during
isotonic shoulder flexion. The order was reversed in the
remaining subject [28]. These data support the notion that
these two muscles work synchronously throughout the ROM
of shoulder flexion, although the exact pattern of recruitment
may vary.

EFFECTS OF WEAKNESS OF THE CLAVICULAR
PORTION OF THE PECTORALIS MAJOR

Weakness of the clavicular portion of the pectoralis major
causes significant reduction in the strength of shoulder flexion

Clinical Relevance

RADICAL MASTECTOMY, A CASE REPORT: Surgical pro¬
cedures for the treatment of breast cancer include removal
of breast tissue and sometimes underlying musculature. The
radical mastectomy , rarely performed any longer ; involved
the removal of all or part of the pectoralis major. Although
weakness was demonstrated following surgery , in some indi¬
viduals surprisingly little dysfunction followed. A 62-year-old
female had undergone bilateral radical mastectomies and
total resection of the pectoralis major muscle bilaterally in
the 1960s. Yet 10 years later she was the reigning female
champion of her local tennis club. The absence of profound
loss of function is consistent with the fact that the pectoralis
major provides additional strength to the shoulder but no
additional motions that are not available from contractions
of other muscles.

EFFECTS OF TIGHTNESS OF THE PECTORALIS MAJOR

Tightness of the pectoralis major is often detected following
thoracic surgery or breast surgery [72]. Tightness limits ROM
of the shoulder in lateral rotation and horizontal abduction. It
may limit shoulder flexion ROM as well. This is explained in

Figure 9.37: Clavicular portion of the pectoralis major. Contraction
of the clavicular portion of the pectoralis major is visible during
resisted shoulder flexion.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

181

and may contribute to a decrease in the strength of medial
rotation of the shoulder.

EFFECTS OF TIGHTNESS OF THE CLAVICULAR
PORTION OF THE PECTORALIS MAJOR

Because the weight of the upper extremity tends to keep the
shoulder in a neutral sagittal plane position, tightness of only
the clavicular head of the pectoralis major is unlikely
However, as a part of the whole pectoralis major, tightness of
the clavicular head may contribute to decreased ROM of lat¬
eral rotation of the shoulder.

ACTIONS OF THE PECTORALIS MAJOR—STERNAL
PORTION

MUSCLE ACTION:

Actions

Evidence

Shoulder extension

Supporting

Shoulder flexion

Supporting

Shoulder adduction

Inadequate

Shoulder medial rotation

Inadequate

Shoulder depression

Supporting

Most authors report that the sternal portion of the pectoralis
major extends the shoulder against resistance. Note that in
the upright position the weight of the upper extremity tends
to extend the shoulder, and no additional muscle force is
needed. However, when the subject pushes down onto a
piece of furniture with the shoulder flexed to 90° the sternal
portion is active (Fig. 9.38).

Only Inman and colleagues report any activity of the pec¬
toralis major, sternal portion, during flexion, and they note

Figure 9.38: Sternal portion of the pectoralis major. Contraction
of the sternal portion of the pectoralis major is apparent during
resisted shoulder extension.

Clinical Relevance

MANUAL MUSCLE TEST OF THE STERNAL PORTION
OF THE PECTORALIS MAJOR: The standard position for
the subject during manuai muscle testing of the sternal por¬
tion of the pectoralis major is supine with the shoulder flexed
(Fig. 9.39) [35]. In this position , the weight of the upper
extremity tends to keep the shoulder flexed; that is; the
weight creates a flexion moment at the shoulder. Therefore ’
the muscles must create an extensor moment to counteract
the weight of the upper extremity. The sternal portion of the
pectoralis major is recruited as the subject attempts to return
the upper extremity to the neutral position.

that the activity is found in the most superior portion and gen¬
erally through the midrange of flexion only [27]. The remain¬
ing activities of the sternal portion of the pectoralis major
muscle appear to be widely accepted but apparently untested
[24,35,63,71,84].

EFFECTS OF WEAKNESS OF THE STERNAL PORTION
OF THE PECTORALIS MAJOR

Weakness of the sternal portion of the pectoralis major causes
a loss of strength in shoulder extension from the flexed
position and perhaps in medial rotation and adduction.

EFFECTS OF TIGHTNESS OF THE STERNAL PORTION
OF THE PECTORALIS MAJOR

Tightness of the sternal portion of the pectoralis major is likely
to restrict shoulder abduction and flexion ROM as well as
lateral rotation ROM of the shoulder.

Figure 9.39: Manual muscle test of the sternal portion of the pec¬
toralis major. When the individual is supine with the shoulder
flexed, the weight of the upper extremity tends to flex the shoul¬
der. Extension of the shoulder from this position requires a con¬
centric contraction of the sternal portion of the pectoralis major.

182

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Latissimus Dorsi

The latissimus dorsi is a broad flat muscle with an extensive
attachment on the spine and pelvis, suggesting that this mus¬
cle is capable of generating large forces (Muscle Attachment
Box 9.16).

ACTIONS OF THE LATISSIMUS DORSI MUSCLE
MUSCLE ACTION:

Actions

Evidence

Shoulder extension

Supporting

Shoulder adduction

Supporting

Shoulder medial rotation

Supporting

Shoulder depression

Supporting

EMG studies confirm the role of the latissimus dorsi in all of
these motions with and without resistance, unlike the teres
major that appears to participate in extension, adduction, and
medial rotation activities only when resistance is present
[3,57]. EMG studies verify the latissimus dorsis role with the
pectoralis major in shoulder depression [18,59]. EMG studies
also suggest that the latissimus dorsi is active in both forced
inspiration and forced expiration [84].

EFFECTS OF WEAKNESS OF THE LATISSIMUS DORSI

Weakness of the latissimus dorsi contributes to decreased
strength in the motions listed above.

MUSCLE ATTACHMENT BOX 9.16

ATTACHMENTS AND INNERVATION
OF THE LATISSIMUS DORSI

Proximal attachment: By tendinous slips to spinous
process of the lower six thoracic vertebrae, anterior
to the trapezius, by the thoracolumbar fascia to the
spines and supraspinous ligaments of the lumbar
and sacral vertebrae, to the outer lip of the posterior
aspect of the iliac crest lateral to the erector spinae,
and to the lower three or four ribs.

Distal attachment: The floor of the intertubercular
groove. The muscle may also attach to the lateral
aspect of the scapula's inferior angle as the muscle
passes over the scapula.

Innervation: Thoracodorsal nerve, C6-8.

Palpation: The latissimus dorsi is most easily palpated
along its lateral border on the axillary line of the
trunk. With the teres major muscle, the latissimus
dorsi muscle forms the posterior axillary wall.

Clinical Relevance

LATISSIMUS DORSI PEDICLE FOR RECONSTRUCTIVE
SURGERY: Because of its size and vascular supply from
multiple arteries; the latissimus dorsi is a frequent source of
grafting material for reconstructive surgery , including
wound closures and breast reconstruction. Such surgery can
significantly impair the strength of the shoulder from which
the latissimus dorsi is taken [16].

EFFECTS OF TIGHTNESS OF THE LATISSIMUS DORSI

The latissimus dorsi is an important muscle in swimming and
is very strong and perhaps overdeveloped in competitive
swimmers. Tightness of the latissimus dorsi limits shoulder
ROM in flexion, lateral rotation, and perhaps abduction.
Attached to the pelvis and lumbar spine posteriorly and to the
anterior aspect of the humerus, the latissimus dorsi crosses
from the posterior to the anterior surface of the trunk.
Consequently, tightness of the latissimus dorsi also may con¬
tribute to flexion of the upper thoracic spine (Fig 9.40).

Figure 9.40: Latissimus dorsi tightness. A tight latissimus dorsi
may contribute to increased thoracic kyphosis.

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

183

Shoulder Depression

Although there is little movement available in the direction
of shoulder depression, the muscular forces in the direction of
shoulder depression are exceedingly important. The force
of shoulder depression is particularly important when the
upper extremity is used in weight-bearing activities. For
example, as a person uses a cane, the arm is bearing weight
(Fig. 9.41). The reaction force of the cane tends to elevate the
shoulder. Active contraction of the shoulder depressors stabi¬
lizes the shoulder, preventing elevation.

Latissimus dorsi

Figure 9.41: Function of the latissimus dorsi and pectoralis major.
The latissimus dorsi and pectoralis major help stabilize the
shoulder against the upward reaction force from a cane.

Clinical Relevance

UPPER EXTREMITY WEIGHT BEARING: Upper extremity
weight bearing is extremely important in rehabilitation and
aiso in athletic events. An individual who uses a wheelchair
must lift himself off the seat to relieve pressure on the but¬
tocks or to transfer to another seat. Without the use of the
lower limbs, such as occurs following some spinal cord
injuries; the individual lifts almost the entire weight of the
body with the upper extremities, pushing down with the
hands (Fig. 9.42). The wheelchair exerts a reaction force on
the upper extremities in an upward direction. This force
tends to elevate the shoulders, therefore, the shoulder
depressor muscles are required to "depress" the shoulder, or,
more accurately, "fix" the shoulder, preventing it from being
elevated by the reaction force. By stabilizing the shoulder
girdle, the shoulder depressor muscles allow the upward
chair reaction force to be transmitted to the rest of the body
to lift it from the chair. Similarly, a gymnast supports the
weight of the body through the upper extremities during
many gymnastic movements (Fig. 9.43). In both cases the
pectoralis major and latissimus dorsi muscles are the pri¬
mary muscles lifting the body weight by "depressing the
shoulder." These two muscles can be assisted by additional
shoulder depressors, including the pectoralis minor and sub-
clavius muscles.

Summary of the Axiohumeral Muscles

These two muscles, the pectoralis major and the latissimus
dorsi, are large, powerful muscles that cross all of the joints
of the shoulder complex. Yet the actions they cause at the

Figure 9.42: Weight relief in a wheelchair. A person who uses a
wheelchair and has the inability to use the lower extremities uses
the upper extremities to lift the body weight up off the but¬
tocks, relieving pressure. The wheelchair pushes up on the upper
extremities, tending to raise the shoulders. Shoulder depressor
muscles fix the shoulder to transfer the force to lift the body.

184

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 9.43: Floor exercises in gymnastics. The gymnast lifts body
weight with the upper extremities in many floor exercises.

shoulder are also provided by other muscles of the shoulder.
These two muscles have no unique actions at the shoulder.
However, these two muscles add significant strength to all
motions of the shoulder except lateral rotation. In addition,
they are the primary shoulder depressors that provide essen¬
tial strength and stability in activities when the upper
extremity bears weight. Weakness of these muscles is most
likely manifested in forceful activities such as weight bearing
or in sporting activities such as gymnastics and golf [60].

Tightness of these muscles may limit all but medial rotation
ROM of the shoulder complex. Therefore, impairment of
either of these muscles may significantly affect some func¬
tions of the shoulder.

MUSCLE STRENGTH COMPARISONS

An understanding of the relative strength of the muscles of
the shoulder in individuals with no shoulder pathology pro¬
vides insight into the functional requirements of the shoul¬
der during daily life. It also provides a perspective for the cli¬
nician who is trying to judge the functional significance of
weakness in the shoulder musculature. Several studies exam¬
ine the strength of various muscle groups of the shoulder and
assess the factors affecting those strengths. While two stud¬
ies report a trend toward increased strength in the shoulder
on the dominant side, these studies deny any statistically sig¬
nificant differences in shoulder strength between the domi¬
nant and nondominant sides [54,70]. Not surprisingly, shoul¬
der strengths in men are substantially larger than those in
women.

Shklar and Dvir list the strength of the shoulder muscle
groups in descending order: extensors, adductors, flexors,
abductors, medial rotators, and lateral rotators [70]. These
authors note that in men this order is unchanged by direction
of contraction (concentric or eccentric). However, in the
women studied, the concentric strength of the flexors is
greater than that of the adductors. Two other studies report
that the strength of the shoulder flexors exceeds that of the
extensors in men, while the two are essentially equal in
women [54]. The differences in the results from these studies
demonstrate an important aspect of muscle comparisons. The
study by Shklar and Dvir examines peak torque during con¬
centric and eccentric contractions throughout the shoulders
ROM from neutral to the flexed position. The study by
Murray and colleagues examines isometric strength with the
shoulder in the neutral position [54,70]. Williams and
Stutzman report isometric strength in various shoulder posi¬
tions and demonstrate the effect of shoulder position on the
isometric strength of the shoulder flexors and extensors [83].
These data support the notion that peak isometric force is
greater in the flexor than in the extensor muscles. This peak
occurs when the shoulder is hyperextended (i.e., when the
flexor muscles are stretched). Neither of the previously cited
studies report strength in this position. With the shoulder at
neutral, the shoulder flexors produce a larger force than the
extensors, supporting the conclusions by Murray et al. [54].
However, when the shoulder is flexed, the extensors produce
more force than the flexors, as reported by Shklar and Dvir.
Thus joint position and the mode of contraction are likely to
affect comparative muscle strengths. The clinician must con¬
sider all of the factors influencing muscle force when inter¬
preting the results of muscle strength tests (Chapter 4).

Several studies compare the strengths of the medial and
lateral rotator muscle groups of the shoulder and consistently

Chapter 9 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE SHOULDER COMPLEX

185

find the medial rotators stronger than the lateral rotators
regardless of the speed of contraction or the position of the
shoulder [15,37,46,54,69,70]. These findings are quite under¬
standable when the number and total physiological cross-
sectional area of the medial rotator muscles is compared with
the number and total physiological cross-sectional area of the
lateral rotator muscles.

It is frequently suggested that the strength of the shoulder
is greater when tested in the plane of the scapula. Although
few studies have assessed this directly, two studies report no
difference in strength of abduction or rotation when tested in
the frontal plane and in the plane of the scapula [23,81].
Although there may be issues of stability and comfort that are
optimized by measuring strength of the shoulder in the plane
of the scapula, there is no support at the present time for the
notion that this position enhances strength. These studies
serve to remind the clinician of the complexities affecting
muscle performance. Evaluation of muscle performance at
the shoulder and an understanding of a muscle s contribution
to shoulder impairment requires the clinician to have a broad
understanding of the normal performance of these muscles
and the factors that influence their output.

SUMMARY

This chapter discusses the individual muscles of the shoulder.
They are presented in the functional groups of axioscapular
and axioclavicular, scapulohumeral, and axiohumeral. These
groups, named according to the attachments of the muscles in
the respective group, have unique functional responsibilities
at the shoulder. The axioscapular and axioclavicular muscles
position the scapulothoracic and sternoclavicular joints.
Similarly, the muscles of the scapulohumeral group position
the glenohumeral joint. Finally, the axiohumeral muscles add
power to the motions of the shoulder.

Impairments of muscles within these groups produce pre¬
dictable effects on shoulder function. Impairments within the
axioclavicular and axioscapular groups impair the ability to
position the scapula during active shoulder elevation.
Impairments in the scapulohumeral muscle group impair the
ability to position the glenohumeral joint, and impairments in
the axiohumeral muscles impair the ability to exert large
muscle forces on the shoulder, particularly during upper
extremity weight-bearing activities. An understanding of the
functional role of each muscle allows the clinician to evaluate
the contribution of individual muscles to function and dysfunc¬
tion of the shoulder. Comparisons of group muscle strengths
reveal how joint position and contraction mode affect muscle
force production at the shoulder. Peak shoulder flexion
strength is greater than peak extension strength, and medial
rotation strength is greater than lateral rotation strength.

The following chapter discusses the forces sustained by
the shoulder joints and the surrounding muscles during daily
activities as well as during more vigorous activities such as
sports.

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CHAPTER

Analysis of the Forces on the
Shoulder Complex during Activity

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE FORCES ON THE GLENOHUMERAL JOINT.188

MECHANICAL DEMANDS PLACED ON STRUCTURES THROUGHOUT THE SHOULDER COMPLEX.193

FORCES ON THE SHOULDER COMPLEX WHEN THE UPPER EXTREMITY IS USED FOR PROPULSION .194

CONNECTIONS BETWEEN ANALYSES OF JOINT AND MUSCLE FORCES AND CLINICAL PRACTICE.195

SUMMARY .195

T he preceding two chapters describe the structure of the bones, joints, and muscles of the shoulder complex.
The purpose of the present chapter is to discuss the mechanical demands placed on these structures during
daily activities. This discussion helps the clinician comprehend the daily loads sustained by the articular struc¬
tures of the shoulder complex and the forces that must be generated by the muscles of the shoulder under normal
conditions of activity. By understanding the demands placed on the shoulder complex under normal conditions, the
clinician can appreciate how various pathological conditions can affect the loads to which the shoulder complex is
subjected.

Specifically, the goals of this chapter are to

■ Review a simplified two-dimensional analysis used to estimate the forces sustained by the glenohumeral
joint while maintaining a static position

■ Examine the forces sustained by structures throughout the shoulder complex

■ Use mechanical analyses to consider the effects of discrete joint and muscle impairments on the loads
sustained by the unimpaired structures

■ Consider the loads on the shoulder when the upper extremity is used for propulsive activities

TWO-DIMENSIONAL ANALYSIS OF THE
FORCES ON THE GLENOHUMERAL JOINT

Examining the Forces Box 10.1 outlines a classic two-
dimensional model to calculate the forces generated at the
glenohumeral joint during abduction of the shoulder and
presents a free-body diagram identifying the forces present in
this activity. It also provides the two-dimensional analysis
used to determine the loads on the head of the humerus dur¬
ing isometric contractions at a given position of abduction.

This analysis yields the loads required of the abductor mus¬
cles to support the upper extremity at that position. By
repeating the analysis at different positions, the loads through
the entire range of motion (ROM) can be approximated.
Figure 10.1 presents estimates of such analyses based on data
by Inman et al. [7].

The analysis presented in Examining the Forces Box 10.1
uses several simplifications. First, only the glenohumeral joint
is examined. Yet upward rotation of the scapula may allow
the glenoid fossa to support the inferior aspect of the joint,

188

Chapter 10 I ANALYSIS OF THE FORCES ON THE SHOULDER COMPLEX DURING ACTIVITY

189

EXAMINING THE FORCES BOX 1

TWO-DIMENSIONAL ANALYSIS OF THE
FORCES ON THE HEAD OF THE HUMERUS
WITH THE SHOULDER ABDUCTED TO 90°
AND THE ELBOW EXTENDED

The following dimensions are based on a well-
conditioned male who is 6 feet tall and weighs 180 lb.
The limb segment parameters are extrapolated from
the anthropometric data of Braune and Fischer [2].

L is the length of the upper extremity, 0.8 m

W is the weight of the upper extremity

The weight of the upper extremity is located at the
center of gravity of the limb, located approximately
48% of the limb's length from the shoulder, 0.38 m

F is the force of the abductor muscles

The moment arm of the abductor muscles is 0.05 m

The muscles' angle of application is 30°

J is the joint reaction force

Solve for abductor force (F):

SM = 0

(F X 0.05 m) - (W X 0.38 m) = 0
(F X 0.05 m) = (W X 0.38 m)

F = 7.6 W

Calculate the forces on the head of the humerus

£F X : F x + J x = 0

J x = — F x where F x -F(cos 30°)

J x = F(cos 30°)

J x = 6.6 W

£F y : F y - W + J Y = 0

J Y = W - F y where F Y = F(sin 30°)

J Y = -2.8 W

Using the Pythagorean theorem:

J 2 = J X 2 + V

J « 7.2 W

Assuming the weight of the upper extremity is 0.05
times body weight (BW), J * 0.4 BW

Using trigonometry, the direction of J can be
determined:

cos a = J x /J

a ~ 24° from the horizontal

190

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 10.1: Muscle and joint reaction forces during abduction
of the shoulder. These forces are largest when the shoulder is
abducted to 90°.

particularly later in the joints ROM. By ignoring the position
of the scapula, the shear forces on the head of the humerus
may be overestimated. On the other hand, the analysis also
lumps all of the muscle and ligament forces into one, the del¬
toid muscle force. This simplification is necessary to decrease
the number of “unknowns,” so that the number of unknowns
is equal to the number of equations available. As described in
Chapter 1, a two-dimensional analysis provides only three
equations to analyze the relevant forces: 2F x = 0, 2F y = 0,
and 2M = 0. Thus only three unknown quantities can
be determined, the deltoid force, the compressive force on
the head of the humerus, and the shear force on the head
of the humerus. However, Chapter 9 offers convincing evi¬
dence that several muscles are active simultaneously during
both shoulder flexion and abduction, rotating and stabilizing
the glenohumeral joint. By lumping these forces together
the analysis is likely to underestimate the total force on the
humeral head. These muscles balance each other out when
producing a movement. For example, the superior glide from
the deltoid muscle and the inferior glide from the infraspina¬
tus muscle balance each other so that there is virtually no net
translation of the humeral head. However they both exert a
force on the humeral head that compresses it against the gle¬
noid fossa (Fig. 10.2). The additional compressive forces by
muscles cocontracting with the deltoid muscle are undetected
in this simplified model.

Despite the models shortcomings, its results offer valuable
insights. Inman and colleagues estimate that the peak com¬
pressive load on the head of the humerus is approximately 10
times the weight of the upper extremity. Assuming that the
upper extremity is about 5% of body weight, this means that
the head of the humerus is subjected to loads of approxi¬
mately one half of total body weight during the simple task of
holding the shoulder abducted [2,4]. It is easy to imagine,
then, the much greater loads on the muscles and joints when
holding a 15-lb infant at arms length and coaxing it to smile
(Fig. 10.3).

Figure 10.2: The effect of cocontraction on the joint reaction
force. The vector sum of the deltoid and infraspinatus muscle
forces increases the load on the joint.

Figure 10.3: Task analysis. Holding a baby in outstretched arms
produces large loads on the shoulder, since the weight (I/I/) of
the baby acts at a large distance (/) from the shoulder joint,
requiring large muscle forces for equilibrium.

Chapter 10 I ANALYSIS OF THE FORCES ON THE SHOULDER COMPLEX DURING ACTIVITY

191

Figure 10.4: The moment arm of the weight of the upper extremity. Comparing these three positions, the moment arm of the weight
of the upper extremity (W) is smallest when the shoulder is abducted to 60° and largest at 90°.

Peak joint reaction force occurs when the shoulder is
abducted to 90°, which is not surprising, since at 90° of
abduction, the adduction moment due to the weight of the
upper extremity is at its maximum (Fig. 10.4). Therefore, the
abductor muscle force must be greatest to generate a
moment to counteract the adduction moment created by the
weight of the upper extremity. This large abduction muscle
force, estimated to be approximately eight times the weight of
the upper extremity, is the major factor influencing the joint
reaction force. Maximum shear forces of slightly less than
50% of body weight are also reported but occur earlier in the
ROM, at approximately 60° of abduction [7].

Examining the Forces Box 10.2 demonstrates the effect
of flexing the elbow to 90°, shortening the moment arm of
the weight of the upper extremity. Similarly, holding the
infant with flexed elbows decreases the moment arm of the
weight of the baby (Fig. 10.5). The results of these analyses,
based on a significant oversimplification of the
mechanical reality manifested by the shoulder com¬
plex, still challenge the traditional view that the
shoulder is “non-weight-bearing.” While under normal cir¬
cumstances humans do not walk on their hands, these data
clearly suggest that the shoulder complex must bear large
and repeated forces during everyday activity.

More recent mechanical analyses have been used to
improve on the classic analysis of Inman et al. These results
suggest that the shoulder is subjected regularly to even larger
forces than suggested by Inman et al. [17]. These increased
estimates reflect the greater sophistication of the modeling
process. A model with a more accurate representation of the
rotator cuff muscles suggests that the maximum compressive
load on the humeral head during abduction of the shoulder in
the plane of the scapula is almost 90% of total body weight,
almost twice the estimates in the earlier study, and maximum
shear is almost 50% of body weight! The addition of only a
1-kg load in the hand results in a 60% increase in joint reac¬
tion forces. It is not surprising then that activities of the upper
extremities can result in large joint forces and pain in individ¬
uals with arthritis. Many activities of daily living (ADL) require
considerable shoulder motion. Many of these activities occur
with simultaneous elbow flexion, effectively decreasing the
external moment on the shoulder [12]. Consequently, the
reported forces on the shoulder during activities such as drink¬
ing from a mug or lifting a low-weight block to shoulder height
range from only 8-50 newtons (approximately 2-11 pounds) in
the superior direction. Teaching the patient ways to reduce
loads on the shoulder during daily activities is an important
part of effective treatment [3].

192

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

EXAMINING THE FORCES BOX 10.2

TWO-DIMENSIONAL ANALYSIS OF THE
FORCES ON THE HEAD OF THE HUMERUS
WITH THE SHOULDER ABDUCTED TO 90°
AND THE ELBOW FLEXED TO 90°

The free body diagram of the shoulder abducted to
90° and the elbow flexed to 90° demonstrates that the
moment arm due to the weight of the upper extremity
is shorter with the elbow flexed. Consequently, less
muscle force is required to support the limb, and a
smaller joint reaction force is produced.

Length of arm (shoulder to elbow): 0.4 m

Center of gravity of the arm is 44% of the length
of the arm from the proximal end: 0.18 m

Weight of the arm: 0.028 body weight (BW)

Weight of the forearm and hand: 0.022 body
weight (BW)

Moment arm of the deltoid: 0.05 m

Angle of application of the deltoid muscle: 30°

Solve for the abductor force (F):

SM = 0
(F

(F X 0.05 m) = (0.028 BW X 0.18 m)

+ (0.022 BW X 0.4 m)

F = 0.28 BW

Calculate the forces on the head of the humerus

SF x : F x + J x - 0
J X = — F x

where F x -F(cos 30°)

J x = F(cos 30°)

J x = 0.24 BW

£F y : F y - W A - W F + J Y = 0

J Y = W A + W F - F y

where F Y = F(sin 30°

-0.09 BW
Using the Pythagorean theorem:

J 2 = J x 2 + J Y 2

J « 0.26 BW

Using trigonometry, the direction of J can be
determined:

cos a = J x /J

Chapter 10 I ANALYSIS OF THE FORCES ON THE SHOULDER COMPLEX DURING ACTIVITY

193

Figure 10.5: Application of the principles to a patient problem. An
individual with shoulder pain should be instructed to avoid hold¬
ing a baby at arm's length (/). Holding the child with shoulders less
flexed and elbows more bent reduces the load on the shoulder by
reducing the moment created by the weight (1/1/) of the child.

Clinical Relevance

ARTHRITIC CHANGES IN THE GLENOHUMERAL
JOINT: Rheumatoid arthritis frequently affects the gleno¬
humeral joint , resulting in significant pain and disability [10].
The large joint loads sustained by the humeral head during
simple active ROM provide ample justification for the
patient's complaints of pain. The benefits of exercise to
maintain mobility and to increase strength must be weighed
against the risks of increasing the joint loads and pain as
well as perhaps hastening joint destruction. The clinician
must investigate joint positions and modes of exercise that
minimize the risks to the joint while maximizing the physio¬
logical benefits. For example ’ active ROM activities per¬
formed in the supine position or in water decrease the
moment generated by the weight of the upper extremity.
Therefore, less muscle force is needed to move the shoulder.
Consequently , the joint reaction force is smaller. The
decrease in joint reaction force is one reason why patients
with arthritis tolerate these exercises more readily.

MECHANICAL DEMANDS PLACED
ON STRUCTURES THROUGHOUT
THE SHOULDER COMPLEX

Investigators have vigorously pursued models that more accu¬
rately portray the morphology and behavior of the whole
shoulder complex [1,5,9,13,14,21]. As stated earlier, the clas¬
sical approach to the analysis of forces in a joint has been to
use simplifying assumptions that reduce the number of
unknowns to a number equal to the number of equations
available to describe the phenomenon. However, the reality is
that at any joint in the human body, there are far more
unknowns than available equations. This is known as redun¬
dancy, and the system with these unknowns is said to be
indeterminate, possessing an infinite number of solutions to
the equations. However, sophisticated mathematical algo¬
rithms are available that allow investigators to determine the
“best” or optimal solution, based on some predetermined
optimization criteria. By using this approach, numerous mod¬
els have been developed to calculate the forces in the muscles
and ligaments of the glenohumeral joint as well as in the other
joints of the shoulder complex. The following briefly presents
data from some of these models. These results remain mere
approximations of the real loads sustained by the shoulder
structures. However, they can provide the clinician with at
least a perspective on the requirements and consequences of
activity and exercise on discrete structures of the shoulder
complex.

In a greatly more complex model of the shoulder, van der
Helm [20] reports peak medial-lateral joint reaction forces at
the glenohumeral joint of approximately 300 N (67 lb) and
100 N (22.5 lb) for abduction and flexion, respectively (One
kilogram equals 9.81 newtons (N), or 1 lb equals 4.45 N.)
The anterior-posterior and longitudinal joint reaction forces
are slightly smaller. Unlike the joint reaction forces estimated
by previous studies, this study presents the separate three-
dimensional components rather than the total reaction force.
Therefore the magnitudes cannot be compared directly
However, like the previous studies, the peaks appear at
approximately 90°, when the moment due to the upper
extremity weight is greatest. In this same study, reported
peak joint reaction forces in the sternoclavicular and
acromioclavicular joints are approximately 50 and 120 N (11
and 27 lb), respectively, and are in the medial-lateral direc¬
tion. This study provides analytical evidence for the integral
role of the scapulothoracic joint in the mobility and stability
of the whole shoulder complex. It also is one of the few stud¬
ies to provide any estimate of the loads sustained at the other
joints of the shoulder. A similar, albeit less detailed, model
predicts muscle loads up to 150 N (34 lb) in the deltoid mus¬
cle and over 100 N (22.5 lb) in the supraspinatus during
abduction with a 1-kg weight in the hand [9]. This study also
reports peak joint reaction forces of approximately 80% of
body weight in the middle of the ROM.

194

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

The studies described so far have used mathematical
analyses with standard Newtonian mechanics to estimate
the loads on the joints and soft tissues of the shoulder com¬
plex. Another approach uses anatomically based models
that mimic muscle behaviors. By creating realistic physical
models of muscles of the shoulder or investigating the rel¬
ative activity of muscles of the shoulder, these studies pro¬
vide insight into the comparative difficulty of tasks as well
as contributions of individual muscles to certain activities.
One such study reports that the average peak force of the
deltoid muscle required to abduct the shoulder in the plane
of the scapula is approximately 250 N (±34.5), approxi¬
mately 56 lb [25]. The effects of the absence of the
supraspinatus, the whole rotator cuff, and the deltoid mus¬
cles also are recorded. Total active shoulder elevation
decreases by 6, 16, and 25%, respectively, with selective
cutting of the supraspinatus, the other rotator cuff muscles,
or the deltoid. A similar model using a physical model of
the muscles of the shoulder reports that the force required
of the deltoid to elevate the upper extremity in the absence
of the rotator cuff muscles increases by 17% [16]. Another
study uses electromyography (EMG) and intramuscular
pressure to examine the relative activity of the supraspina¬
tus and verifies what is reported in mathematical analyses,
that is, that the activity of the supraspinatus muscle increases
as the shoulder is abducted from 0° to 90° [8]. These stud¬
ies help identify the relative contributions and importance
of given structures to the overall function of the shoulder.
They consistently demonstrate that the loss of function in
one muscle results in either impaired motion or an
increased load in the remaining muscles. These results pro¬
vide a theoretical basis to explain such clinical observations
as a patient s complaints of shoulder weakness and fatigue
in the presence of a rotator cuff tear.

Two additional studies serve as useful examples of how
studies investigating the loads on muscles can provide
insight that may help the clinician understand the
mechanical basis for a patient s complaints. The first study
uses a mathematical model of the shoulder and EMG
recordings of the shoulder muscles to determine the fati¬
gability of some of these muscles [14]. These authors sug¬
gest that the deltoid, infraspinatus, and supraspinatus are
the first to show signs of fatigue during prolonged isomet¬
ric contraction of the shoulder at 90° of flexion against a
4-kg weight. The trapezius appears more resistant to
fatigue in this study. The second study investigates the
level of EMG activity in shoulder muscles at different
shoulder and elbow positions with and without perform¬
ing a low-resistance manual task [19]. The presence of a
manual task results in increased EMG activity in almost
all positions and muscles. While neither of these studies
provides a direct measurement of the forces generated in
the muscles of the shoulder, together they suggest that
even small increases in the loads carried by the upper
extremity may significantly increase the loads sustained by
muscles.

Clinical Relevance

CASE REPORT: A thirty-something female came to physical
therapy complaining of a gradual onset of shoulder pain.

She was an artist whose primary art form was oil painting.
She was working on a new project using a very large canvas
that required prolonged elevation of her painting hand
above the level of her head. The patient began to notice
shoulder pain while working. She reported that the pain
began in the first week of the new project and generally
appeared only after a few hours of work. However ; the pain
was growing more intense and lasting longer after each
painting session.

The patient's initial evaluation occurred on a Monday
morning. She had not painted for 3 days. She denied pain
at the time of the evaluation, and no tests elicited pain. She
was instructed to return to physical therapy after several
days of painting. She was also instructed to schedule the
visit after a full day of painting. At the time of the patient's
second visit she had slight pain with palpation at the superior
aspect of the greater tubercle. ROM was full and pain free,
and isometric contractions of the shoulder in the neutral
position were strong but slightly painful. Resisted shoulder
abduction was mildly painful, especially in midrange. The
pain increased with repetitions.

These findings were consistent with mild impingement or
with irritation of the supraspinatus tendon. The therapist
hypothesized that the task of painting on such a large can¬
vas was fatiguing the rotator cuff muscles, which gradually
lost their ability to stabilize the glenohumeral joint. As stabil¬
ity decreased, superior glide of the glenohumeral joint
increased and gradually allowed impingement of the ten¬
don. This hypothesis is consistent with the findings reported
in the literature. The patient's history revealed that the job
required prolonged periods of increased shoulder elevation,
which was a new activity for her, so the muscles were
untrained for this strenuous activity. (The patient had not
recognized this as a new or strenuous activity.) She was
treated with strengthening and endurance exercises for the
rotator cuff muscles and was instructed to take frequent
rests while painting, to avoid excessive fatigue. The patient
reported decreased pain in 7 week and denied any pain
while painting after 4 weeks.

FORCES ON THE SHOULDER COMPLEX
WHEN THE UPPER EXTREMITY IS USED
FOR PROPULSION

In Chapter 9 the role of the muscles that depress the shoul¬
der is discussed. These muscles are particularly important in
activities in which the upper extremity bears weight, such
as pushing up from a chair or crutch walking. The upper

Chapter 10 I ANALYSIS OF THE FORCES ON THE SHOULDER COMPLEX DURING ACTIVITY

195

extremity is particularly important as a weight-bearing struc¬
ture when the function of the lower extremities is impaired.
It is reasonable to hypothesize that the task of using crutches
or propelling a wheelchair subjects the muscles, ligaments,
and articular surfaces of the shoulder to considerably larger
forces than tasks such as holding the upper extremity flexed.
However, only a few studies actually offer any analysis of the
loads sustained during these activities. Several studies provide
analyses of the loads sustained during wheelchair activities.
Investigators report average peak contact forces on the gleno¬
humeral joint ranging from approximately 110-200 N (25-45
pounds) during wheelchair propulsion by individuals with
spinal cord injuries [11,22]. Average peak moments of 20-35
Nm are also reported during propulsion [11,18].

The weight relief maneuver wheelchair users employ to lift
their buttocks off the wheelchair seat and avoid pressure sores
on the sacrum generates loads on the shoulder of approxi¬
mately 1,000-1,500 N (225-337 pounds) [22,23]. These data
demonstrate the burden the shoulder sustains in habitual
wheelchair users and may explain why complaints of shoulder
pain are common in these individuals. Careful analysis of the
task and the wheelchair itself will help researchers, clinicians
and wheelchair uses to develop strategies and equipment to
protect the shoulders of wheelchair users.

In a study of crutch walking using a swing-through gait,
peak flexor moments at the shoulder normalized by body
weight were reported to be an average of 0.4 N-m/kg in five
individuals with paraplegia, compared with average peak
moments of slightly more than 0.2 N-m/kg in eight individuals
without paraplegia [15]. The moments reported during these
activities are approximately three times the moments reported
during isometric shoulder abduction at 90° with the elbow
extended [6]. None of these studies report actual calculations of
joint reaction forces, but it is probable that the moments during
weight bearing, which are more than three times the moments
generated during static postures without resistance, result in
similarly large increases in joint reaction forces. Despite such
apparently large loads sustained during crutch walking, a study
of 10 subjects with a mean duration of crutch-aided ambulation
of 8.7 years reveals no degenerative changes at the
shoulder bilaterally [24]. These data emphasize the
remarkable resilience of the shoulder complex.

CONNECTIONS BETWEEN ANALYSES
OF JOINT AND MUSCLE FORCES
AND CLINICAL PRACTICE

This chapter presents the results of several studies investigat¬
ing the forces sustained by the joints and muscles of the
shoulder complex. The analyses use simplifying assumptions
or uncomplicated physical representations of complex
anatomical structures. Consequently, these results are at best
an estimation of the real loads to which the shoulder is sub¬
jected. Comparing the absolute values of forces reported by
these studies with the maximum sustainable loads for

cartilage, bone, and muscle can help the clinician assess
the potentially detrimental effects of an activity or exercise.
Similarly, such knowledge is essential in the design of suit¬
able joint replacement devices. However, in the broader sense,
these studies offer the clinician a theoretical framework from
which to analyze any patients complaints. Even a simplistic
model representing the forces involved in an activity allows
the clinician to ask the question, How much muscle force is
required to lift this 20-lb baby? and perhaps more importantly,
Is there another way to lift the baby so that less muscle force is
required? Similarly, the clinician can ask, What is the load on
this inflamed joint during this strengthening exercise? Can the
exercise be performed differently to reduce the force on the
joint? Although few clinicians have the opportunity to answer
these questions quantitatively, an understanding of the basic
approach to the analysis enables the clinician to generate hypo¬
thetical answers to these questions. Clinical observations can
then support or refute these estimates.

SUMMARY

In this chapter the basic two-dimensional approach to calculat¬
ing muscle forces and joint reaction forces is presented. A sim¬
plified model demonstrates that the shoulder sustains loads of
approximately 50% of body weight during unresisted active
abduction. Results from more-sophisticated analyses predict
even higher loads, and weight-bearing activities can be
expected to generate still larger loads on the shoulder.
Impairments within the shoulder complex also are likely to alter
the direction and magnitude of the loads on the shoulder.
Although the published data offer only estimates of the forces in
the shoulder, the clinical use of the theoretical framework used
in these analyses is discussed, and a patient example demon¬
strates the clinical relevance of some of the data presented.

The preceding two chapters present the structure and
functions of the bones, joints, and muscles of the shoulder
complex. The effects of impairments of these structures are
also discussed. The current chapter presents a scheme to con¬
ceptualize the shoulder as a mechanical system that sustains
variable loads that depend on the nature of the activity. Such
a framework offers the clinician a method for identifying the
underlying mechanisms that cause the abnormal perform¬
ance of the bones, joints, and muscles of the shoulder and the
theoretical basis for prescribing treatment regimens to
improve or restore normal function. This same framework of
mechanical analysis is repeated in the rest of the anatomical
regions presented in this book.

References

1. Bassett RW, Browne AO, Morrey BF, An KN: Glenohumeral
muscle force and moment mechanics in a position of shoulder
instability. J Biomech 1990; 23: 405-415.

2. Braune W, Fischer O: Center of gravity of the human body. In:
Human Mechanics; Four Monographs Abridged AMRL-TDR-
63-123. Krogman WM, Johnston FE, eds. Wright-Patterson Air

196

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Force Base, Ohio: Behavioral Sciences Laboratory, 6570th
Aerospace Medical Research Laboratories, Aerospace Medical
Division, Air Force Systems Command, 1963; 1-57.

3. Cordery J, Rocchi M: Joint protection and fatigue management.
In: Rheumatologic Rehabilitation Series. Melvin M, Jensen GM,
eds. Bethesda: American Occupational Therapy Association,
1998; 279-322.

4. Dempster WT: Space requirements of the seated operator. In:
Human Mechanics; Four Monographs Abridged AMRL-TDR-
63-123. Krogman WM, Fischer O, eds. Wright-Patterson Air
Force Base, Ohio: Behavioral Sciences Laboratory, 6570th
Aerospace Medical Research Laboratories, Aerospace Medical
Division, Air Force Systems Command, 1963; 215-340.

5. Hogfers C, Karlsson D, Peterson B: Structure and internal con¬
sistency of a shoulder model. J Biomech 1995; 28: 767-777.

6. Hughes RE, An K: Force analysis of rotator cuff muscles. Clin
Orthop 1996; 330: 75-83.

7. Inman VT, Saunders JB, Abbott LC: Observations of the func¬
tion of the shoulder joint. J Bone Joint Surg 1944; 42: 1-30.

8. Jarvholm U, Palmerud G, Herberts P, et al.: Intramuscular pres¬
sure and electromyography in the supraspinatus muscle at
shoulder abduction. Clin Orthop 1989; 245: 102-109.

9. Karlsson D, Peterson B: Towards a model for force predictions
in the human shoulder. J Biomech 1992; 25: 189-199.

10. Klippel JH: Primer on the Rheumatic Diseases. Atlanta:
Arthritis Foundation, 2001.

11. Koontz AM, Cooper RA, Boninger ML, et al.: Shoulder kine¬
matics and kinetics during two speeds of wheelchair propulsion.
J Rehab Res Dev 2002; 39: 635-650.

12. Murray IA, Johnson GR: A study of the external forces and
moments at the shoulder and elbow while performing every day
tasks. Clin Biomech 2004; 19: 586-594.

13. Niemi J, Nieminen H, Takala EP, Viikari-Juntura E: A static shoul¬
der model based on a time-dependent criterion for load sharing
between synergistic muscles. J Biomech 1996; 29: 451^60.

14. Nieminen H, Takala EP, Viikari-Juntura E: Load-sharing pat¬
terns in the shoulder during isometric flexion tasks. J Biomech
1995; 28: 555-566.

15. Noreau L, Comeau F, Tardif D, Richards CL: Biomechanical
analysis of swing-through gait in paraplegic and non-disabled
individuals. J Biomech 1995; 28: 689-700.

16. Payne LZ, Deng XH, Craig EV, et al.: The combined dynamic
and static contributions to subacromial impingement. Am J
Sports Med 1997; 25: 801-808.

17. Poppen N, Walker PS: Forces at the glenohumeral joint in
abduction. Clin Orthop 1978; 135: 165-170.

18. Robertson RN, Boninger ML, Cooper RA, Shimada SD:
Pushrim forces and joint kinetics during wheelchair propulsion.
Arch Phys Med Rehabil 1996; 77: 856-864.

19. Sporrong H, Palmerud G, Kadefors R, Herberts P: The effect of
light manual precision work on shoulder muscles—an EMG
analysis. J Electromyogr Kinesiol 1998; 8: 177-184.

20. van der Helm FCT: A finite element musculoskeletal model of
the shoulder mechanism. J Biomech 1994; 27: 551-569.

21. van der Helm FCT: Analysis of the kinematic and dynamic
behavior of the shoulder mechanism. J Biomech 1994; 27: 5:
527-569.

22. van Drongelen S, van der Woude LH, Janssen TW, et al.:
Glenohumeral contact forces and muscle forces evaluated in
wheelchair-related activities of daily living in able-bodied sub¬
jects versus subjects with paraplegia and tetraplegia. Arch Phys
Med Rehabil 2005; 86: 1434-1440.

23. van Drongelen S, van der Woude LHV, Janssen TWJ, et al.:
Glenohumeral joint loading in tetraplegia during weight relief
lifting: a simulation study. Clin Biomech 2006; 21: 128-137.

24. Wing PC, Tredwell SJ: The weightbearing shoulder. Paraplegia
1983; 21: 107-113.

25. Wuelker N, Wirth CJ, Plitz W, Roetman B: A dynamic shoulder
model: reliability testing and muscle force study. J Biomech
1995; 28: 489-499.

UNIT 2

ELBOW UNIT

A n the previous unit, the structure and function of the shoulder are presented. It is shown that the purpose

of the shoulder, to position the upper extremity in space, requires that the shoulder complex possess remarkable
flexibility. Such flexibility is provided by the unique coordination of four separate joints as well as by the
extreme flexibility available at the glenohumeral joint itself. However, such mobility comes at a cost to stability. The
glenohumeral joint has several unique anatomical features and structures to enhance stability, particularly the rotator
cuff muscles.

In contrast, the function of the elbow is simpler. The role of the elbow is primarily to shorten or lengthen the upper
extremity, allowing the hand to move away from and toward the body during such activities as reaching into the
refrigerator and bringing a snack to the mouth. In addition, the elbow assists in turning the hand toward or away
from the body. These simplified functional demands are paralleled by decreased structural complexity. A reduction
in available motion is accompanied by a significant increase in inherent stability. The following three chapters review
the structure and the functional requirements of the elbow joint and demonstrate how issues of mobility and stability
at the elbow differ from those of the shoulder.

The purposes of this three-chapter unit on the elbow are to

■ Present the structure of the elbow joint and discuss its effects on the mobility and stability of the joint
■ Discuss the role of muscles in the mechanics and pathomechanics of the elbow joint
■ Analyze the forces to which the elbow is subjected and the factors that influence those forces

197

CHAPTER

Structure and Function

of the Bones and Noncontractile

Elements of the Elbow

CHAPTER CONTENTS

STRUCTURE OF THE BONES OF THE ELBOW .198

Distal Humerus .198

Proximal Ulna .202

Proximal Radius.203

ARTICULATIONS AND SUPPORTING STRUCTURES OF THE ELBOW .204

Humeroulnar and Humeroradial Articulations.204

Superior Radioulnar Joint .210

Motion of the Elbow Joint.212

Comparison of the Shoulder and the Elbow .216

SUMMARY .216

T he focus of this chapter is the bony architecture and supporting structures of the elbow joint and their contri¬
butions to function. Specifically, the purposes of the present chapter are to

■ Discuss the structure of the bones that constitute the elbow and their effect on joint mobility and stability.

■ Present the functional units of the elbow and the noncontractile structures that support them.

■ Examine the normal movement of the elbow joint.

■ Compare the structure and function of the elbow with those of the shoulder.

STRUCTURE OF THE BONES
OF THE ELBOW

The elbow joint consists of the articulations among the distal
humerus, the proximal ulna, and the proximal radius
(Fig. 11.1). The relevant details of each bone are presented
below. As in the preceding unit on the shoulder complex, only
the details of each bone that apply to the elbow are present¬
ed. Thus the current chapter provides a discussion of the dis¬
tal humerus and proximal radius and ulna. Chapter 8 presents
a detailed discussion of the structure of the proximal humerus

because it is directly associated with the shoulder. Similarly,
the distal radius and ulna are discussed in Chapter 14, which
presents the bones and joints of the wrist and hand.

Distal Humerus

Chapter 8 describes the humerus to the level of the deltoid
tuberosity and radial groove, which are located in the mid¬
shaft of the humerus. The shaft of the humerus is approxi¬
mately round in cross section proximally but gradually flattens
anteriorly and posteriorly and widens medially and laterally as

198

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

199

Proximal

radius

Distal

humerus

Proximal ulna

Figure 11.2: A cross-sectional view of the shape of the midshaft
of the humerus and of the distal humerus reveals how the distal
humerus flattens anteriorly and posteriorly.

Figure 11.1: The elbow joint complex is composed of the distal
humerus, proximal ulna, and proximal radius.

the shaft continues distally (Fig. 11.2). The distal shaft also
curves slightly anteriorly, directing its articular surfaces more
anteriorly, thus positioning the articular surfaces in a way that
favors flexion mobility (Fig. 11.3). The flattening of the distal
shaft of the humerus gives rise to the medial and lateral
supracondylar ridges.

The distal end of the humerus consists of the articular sur¬
face, including the trochlea and the capitulum, and the nonar¬
ticulating surfaces, the medial and lateral epicondyles, as well
as the olecranon fossa and the coronoid and radial fossae
(Fig. 11.4). The medial and lateral epicondyles are prominent
projections that are distal culminations of the medial and lat¬
eral supracondylar ridges. Although both epicondyles are pal¬
pable, the medial epicondyle is more prominent than the lat¬
eral. It encompasses approximately one third of the distal
humerus. It is grooved posteriorly by a shallow sulcus for the
ulnar nerve. As the ulnar nerve travels in this groove it lies
directly against the bone and is susceptible to compression
against the humerus by a blow to the medial elbow. The
groove is covered by a fascial roof running from the medial
epicondyle to the proximal end of the ulna’s olecranon process.
This roof forms the cubital tunnel for the ulnar nerve [53].

Figure 11.3: A sagittal view of the humerus reveals the anterior
curvature of its distal end.

200

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.4: Distal humerus. A. Anterior view. B. Posterior view.

Clinical Relevance

THE CRAZY BONE OF THE ELBOW: Nerves are most
susceptible to injury in locations where they lie against
rigid structures or in rigid spaces. The radial nerve is par¬
ticularly vulnerable as it travels along the humerus in the
radial (spiral) groove. Similarly , the ulnar nerve is at risk
as it wraps around the medial epicondyle at the elbow.
Few individuals have escaped the characteristic pain and
tingling that radiate distally through the medial aspect of
the forearm and hand when the medial aspect of the
elbow (the crazy bone) hits a door or piece of furniture
(Fig. 11.5). More serious and lasting injuries to the ulnar
nerve also occur as the nerve travels through the restrict¬
ed space of the cubital tunnel. Preliminary studies suggest
that the cubital tunnel narrows during elbow flexion, as a
result of a stretch to the fascial covering. This narrowing
apparently is accompanied by a stretch to the nerve
itself. The combination of tunnel narrowing and nerve
stretch may contribute to some ulnar nerve neuropathies
at the elbow.

The medial epicondyle provides important attachments
for the joint capsule and medial (ulnar) collateral ligament of
the elbow as well as for the superficial flexor muscles of the
forearm. The lateral epicondyle is prominent posteriorly par¬
ticularly in elbow flexion. It gives rise to the lateral collateral lig¬
ament and to the superficial extensor muscles of the forearm.

The radial and coronoid fossae of the humerus are shallow
depressions on the anterior surface of the distal humerus
just proximal to the articular surfaces of the capitulum
and trochlea, respectively These depressions allow close

Figure 11.5: Sensory distribution of the ulnar nerve. A. Palmar
view. B. Dorsal view.

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

201

Figure 11.6: The role of the olecranon, radial and coronoid fossae of the humerus. A. The elbow joint reveals the olecranon in the
olecranon fossa in extension. B. The radial head and the coronoid process in the radial and coronoid fossae, respectively, in flexion.

approximation between the humerus and the radius and ulna
during maximum elbow flexion (Fig. 11.6). The olecranon
fossa is a deep depression on the posterior surface of the dis¬
tal humerus, proximal to the trochlea. The proximal aspect of
the ulna’s olecranon process fits into this notch when the
elbow is extended.

The articular surfaces of the distal humerus form the lat¬
eral two thirds of its distal aspect. The trochlea lies in the mid¬
dle third, and the capitulum lies in the lateral one third. The
capitulum forms approximately a hemisphere and is situated
on the anterior and distal aspects of the humerus but does not
extend onto the posterior surface (Fig. 11.4). The trochlea
is a pulley-shaped surface that extends over the anterior,
distal, and posterior aspects of the humerus, forming almost
330° of a circle [59,60]. The medial portion of the trochlea
expands farther distally than the lateral, which helps to
explain the lateral orientation of the ulna with respect to the
humerus. This orientation, described as the carrying angle,
is discussed in greater detail later in this chapter.

The articular surfaces of both the trochlea and capitulum
are covered by hyaline cartilage. Average cartilage thickness
on the capitulum from 12 cadaver specimens ranges from 1.06
to 1.42 mm (± 0.24—0.30 mm) [51]. Mineralization and density
of the subchondral bone appear to be greatest anteriorly on
the capitulum and distally and anteriorly on the trochlea

[16,18,19] (Fig. 11.7). According to Wolffs law, the mineral¬
ization and density of the distal humerus suggests that the
distal humerus sustains its largest loads anteriorly and distally.
(See Chapter 3 for more details about Wolffs law.)

Figure 11.7: Areas of increased mineralization on the distal
humerus. The areas of greatest bone mineralization on the distal
humerus are (A) anterior on the capitulum and ( B ) anterior and
distal on the trochlea.

202

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.8: Anterolateral view of the proximal ulna reveals
the relevant landmarks and articular surfaces of the proximal
ulna.

Proximal Ulna

The proximal end of the ulna is considerably larger than the
distal end. Like the distal end of the humerus, the proximal
ulna is curved anteriorly. It consists primarily of the olecranon
and coronoid processes and the trochlear notch they create
(Fig. 11.8). The notch articulates with the trochlea of the
humerus. The olecranon is a hooklike projection extending
proximally and then anteriorly. It is smooth and easily palpated
posteriorly when the elbow is extended, positioned between
the two humeral epicondyles. When the elbow is flexed, the
pointed junction between the posterior and superior surfaces
of the olecranon process is found distal to the two epicondyles,
forming a triangle with these two bony landmarks (Fig. 11.9).
The olecranon is continuous distally with the posterior border of
the ulna, also known as the ulnar crest, which is palpable the
length of the ulna.

The coronoid process lies on the anterior aspect of the
proximal ulna, and its superior surface forms the floor of
the trochlear notch. The distal aspect of the anterior surface of
the process is known as the tuberosity of the ulna. On the lat¬
eral aspect of the coronoid process is a smooth oval facet. This
facet, the radial notch, is the site for articulation with the head
of the radius. Just distal to this facet is a fossa that provides

Figure 11.9: Relationship of the olecranon process to the
humeral epicondyles. A posterior view of the elbow joint reveals
the relationships among the olecranon process and the medial
and lateral condyles in (A) elbow extension and (B) elbow
flexion.

attachment for the supinator muscle. This fossa is limited poste¬
riorly by the supinator crest.

The trochlear notch is formed by the anterior surface of the
olecranon process and the superior surface of the coronoid
process. The trochlear notch itself is covered with articular car¬
tilage and has a central ridge running proximally and distally
through the length of the notch. It fits into the deepest part of
the trochlea on the humerus. The junction of olecranon and
coronoid processes in the trochlear notch is somewhat nar¬
rowed medially and laterally. The articular surface of the
trochlear notch frequently is separated into two distinct articu¬
lar surfaces proximally and distally, divided by a nonarticular
roughened area [19,62,69] (Fig. 11.10). The hyaline cartilage
covering the trochlear notch is thinnest medially and laterally,
thickening toward the midline of the surface, with an average
maximum thickness of approximately 2 mm in 14 cadaver spec¬
imens [38]. However, the pattern of cartilage thickness appears
to vary proximally and distally along the surface and seems to
depend on whether the articular surface of the trochlear notch
is continuous or separated into individual articular surfaces. As
in the humerus, the degree of subchondral bone mineralization
also varies across the trochlear notch, greater in the proximal
and distal regions than centrally, again suggesting that the bones
architecture depends on the loads it sustains [38].

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

203

Figure 11.10: Articular surface of the trochlear notch. An anterior
view of the trochlear notch demonstrates two sites of contact,
with no contact in the deepest part of the notch, a common pat¬
tern of contact with the trochlea.

Figure 11.11: An anterior view of the proximal radius reveals the
articular surfaces and important landmarks of the proximal radius.

Proximal Radius

The proximal radius includes the radial head, neck, and
tuberosity (Fig. 11.11). The radial head is a disc-shaped
expansion of the proximal end of the radius. The proximal
surface of the head is concave and known as the fovea of the
radius, which articulates with the capitulum [61]. The periph¬
eral surface, or rim, of the head is also articular, rotating in the
radial fossa of the ulna. In the transverse view, the rim of the
radial head can be either circular or ellipsoid [9,65]. The exact
shape of the rim dictates the path of the distal radius during
pronation and supination. The rim is highest medially and
more shallow laterally [1,69]. The rim of the radial head is
palpable at the lateral aspect of the elbow, just distal to the lat¬
eral epicondyle of the humerus.

Like that of the humerus and ulna, the articular surface of
the proximal radius, including the head and rim, is covered
with hyaline cartilage, with thicknesses in the foveae of
cadaver specimens ranging from about 0.9 to 1.10 mm [38].
The mineralization of the subchondral bone reportedly is
thickest in the central part of the fovea [19].

Distal to the head of the radius, the diameter of the radius
decreases, forming the neck of the radius. In adults the head of
the radius is expanded beyond the circumference of the neck,
creating a constriction at the neck into which the annular liga¬
ment fits. The radial tuberosity is distal to the radial neck, on
the medial aspect of the radius. The shaft of the radius is slightly
bowed, with the maximum bend found approximately midshaft
where the pronator teres attaches (Fig. 11.12). The radius can

Figure 11.12: Bow shape of the radius. The bowing of the radius
effectively increases the moment arm of the pronator teres that
attaches at the peak of the bow.

204

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

then function as a crank to alter the moment arm of the prona¬
tor teres (Chapter 12) [6].

The bones of the elbow possess several landmarks that are
identifiable with palpation. Reliable identification of
these structures is an essential ingredient of a valid
physical examination. The following bony structures
are identifiable through palpation:

• Medial epicondyle of the humerus

• Lateral epicondyle of the humerus

• Medial supracondylar ridge of the humerus

• Lateral supracondylar ridge of the humerus

• Olecranon process

• Olecranon fossa of the humerus

• Crest of the ulna

• Radial head

ARTICULATIONS AND SUPPORTING
STRUCTURES OF THE ELBOW

Although the elbow is enclosed by a single joint capsule, there
are three distinct articulations within that capsule: the
humeroulnar, humeroradial, and superior radioulnar
joints. The term elbow refers to the humeroulnar and
humeroradial articulations. However, because the superior
radioulnar joint is so intimately related to the other articula¬
tions, the term sometimes is used to include the superior
radioulnar joint. So the clinician must take care to clarify
whether elbow also refers to the superior radioulnar joint.
The following discussion separates the presentation of the
articulations involving the humerus from that between only
the radius and ulna. This separation results from the func¬
tional distinctions between these two systems. The humeral
articulations with the ulna and radius are the source of flexion
and extension. The superior radioulnar joint allows pronation
and supination.

Humeroulnar and Humeroradial
Articulations

The humeroulnar and humeroradial articulations are dis¬
tinct from one another. However, together they form the
elbow articulation that is described as a hinge joint, produc¬
ing the motion of flexion and extension. They also share
some of the supporting structures. Therefore, the articular
surfaces of each unit are described separately, but the sup¬
porting structures for both joints are presented together.
The motions allowed at the articulations are described
together following the descriptions of the joints and sup¬
porting structures.

HUMEROULNAR ARTICULATION

The humeroulnar articulation consists of the trochlear notch
of the ulna surrounding the trochlea of the humerus. The
reciprocal articular surfaces are generally congruent, with the

Figure 11.13: Congruence of the articular surfaces of the elbow.
An anterior view of the elbow complex reveals that the articu¬
lar surfaces of the humerus, radius, and ulna fit very well
together.

ridge of the trochlear notch of the ulna fitting well into the
groove of the trochlea (Fig. 11.13). However, close examina¬
tion reveals that the fit is not perfect. Assessment of 15 cadaver
specimens sustaining a load of 10 N (approximately 2.25 lb)
suggests that the joint space varies from 0.5 to 1.0 mm in
the depth of the trochlear notch and may reach 3.0 mm
medially and laterally [17]. In these same specimens, much
smaller joint spaces are reported anteriorly and posteriorly
(Fig. 11.14). The joint congruity increases with increasing
joint load as the articular cartilage deforms.

The anterior curve of the distal humerus and the similar
bend in the proximal ulna help define the relative amounts
of flexion and extension motion at the humeroulnar joint.
The forward bend of both bones positions the articular sur¬
faces to favor flexion excursion over extension excursion
(Fig. 11.15). A more superior orientation of these surfaces
would allow more extension range of motion (ROM) by
increasing the distance the ulna could travel before the ole¬
cranon enters the olecranon fossa. However, flexion would
be limited earlier by the coronoid process entering the coro-
noid fossa.

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

205

Figure 11.14: Humeroulnar articular surfaces. A sagittal view of
the humeroulnar articulation reveals an asymmetrical joint space,
deeper in the middle and narrower at the superior and inferior
limits of the joint.

Clinical Relevance

CHANGES IN BONY ALIGNMENT FOLLOWING
FRACTURE: Fractures of the distal humerus or proximal
ulna can alter the normal orientation of the articular sur¬
faces of the humeroulnar articulation. Changes in the relative
alignment of these surfaces can have a significant influence
on the available ROM at the elbow following the fracture. Of
course\ stretching exercises cannot ameliorate motion restric¬
tions due to bony malalignments. Therefore ’ clinicians must
distinguish between restrictions secondary to soft tissue limi¬
tations and those due to bony blocks.

The alignment of the ulna and humerus in the frontal plane
also is related to the shape of their articulation. The medial
flare of the trochlea extends more distally than the lateral
flare. This expansion places the medial aspect of
the trochlear notch of the ulna farther distally as well, resulting
in a lateral deviation of the ulna with respect to the humerus
(Fig. 11.16). Although this orientation is typically described
as the carrying angle, a more generic term for the alignment
is valgus. Valgus is defined as a lateral deviation of a distal
segment with respect to the segment proximal to it. Varus is
the opposite, that is, a medial deviation of a limb segment

Figure 11.15: The effect of the curves of both the distal humerus and the proximal ulna on elbow ROM. A. The mutual anterior curves
of the distal humerus and proximal ulna allow flexion ROM but limit extension ROM. B. A hypothetical increase in the superior orienta¬
tion of the trochlear notch increases extension ROM and decreases flexion ROM. C. A theoretical increase in anterior orientation of the
ulna decreases extension ROM and increases flexion ROM.

206

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.16: Carrying angle of the elbow. The distal expansion
of the humeral trochlea contributes to the lateral deviation of
the ulna defined as the normal carrying angle.

with respect to the proximal segment. The neutral position
between varus and valgus is achieved when the angle
between the proximal and distal segments is 180° (usually
described as 0°) (Fig. 11.17). The valgus alignment, or car¬
rying angle, of the elbow has been the focus of considerable
study Average carrying angles of 10-15° are reported
[27,60]. Although texts frequently report that the carrying
angle is greater in women than in men [49,59], careful
measurements suggest that there is no statistically signifi¬
cant difference in carrying angle between the sexes [27,68].

HUMERORADIAL ARTICULATION

The humeroradial articulation consists of the radial head
resting on the capitulum of the humerus. Because the capit-
ulum lies on the anterior surface of the distal humerus, the
head of the radius articulates with only a portion of the
capitulum when the elbow is extended (Fig. 11.18). Contact
between the humerus and radius increases with elbow flex¬
ion [68]. Chapter 2 defines stress as force/area. Thus for a
given load at the humeroradial joint, the stress at the joint is
less when the elbow is flexed than when it is in maximum
extension because the contact between the bones is greater
with flexion.

Figure 11.17: Alignment of the elbow in the frontal plane.
A. Valgus. B. Varus.

Structures Stabilizing the Humeroulnar
and Humeroradial Articulations

The first source of support to the humeroulnar and
humeroradial articulations consists of the bony surfaces
themselves. Although, as noted above, there is not perfect
congruency among the humerus, ulna, and radius, a frontal
view of the three bones reveals an almost tongue-and-groove
fit among the three bones (Fig. 11.13). This fit makes medial
and lateral glide between proximal and distal surfaces almost
impossible. Conversely, the reciprocal concave-convex sur¬
faces serve as guides to flexion and extension much like the
rails of a train track, where derailment occurs by the train tip¬
ping to one side or another.

Clinical Relevance

HUMEROULNAR DISLOCATIONS: Dislocations of the
humeroulnar articulations can occur posteriorly where there
is little bony limitation to the trochlear notch being pushed
off the trochlea. More frequently , dislocations occur in a
combination of lateral and posterior movement of the fore¬
arm resulting from a force directed laterally on the distal
forearm [27] (Fig. 1149). Such dislocations are usually accom¬
panied by tears of the supporting ligaments; described below.

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

207

Figure 11.18: Sagittal view of the
elbow. A. In extension, the capitu-
lum articulates with only the anteri¬
or half of the head of the radius.

B. In flexion, the capitulum articu¬
lates with the entire radial head.

Figure 11.19: Lateral dislocations of the elbow occur as the ulna
rotates laterally because lateral translation is prevented by the
congruent surfaces of the humerus, ulna, and radius.

The primary supports to the elbow joint are the cap¬
sule; the medial, or ulnar, collateral ligament (MCL); and
the lateral collateral ligament (LCL). The annular liga¬
ment supports the superior radioulnar joint and is
described with that joint. The capsule of the elbow joint
surrounds all three articulations, the humeroulnar,
humeroradial, and superior radioulnar joint (Fig. 11.20).
The capsule is attached proximally to the humerus at the
margins of the olecranon and coronoid and radial fossae as
well as to the anterior and posterior surfaces of the medial
epicondyle. It also attaches to the posterior surface of the
capitulum. Distally, the capsule attaches to the border of
the olecranon and coronoid processes and to the annular
ligament.

The capsule is by necessity somewhat loose anteriorly
and especially posteriorly. Like the capsule of the gleno¬
humeral joint, the elbow joint capsule has folds that
unfold and refold during flexion and extension to allow
full ROM. In flexion, the posterior capsule unfolds to
allow full excursion; in extension, the anterior capsule
unfolds as the posterior capsule refolds. These folds allow
large flexion and extension excursions but provide little
joint stability. Cadaver dissection reveals no increase in
joint laxity with isolated transection of the elbow joint cap¬
sule [43]. Data collected from 13 cadaver specimens sug¬
gest that the entire elbow joint capsule is most lax in 80°
of elbow flexion [44]. The tension on the joint capsule in
the presence of joint effusion appears to be minimized in
this position.

208

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.20: Elbow joint capsule. A. Medial view. B. Lateral view.

Clinical Relevance

JOINT SWELLING AND ELBOW FLEXION
CONTRACTURES: Patients with inflammation of the
elbow joint frequently find that the position of comfort is sig¬
nificant elbow flexion. This clinical finding is consistent with

the evidence suggesting that the tension in the joint capsule
is minimized with the elbow flexed to 80° [44]. It is likely
that patients seek a position that minimizes the tension on
the joint capsule ’ thus relieving pain associated with a
stretch of the capsular ligament. However ; prolonged posi¬
tioning in flexion in the presence of inflammation may result
in adaptive changes in the surrounding musculature as well
as structural changes in the capsule itself resulting in a flex¬
ion contracture. Treatment to reduce the inflammation is
critical to maintaining joint function.

The MCL and LCL reinforce the capsule medially and later¬
ally, respectively. The MCL is the larger of the two collateral
ligaments. It consists of distinct anterior and posterior parts
and a smaller transverse portion (Fig. 11.21). The MCL is
attached proximally to the distal surface of the medial epi-
condyle. The anterior portion of the MCL attaches distally
to the coronoid process, and the posterior portion attaches
to the olecranon process. The transverse portion actually
spans the medial aspect of the trochlear notch, with no attach¬
ment on the humerus. The MCL as a whole resists valgus
forces that tend to deviate the forearm laterally.

The normal valgus alignment of the elbow predisposes it to
valgus stress. Overhead and throwing activities increase the val¬
gus stresses even more (Fig. 11.22). Thus it is not surprising that
the MCL is a more extensive and complex ligament than the
LCL. The organization of the MCL provides protection against
excessive valgus throughout the range of flexion and extension.

Figure 11.21: Medial collateral ligament. A. The medial collateral ligament consists of an anterior, posterior, and transverse section.
B. The medial collateral ligament resists valgus stresses.

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

209

Figure 11.22: Valgus stress on the elbow. Many everyday activities
produce large loads that tend to push the elbow into valgus.

Studies repeatedly demonstrate that the posterior portion of
the MCL (PMCL) is taut when the elbow is flexed [8,24,48].
Similarly, the anterior portion of the MCL (AMCL) is taut in
extension [8,24,40,48,55]. Closer inspection of the AMCL sug¬
gests that it has three separate portions, which provide support
through distinct regions of joint excursion [24,48]. The anteri-
ormost segment of the AMCL is tight in extension, the middle
portion is taut in midrange of flexion, and the posteriormost
portion of the AMCL is taut with the PMCL in the later stages
of flexion. Studies suggest that the AMCL is the main protec¬
tion against excessive valgus through extension and moderate
flexion ROM [8,21,47]. This complex organization of the MCL
ensures that the elbow is protected from valgus displacement
throughout its entire ROM.

Clinical Relevance

PITCHERS' ELBOW: The throwing motion puts a signifi¬
cant vaigus stress on the eibow and consequently on the
medial collateral ligament (MCL) (Fig. 11.23). The repetitive
stresses sustained by baseball pitchers from Little League
players to Major League baseball pitchers can and frequently
do lead to injuries to the MCL When the injury includes
tears of the MCL surgical repair may be indicated. The
most common , known as Tommy John surgery , named for
the major league pitcher whose career was saved by the
surgery , reconstructs the torn MCL with a tendon , usually

Figure 11.23: Valgus stress on the elbow while pitching. The large
shoulder lateral rotation used by most baseball pitchers produces
large valgus stresses on the pitching elbow.

from the palmaris longus or the plantaris muscles , small
muscles from the forearm or leg , respectively The best way
to prevent such injuries in children is to limit the number of
pitches the child throws.

The LCL attaches to the lateral epicondyle of the humerus
and can be divided into three discrete bundles (Fig. 11.24).
One portion, known as the lateral ulnar collateral ligament
(LUCL), inserts on the proximal portion of the supinator crest
of the ulna. The radial portion of the LCL is known as the
radial collateral ligament (RCL) and extends from the lateral
epicondyle to the annular ligament in a fan-shaped arrange¬
ment with anterior, middle, and posterior segments [48]. The
third component of the LCL, known as the accessory LCL
(ALCL), runs from the annular ligament to the supinator crest
[60]. These three portions of the LCL provide stability against
excessive varus deviation. Like those of the MCL, the distinct
segments appear to have individual roles in providing stability.
Specifically, the middle portion of the RCL is taut throughout
the range of flexion and extension, while the anterior and

210

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.24: The lateral collateral ligament consists of the radial
collateral, lateral ulnar collateral, and the accessory lateral collat¬
eral ligament.

posterior segments are taut in extension and flexion, respec¬
tively. The LUCL is taut in the extremes of elbow flexion but
is pulled tight with an additional varus stress anywhere in the
range of flexion or extension. A study of 30 cadaver specimens
suggests that the RCL provides the primary support to the lat¬
eral aspect of the elbow [45]. However, in a study of only four
cadaver specimens, Morrey and An suggest that the LCLs
contribution to elbow stability was less than that provided by
the bony articulations themselves [40]. However, these
authors tested the elbow specimens only in 0° and 90° of flex¬
ion. Additional studies are needed to clarify the functional sig¬
nificance of each ligamentous element to the overall stability
of the elbow joint. Regardless of the outcomes of future stud¬
ies in this area, the elbow appears to be protected from exces¬
sive varus and valgus excursions by ligamentous and bony sup¬
port throughout the ROM.

The collateral ligaments also appear to limit medial and
lateral rotation of the ulna on the humerus. In a study of
10 elbows from five cadavers, average maximal lateral rotation
at the humeroulnar articulation is almost 10°, and average
peak medial rotation is less than 5° [63]. This same study
demonstrates that sudden high-velocity loads to the elbow in
hyperextension and supination lead to tears of the anterior
capsule and to both collateral ligaments. These lesions result
in increased hyperextension and valgus and rotational laxity.
The rotational laxity is more apparent than the valgus laxity,

and consequently, rotational laxity of the elbow may be an
important sign of ligamentous damage.

The bones of the elbow clearly contribute to the medial and
lateral stability of the elbow joint. The radial head is reported
to be an important limit to valgus excursion at the elbow
[30,40,51]. One study reports approximately a 30% reduction
in stability following a radial head resection in 30 cadaver spec¬
imens [30]. However, another cadaver study suggests that iso¬
lated absence of the radial head results in no significant
increase in elbow movement [42]. In contrast, the cadaver
specimens exhibited an increase of 6-8° of valgus excursion
with a MCL ligament lesion. A lesion to both the MCL and
radial head appears to result in gross valgus instability.

Clinical Relevance

RADIAL HEAD EXCISION: Removal of the radial head , or
radial head excision , is a surgical procedure considered in
the presence of a radial head fracture or severe arthritic
changes [23]. Some studies suggest there may be little
increase in joint laxity as a result , while others report more
significant instability There appear to be few functional
deficits as a result. Elbow instability after radial head resec¬
tion may indicate more extensive soft tissue damage [28].
Injuries involving both the radial head and MCL ligament
frequently require extensive surgical intervention , and the
functional consequences are more severe [68].

Thus the humeroulnar and humeroradial articulations, which
compose the elbow joint proper, are supported by the bony
surfaces involved as well as by ligamentous structures includ¬
ing the capsule and the collateral ligaments.

Superior Radioulnar Joint

The superior radioulnar joint is mechanically quite distinct
from the humeral articulations despite its enclosure within the
capsule of the elbow joint (Fig. 11.25). The articulation is
described as a pivot joint with a single axis of motion. Unlike
the humeral articulations, the bony architecture of the superior
radioulnar joint, which includes the rim of the radial head and
the radial facet on the ulna, provides little or no support to the
joint. Therefore, the support of the superior radioulnar joint
comes from the surrounding connective tissue, including the
capsule and LCL, the annular ligament, the interosseous mem¬
brane, and the oblique cord. The capsule and LCL have been
described and need no further review. The following presents
the structure and function of the remaining ligaments.

ANNULAR LIGAMENT

The annular ligament is a broad, tough, fibrous band sur¬
rounding the neck of the radius, attaching to the anterior and
posterior margins of the radial notch on the ulna. Thus it
forms a loop encircling the radius, with its primary attach¬
ments on the ulna, although there are some loose attach¬
ments to the capsule as well as to the posterior aspect of the

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

211

Figure 11.25: The superior radioulnar joint is supported by the
annular ligament that surrounds the neck of the radius and by
the oblique and interosseous membrane. A. Anterior view shows
all three of the supporting structures. B. Superior view reveals
how the annular ligament surrounds the head of the radius.

trochlea and to the radial neck. The deep surface of the annu¬
lar ligament is lined with fibrocartilage, providing additional
stiffness and resilience. The increased mechanical stiffness is
important because unlike most ligaments, which attach
directly to the bones they support, the annular ligament func¬
tions primarily as a sling, acting as a barrier to slippage of the
radius. It has little or no limiting effect on the normal motion
of the superior radioulnar joint.

The annular ligament binds the radius to the ulna, serving
as an effective check to lateral subluxation. In addition, the
annular ligament is the primary protection against distal sub-
luxation or dislocation of the superior radioulnar joint. Such
an injury typically occurs from a traction force pulling the
forearm distally from the elbow, such as that applied when
lifting or swinging a child by the hands (Fig. 11.26).

Clinical Relevance

"PULLED ELBOW" INJURIES: Inferior dislocations of the
superior radioulnar joint most frequently occur in preschool
children and often result from swinging the child by the
hands in play [46,54,57]. Consequently, the injury is known
as the "pulled elbow" or "nursemaid's elbow." The radial head
is pulled through the ring of the annular ligament by the ten¬
sile force applied to the forearm. A common explanation for
this dislocation has been that at this stage of development
the radial head is inadequately formed and is no wider than
the neck of the radius; thus the annular ligament cannot
serve as a satisfactory noose to prevent slippage of the radial
head. More-recent data suggest that the radial head is larger
than the radial neck throughout development. However, in
young children, the annular ligament is weaker and more
easily torn [50]. In addition, it appears that the more narrow
lateral aspect of the radial head slips out easily when the
elbow is extended and the forearm pronated [if The injury
may also be more prevalent in children with hypermobility. As
the child develops, the annular ligament becomes stronger as
does the surrounding musculature. The injury rarely occurs
after age six or seven. The incidence of injury can be reduced
by cautioning parents and other caregivers to avoid swinging
or pulling young children by their hands.

OBLIQUE CORD AND INTEROSSEOUS MEMBRANE

The oblique cord is a thin bundle of fibers running distally
from the tuberosity of the ulna to the radius, just distal to its
tuberosity (Fig. 11.25). Its functional significance is unclear.
The interosseous membrane attaches to the length of the
medial surface of the shaft of the radius and passes medially
and distally to the interosseous border of the ulna. The fibers
of the interosseous border run perpendicular to the fibers of
the oblique cord. One clear role of the interosseous mem¬
brane is to bind the radius and ulna together through the
length of the forearm. Another role, directly related to the
membranes fiber orientation, has also been identified [4,14].
At the elbow the ulna transmits most of the load to or from
the humerus. At the wrist, the radius transmits most of the
load (approximately two thirds) to or from the hand [56]. The
interosseous membrane plays a role in distributing to the ulna
the loads applied to the distal radius [22]. Such loads are
applied during weight bearing on the hand or during a fall on
the outstretched hand.

Clinical Relevance

LOAD DISTRIBUTION AT THE ELBOW: When an indi¬
vidual falls on an outstretched hand, the radius sustains
large axial loads that could be transmitted directly to the
distal humerus (Fig. 11.27). However, the orientation of the

( continued )

212

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.26: Typical mechanism of
a distal dislocation of the superior
radioulnar joint. The classic mecha¬
nism producing a distal dislocation
of the superior radioulnar joint is a
strong quick pull (P) on the distal
radius, pulling the head of the radius
through the annular ligament as the
weight of the body (1/1/) pulls the
humerus away from the radius.

(Continued)

interosseous membrane aiiows it to disperse some of the
ioad to the ulna, thus decreasing the ioad directed onto the
capituium [4,14,37,68]. The ioad on the radius tends to
push it proximaiiy into the humerus. However, as the radius
tends to move proximaiiy, the interosseous membrane pulls
the ulna proximaiiy as well, thus distributing the axial load
to the ulna and ultimately to the trochlea. Consequently, the
load is spread over a larger area of the humerus, and the
stress (force/area) is decreased, perhaps decreasing the risk
of fracture. However, ulnar head resection as the result of
severe arthritis or fracture effectively eliminates the load-
sharing ability of the ulna when loads are applied through
the hand [56].

In conclusion, all of the articulations of the elbow depend
on the support of noncontractile soft tissue. The humeral artic¬
ulations gain additional support from the congruency of the
bones themselves. How these bony surfaces and ligamentous
structures affect joint mobility is presented in the following
section.

Motion of the Elbow Joint

The elbow joint complex contains both a hinge and a pivot
joint. Therefore, it is described sometimes as a trochoging-
lymus joint. However, the motions of flexion and extension
involve the humeral articulations, and the motions of prona¬
tion and supination occur at the superior radioulnar joint. The
motions are quite independent of one another and are
discussed separately below.

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

213

Figure 11.27: Role of the interosseous membrane in transmitting
a load on the radius onto the ulna. During weight bearing on
the upper extremity, the radius is loaded initially, but the orien¬
tation of the fibers of the interosseous membrane allows the
load to be transmitted to the ulna.

FLEXION AND EXTENSION

Flexion and extension occur about an axis that passes through
the centers of the trochlea and capitulum [34,39]. Chapter 7
presents the concept of the instant center of rotation
(ICR), which is a two-dimensional method to describe the
amount of translation that occurs at a joint during rotation.
The ICR changes very little throughout the elbows range of

flexion and extension, indicating that these motions occur
about an almost fixed axis [10,34,68]. However, there has
been considerable study of the change in the carrying angle
during flexion and extension. Some suggest that the angle
decreases as the elbow flexes [10,31,37,70]. This has been
attributed to the large distal expansion of the medial aspect
of the trochlea as well as to the purported spiral shape of the
groove of the trochlea. However, careful study reveals that
the change in carrying angle depends on the way it is meas¬
ured and reconfirms the notion of a relatively fixed axis of
rotation during normal flexion and extension [2,34].

The humeral articulations have slight medial and lateral
mobility as well as medial and lateral rotation mobility total¬
ing perhaps 10° [59,68]. These motions are apparent during
flexion and extension when a varus or valgus stress is applied.
They may also be important during pronation and supination
[3]. Recognition of the existence of this nonhingelike mobility
has been crucial in the development of viable total elbow
prostheses.

Clinical Relevance

ELBOW JOINT TOTAL ARTHROPLASTY: Early total
elbow replacements used strict hinge joint devices. Such
devices frequently failed because the device began to
loosen. More recent developments include unlinked and
"semiconstrained" elbow joint implants that allow slight
frontal and transverse plane joint mobility during flexion
and extension [29]. These devices have exhibited fewer
problems with loosening [53].

PRONATION AND SUPINATION

Pronation and supination occur at the superior radioulnar joint
but also involve the distal radioulnar joint. The axis of prona¬
tion and supination is a line that runs from close to the center
of the fovea of the radial head to the head of the ulna [68,70].
Like the axis of flexion and extension, the axis of pronation and
supination appears fixed, with the distal radius gliding
about a relatively immovable distal ulna. During prona¬
tion and supination with a fixed ulna, the axis of motion
is located in the head of the ulna [10,70]. When pronation
occurs with the radius moving about a fixed ulna, the hand
must move in space (Fig. 11.28). However, it is possible to
pronate the hand while keeping it fixed in space without com¬
pensation at the shoulder or wrist. This ability suggests that the
ulna moves radially as the radius rotates around it, thus keep¬
ing the hand in the same location. Evidence for the existence
of movement by the ulna during pronation and supination is
found during forearm movement with the hand fixed, such as
turning a screwdriver. Several studies demonstrate that prona¬
tion and supination involves slight radial deviation of
the ulna [3,20,30-33,67,70]. In this case the axis of
motion lies more laterally in the ulna [33]. This complex

214

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 11.28: The motion of the distal radius
and ulna during pronation. A. Pronation
produced by motion of the radius about a
fixed ulna causes the hand to move in
space. B. Pronation with the hand fixed in
space requires movement of the ulna
posteriorly, laterally, and then anteriorly.

movement of both the radius and ulna during pronation and
supination provides further evidence that the elbow joint com¬
plex has more elaborate motion than generally believed. This
awareness also supports the need for carefully designed pros¬
thetic devices that are less rigidly constrained than simple
hinge joints.

RANGES OF ELBOW MOTION REPORTED
IN THE LITERATURE

Ranges of elbow motion reported in the literature are found
in Table 11.1. Like many of the joint ranges of motion reported

in the literature, values frequently are reported without sup¬
porting data. Often the values that are based on controlled
population studies differ from those values commonly accepted
as “normal.” Such is the case at the elbow. Although elbow
hyperextension ROM is commonly noted, studies that actually
measure it report little or no available hyperextension ROM
[5,64,66]. There also is wide variability among the reported
values for normal pronation and supination flexibility. The
large standard deviations reported for pronation and supina¬
tion by Schoenmarklin and Marras suggest that some of the
differences in reported ranges of motion may result from
individual differences [52].

Chapter 11 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ELBOW

215

TABLE 11.1: Normal Passive Range of Motion Values from the Literature (in Degrees)

Flexion

Hyperextension

Pronation

Supination

Steindler [59]

135-140

10-20

US Army/Air Force [15]

150

Boone and Azin [5] a

140.59 ± 4.9

0.3 ± 2.7

75.0 ± 5.3

88.1 ± 4.0

American Academy of Orthopaedic Surgeons [26]

150

Walker et al [66] c

143 ± 11

-4 ± 5 d

71 ± 11

74 ± 14

Youm et al [70] e

140 ± 5

70 ± 5

85 ± 4

Schoenmarklin and Marras [52] f

80 ± 20

100 ± 19

Gerhardt and Rippstein [25]

150

Solveborn and Olerud [58p

143 (141-145)

4 (1-6)

a Based on 56 male subjects greater than 20 years of age (x = 34.9 ± 3.4 years).

^Reported data from Boone and Azin.

c Based on 30 male and 30 female subjects 60 to 84 years old (x = 75.6 ± 7.4 years).

^Negative numbers indicate inability to reach an extension position.
e Based on eight fresh frozen cadaver specimens.

f Based on 39 industrial workers, 22 men and 17 women. Mean age was 41.7 ± 10.5 years.

9 Based on the right elbows of 16 subjects, 12 men and 4 women. Mean age was 46 years. Reported as mean and 95% Cl.

Clinical Relevance

CLINICAL JUDGMENTS FROM ROM MEASUREMENTS:

When making judgments about the quality of a patient's
ROM\ the clinician must remember the possibility of large
intersubject differences. The difference between an individual's
right and left sides may be more important than the differ¬
ence between an individual's ROM and the "normal" value
found in the literature. The clinician must evaluate the unin¬
volved side to help establish the patient's "normal" excursion.

The differences among the studies that report data from
described measurements also may result from differences in
populations studied [5,52,66]. Boone and Azin present data
from only males, and the mean age of the subjects is consider¬
ably lower than the age of the subjects studied by Walker
et al. [5,66]. Walker et al. note significantly more elbow flexion,
extension, and supination in women than in men. Although
these authors deny any age effects on elbow ROM in their pop¬
ulation of subjects, the study examines only a small spectrum of
elderly subjects. Schoenmarklin and Marras report pronation
and supination from men and women whose jobs require very
repetitive hand activities that may influence their ROM [52].
They do not report comparisons based on sex or age. The dif¬
ferences among the values reported by Walker et al., by Boone
and Azin, and by Schoenmarklin and Marras may be the result
of age, gender, and occupational differences.

Investigations into the elbow excursions that occur during
functional activities help put these ROM values in perspective
[7,11,41]. Studies report that most activities of daily living use
the middle ranges of joint excursion for elbow flexion and
extension from about 30° to 130° [11,36,41]. Personal care
tasks such as feeding and personal hygiene may use up to 150°
of flexion but little extension. Activities such as rising from a
chair and tying a shoelace use less flexion and more extension.

Similarly, these studies suggest that most activities use
the midranges of pronation and supination from
approximately 50° of one to 50° of the other.

Clinical Relevance

COMPENSATIONS FOR RESTRICTED ELBOW ROM:

Compensations for restricted elbow ROM during functional
activities include increased shoulder motions [12]. Shoulder
pain can then develop in patients with limited elbow mobility ,
as a result of overuse of the shoulder. Therefore\ clinicians
must carefully assess the shoulder in patients with
yXKi elbow dysfunction. Conversely , the elbow should be
screened in patients with shoulder complaints.

In conclusion, these data suggest that the commonly
accepted ROM of the elbow requires verification by careful
studies. The effects of age and gender must also be examined.
In the meantime, the clinician should draw conclusions care¬
fully regarding the functional implications of altered joint
ROM at the elbow.

STRUCTURES LIMITING NORMAL ROM AT THE ELBOW

Discussion of the elbow joint capsule earlier in this chapter
reveals that the capsule is somewhat loose anteriorly and pos¬
teriorly to allow full flexion and extension ROM. The collat¬
eral ligaments however are taut in flexion and extension.
These collateral ligaments have some effect on normal joint
excursion; however, their primary role is to prevent excessive
ROM. Elbow flexion is limited most by the soft tissue contact
of the forearm and arm muscles. The elbow provides an
opportunity to learn to recognize the structure responsible for
stopping a motion by assessing the end-feel of a joint. End-feel
is the tactile sensation provided at the end of passive ROM.
Bony contact that stops a movement yields a hard end-feel.

216

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

The stretch of a ligament produces a hard but springy end-feel.
Soft tissue approximation causes a soft end-feel [35].

Clinical Relevance

ELBOW MOTION AND END-FEELS: Normal elbow flexion
range has a characteristic soft end-feel resulting from the
contact of the forearm muscles against the relaxed elbow
flexors. Elbow extension has a springier end-feel suggesting
limits from the ligaments and stretch of the elbow flexors.
Bony contact is sometimes reported as the limiting factor in
elbow extension. However ; nerve blocks to the elbow flexors
in healthy individuals have resulted in increased extension
ROM , suggesting that muscles provide the initial limits to
normal elbow extension ROM in most individuals [13]. It is
important to recall the wide range of variability within the
healthy population , as suggested by the standard deviations
presented in Table 11.1. Perhaps individuals with little muscle
mass and generalized hypermobility do have bony limita¬
tions in elbow ROM , particularly extension. Assessment of
end-feel can help determine the structures responsible for
stopping the joint motion.

Elbow pronation and supination ROM also is limited prima¬
rily by the reciprocal stretch of antagonist muscles. The LCL
may contribute some limitation to pronation ROM, and the
interosseous membrane may restrict both pronation and
supination. However, normal pronation and supination excur¬
sions are limited by muscle stretch.

Comparison of the Shoulder
and the Elbow

The links between structure and function are particularly
apparent when regions as diverse as the elbow and shoulder
are compared. The elbow has tightly fitting articular surfaces
that guide and restrict elbow motion. The elbow also possesses
collateral ligaments that serve as important stabilizing struc¬
tures in the mediolateral direction and contribute to the lim¬
its of elbow extension. Although the muscles of the elbow play
some role in stabilizing the joint, their primary responsibility
is to move the elbow. In contrast, the shoulder, the most
mobile unit in the body, depends heavily on muscles for its
stability. In fact, the bony shape of the glenohumeral joint
provides remarkable mobility but offers little assistance in
stability. The ligaments of the glenohumeral joint provide sup¬
port but only at the end ranges of motion. Thus the elbow and
shoulder provide dramatic contrasts in functional require¬
ments and in the architectures that fulfill those requirements.

SUMMARY

This chapter presents the structure of the bones and support¬
ing elements of the elbow joint and the movements available.
The bones offer congruent articular surfaces that provide

significant stability to the elbow complex. The MCL and LCL
provide the primary ligamentous support to the humeroulnar
and radiohumeral articulation. The MCL and LCL resist val¬
gus and varus stresses, respectively, throughout the full range
of flexion and extension. The annular and interosseous liga¬
ments support the superior radioulnar joint.

The variety and magnitude of motion available at the
elbow are much less than at the shoulder, reflecting the dif¬
ference in the elbows structure, which consists of less-
complex articulations and a simpler ligamentous organization
contributing to a joint complex that is inherently more stable
and less mobile. Considerable variability exists in the reported
mobility of the elbow and may indicate effects of gender, age,
and use. The role of the muscles in propelling this joint is pre¬
sented in the following chapter.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity at the Elbow

CHAPTER CONTENTS

ELBOW FLEXOR MUSCLES.220

Biceps Brachii.220

Brachialis .224

Brachioradialis.226

Pronator Teres.226

Comparisons among the Elbow Flexors .228

ELBOW EXTENSORS.233

Triceps Brachii .234

Anconeus.236

SUPINATOR MUSCLES .237

Supinator.237

COMPARISONS OF THE STRENGTH OF ELBOW FLEXION AND EXTENSION.239

SUMMARY .240

T he preceding chapter presents the bones and articulations that constitute the elbow joint complex. Although
it is clear that the structure and function of the elbow joint complex are simpler than those of the shoulder,
the musculature has its own unique specializations. The elbow joint complex consists of two mechanically
distinct articulations, the humeral articulations that allow flexion and extension and the superior radioulnar articula¬
tion that contributes to pronation and supination of the forearm and hand. Therefore, the muscles of the elbow are
organized in a way that allows the elbow to function in virtually any combination of elbow and forearm position.

The purposes of this chapter are to

■ Discuss the architecture and action of each of the primary muscles of the elbow
■ Examine the individual functional roles of each of the elbow muscles
■ Discuss the contributions to functional deficits from impairments of individual muscles
■ Compare the relative strengths of the flexor and extensor muscle groups of the elbow

For the purposes of this chapter, the primary muscles of the elbow are defined as those that cross the elbow and attach
on the forearm with no attachment across the wrist. Most of the muscles acting at the elbow can be characterized as
muscles that flex or extend the elbow. These muscles are the biceps brachii, brachialis, brachioradialis, pronator teres,
triceps brachii, and anconeus. The supinator also is a muscle of the elbow. Although it makes no contributions to flexion
or extension of the elbow, it is an essential muscle of the elbow, functioning solely at the superior radioulnar joint.

219

220

Part II I KINESIOLOGY OF THE UPER EXTREMITY

The brachioradialis, pronator teres, and supinator muscles actually have their proximal attachments with the forearm
muscles. Although the rest of the forearm muscles also affect the elbow, their primary actions are at the wrist. The bra¬
chioradialis, pronator teres, and supinator muscles function at the elbow rather than at the wrist. The current chapter
focuses on all of the muscles whose primary actions are at the elbow. The rest of the forearm muscles are described in
Chapter 15. In that chapter, the role of the remaining forearm muscles is presented, including their effects on the
motions at the humeral and radioulnar articulations.

ELBOW FLEXOR MUSCLES

The primary flexors of the elbow are the biceps brachii,
brachialis, and brachioradialis (Fig. 12.1). The pronator teres
also contributes to active elbow flexion and is included in this

group. The actions attributable to each muscle are presented
below. After each muscle is discussed individually, the current
understanding of their contributions to the coordinated
movement of elbow flexion is presented. This understanding
is based on electromyographic (EMG) data as well as on
mathematical models of the region.

Biceps Brachii

The biceps brachii is a fusiform muscle with two heads
(.Muscle Attachment Box 12.1). Its attachments span both the
shoulder and the elbow, and at the elbow it crosses both the
humeral and radioulnar articulations. Thus contractions of
the biceps brachii affect the glenohumeral, humeroulnar,
and humeroradial articulations as well as the superior
radioulnar joint.

MUSCLE ATTACHMENT BOX 12.1

ATTACHMENTS AND INNERVATION
OF THE BICEPS BRACHII

Proximal attachment: The long head attaches to the
supraglenoid tubercle of the scapula. It is intracap-
sular and covered by a synovial sheath. It may also
have a direct attachment to the anterosuperior part
of the glenoid labrum [4]. The short head of the
biceps brachii attaches to the coracoid process of
the scapula.

Distal attachment: The two tendons merge and
attach together as the biceps tendon onto the
tuberosity of the radius. The tendon has a medial
expansion, the bicipital aponeurosis, which fuses
with the deep fascia of the flexor muscles of the
wrist.

Innervation: Musculocutaneous nerve, C5 and C6.

Palpation: The belly of the biceps brachii is easily
palpated on the volar surface of the arm. The distal
tendon and aponeurosis are also palpable. The ten¬
don of the long head often can be discerned in the
intertubercular groove.

Figure 12.1: The primary flexor muscles of the elbow include the
biceps brachii, brachialis, brachioradialis, and pronator teres.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

221

ACTIONS

MUSCLE ACTION: BICEPS BRACHII

Action

Evidence

Elbow flexion

Supporting

Elbow supination

Supporting

Shoulder flexion

Supporting

Shoulder abduction

Supporting

Stabilization of the glenohumeral joint

Supporting

There is no doubt that the biceps brachii flexes the elbow and
supinates the forearm [6,10,33,41,44,53]. Although both heads
of the biceps brachii contribute to these actions, their relative
contributions are unclear. Basmajian and De Luca suggest that
the long head is more active than the short head during con¬
centric elbow flexion and during unresisted supination in most
subjects [6]. However, Stewart et al. find no difference in the
activity of the two heads during elbow flexion, regardless of
forearm position or contractile speed [47]. A study of 10 peo¬
ple with longstanding (an average of 3.2 years) ruptures of the
tendon of the long head of the biceps reveals strength deficits
of 10-15% for elbow flexion and less than 2% for supination
compared with the unaffected side [49]. EMG data suggest
that the muscles of the elbow have specific individual roles in
elbow motion depending on forearm position, the amount of
resistance during the movement, and the speed of the motion.
The activity of the biceps brachii during elbow motion
depends on these conditions. The specific conditions under
which the biceps brachii participates in elbow flexion and fore¬
arm supination are reviewed following the discussion of the
rest of the elbow flexors.

The biceps brachii frequently is reported as a shoulder
flexor [44,50]. Basmajian and De Luca report EMG data sup¬
porting this commonly held view [6]. These authors note activ¬
ity in both heads of the biceps brachii during shoulder flexion
but more in the long head in most subjects. However, a study
of five cadaver shoulders suggests that the short head of the
biceps brachii has a substantial moment arm for shoulder
flexion, while the long head has a negligible one [7].

Because the biceps brachii crosses both the shoulder and
the elbow, muscle length is affected by position changes at
either joint. Passive elbow flexion puts the muscle in a short¬
ened position, thus putting the muscle on slack. Passive
shoulder flexion does the same. Passive elbow and shoulder
extension put the muscle in a lengthened position, thus
stretching the muscle.

The length-tension relationship suggests that as a
muscle is stretched, its force of contraction is
increased, and as a muscle is shortened, its force of
contraction is decreased. (Details of this relationship are pre¬
sented in Chapter 4.) Because the biceps brachii is a two-joint
muscle, isolated contraction causes elbow and shoulder flexion
together. However, if sufficient elbow and shoulder flexion
occur together, the biceps brachii may be so shortened that it
can generate little force. This is known as active insufficiency

Figure 12.2: Active insufficiency of the biceps brachii occurs as
the result of combined elbow and shoulder flexion putting the
muscle in an excessively shortened position.

(Fig. 12.2). In contrast, shoulder extension lengthens the
biceps brachii and increases the biceps contractile force during
elbow flexion.

Contraction of the biceps brachii with simultaneous con¬
traction of a shoulder extensor muscle produces elbow flexion
and shoulder extension, thus maintaining sufficient length of
the biceps brachii muscle. Allen et al. note the presence of
slight shoulder hyperextension during a maximum voluntary
contraction (MVC) of the elbow flexors (Fig. 12.3 ) [1]. These
authors interpret this finding as a means of increasing the
force of contraction by stretching the biceps in accordance
with the length-tension relationship of a muscle. Clinicians
can use this effect of positioning to facilitate a patient s elbow
flexion strength.

Clinical Relevance

CHANGING SHOULDER POSITION TO AFFECT BICEPS
BRACHII CONTRACTILE FORCE AT THE ELBOW:

Clinicians affect a patient's elbow flexion strength by varying
the position of the elbow or shoulder joint. To correctly iden¬
tify a change in strength as the result of intervention or dis¬
ease, a clinician must standardize the position of both the
shoulder and elbow when testing elbow strength. On the
other hand, shoulder hyperextension is a useful position in
which to exercise a patient with weakness of the
biceps brachii, since the resulting muscle stretch
enhances the muscle's force output.

222

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 12.3: Lengthening the biceps brachii to increase its con¬
traction force. Slight hyperextension of the shoulder during
heavily resisted elbow flexion lengthens the biceps brachii and
increases its force output.

A few reports describe the biceps brachii as an abductor
of the shoulder [6,41]- Sturzenegger et al. [49] report an aver¬
age 8% reduction in shoulder abduction strength in the pres¬
ence of longstanding ruptures of the tendon of the long head
of the biceps brachii. A study of five cadaver shoulders sug¬
gests that both heads of the biceps brachii have abduction
moment arms, implying that the muscle is capable of produc¬
ing abduction at the shoulder. However, there are no known
studies examining the EMG activity of the biceps brachii dur¬
ing shoulder abduction. The cadaver study also suggests that
the long head of the biceps can produce a lateral rotation
moment. However, EMG data show no biceps brachii activity
in lateral rotation but occasional activity of the short head dur¬
ing medial rotation [6]. These studies support the notion that
the biceps brachii may contribute to shoulder abduction and
rotation. Additional research is necessary to clarify these roles.

Many authors identify the long head of the biceps brachii
as an important dynamic stabilizer of the glenohumeral joint

[4.8.37.45] . The proximal end of the tendon of the long head
of the biceps brachii is almost parallel to the supraspinatus
muscle and probably functions similarly to stabilize the gleno¬
humeral joint by compressing the joint [8,37] (Fig. 12.4). A
detailed study of cadavers suggests that the biceps can pro¬
vide important protection against both anterior and posterior
dislocation of the glenohumeral joint, depending on the joints
rotation [8]. Additional cadaver studies support the role of
the biceps in stabilizing the glenohumeral joint in the anterior-
posterior (AP) and in the superior and inferior directions

[29.37.45] . However, EMG data reveal no activity of the
biceps brachii to stabilize the glenohumeral joint against loads

Figure 12.4: The role of the biceps brachii in stabilizing the
glenohumeral joint. The pull of the tendon of the long head
of the biceps brachii is almost parallel to the pull of the
supraspinatus, allowing the biceps brachii to contribute to the
stability of the glenohumeral joint.

to sublux the joint inferiorly in individuals without shoulder
pathology [6].

Although these studies appear to contradict one another, it
is important to recognize the difficulty in comparing these
studies. The cadaver studies demonstrate the potential of the
biceps brachii to stabilize the glenohumeral joint. The EMG
data are from a study of subjects with normal joint stability that
examined movement of the humerus in the inferior direction
only. Additional studies are needed to examine the role of the
biceps brachii in stabilizing the glenohumeral joint in each
direction in live subjects with and without stable shoulders.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

223

Until those data are available, the role of the biceps brachii
in stabilizing the glenohumeral joint in specific directions
remains unclear. However, there is ample evidence to support
its role as a glenohumeral joint stabilizer when other stabiliz¬
ers are impaired.

EFFECTS OF WEAKNESS

Weakness of the biceps brachii causes a loss of strength in
elbow flexion and supination. However, a case report of an
individual with an isolated lesion of the musculocutaneous
nerve with complete denervation of the biceps brachii reveals
an individual with excellent function because of the compen¬
sations provided by other elbow muscles [12]. It must be
noted that “excellent function” is not defined in the report,
nor are strength measures reported. While the elbow has sev¬
eral muscles that produce flexion, the biceps brachii is a pri¬
mary flexor, and weakness in it results in a substantial
decrease in strength. However, the remaining elbow flexor
muscles apparently preserve considerable function. Similarly,
weakness of the biceps brachii produces a significant
decrease in supination strength, although the remaining mus¬
cles that supinate the forearm limit the functional loss [40].

Weakness of the biceps brachii may also be manifested by
slight weakness in shoulder flexion. However, the primary
shoulder flexors are so large and strong that isolated weakness
of the biceps muscle is unlikely to produce a functionally sig¬
nificant loss of shoulder flexion strength. Still, decreased biceps
brachii strength in the presence of rotator cuff pathology may
contribute to even more glenohumeral joint instability.

EFFECTS OF TIGHTNESS

Tightness of the biceps brachii muscle may cause a reduction
in elbow extension and pronation range of motion (ROM)
and, perhaps even shoulder extension ROM. However as a
two-joint, or biarticular, muscle, the effects of tightness at one
joint are altered by the position of the other joint. The inter¬
relationship between shoulder and elbow positions and the
effect this relationship has on the biceps brachii can assist the
clinician in differentiating among various structures that may
limit the extension ROM of the elbow joint.

Clinical Relevance

IDENTIFYING TIGHTNESS OF THE BICEPS BRACHII:

An elbow flexion contracture is the loss of full passive elbow
extension ROM. This may be the result of tightness of the
anterior joint capsule and collateral ligaments or one or all
of the elbow flexor muscles. Appropriate treatment to reduce
the contracture requires correct identification of the offend¬
ing structure. Identification of tightness of the biceps brachii
muscle is based on an understanding of its actions at both
the elbow and the shoulder joints and the clinician's ability
to manipulate the muscle length by changing the position

of the shoulder and elbow. If the elbow joint capsuloligamen-
tous complex is tight, elbow joint ROM is restricted, regardless
of shoulder and forearm position. However, if the biceps
brachii is tight and limiting elbow joint extension ROM, flex¬
ion of the shoulder joint puts the muscle in a slackened posi¬
tion that can allow an increase in elbow joint extension ROM.
Similarly, pronation of the forearm stretches the biceps brachii
and may decrease the elbow extension ROM available
(Fig. 12.5). The biceps brachii muscle is maximally stretched
by shoulder extension combined with elbow extension and
forearm pronation. It is maximally shortened by shoulder and
elbow flexion with forearm supination. Combinations of these
motions can be used to identify any contribution from the
biceps brachii to an elbow flexion contracture (Fig. 12.6).

Figure 12.5: Passive pronation of the forearm stretches the biceps
brachii. A. Tightness of the biceps brachii limits elbow extension
with the forearm supinated. B. An additional pull on the biceps
brachii exerted when the forearm is moved passively into pronation
causes the biceps brachii to pull the elbow into additional flexion.

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Part II I KINESIOLOGY OF THE UPER EXTREMITY

Brachialis

The brachialis is a pennate muscle with a broad attachment to
the distal humerus (Muscle Attachment Box 12.2). This exten¬
sive attachment indicates that the muscle is large and capable
of generating significant force [22].

ACTIONS

MUSCLE ACTION: BRACHIALIS

Action

Evidence

Elbow flexion

Supporting

The brachialis is a one-joint, or monarticular, muscle.
Consequently, its actions are at the elbow only. The reported
action of the brachialis is elbow flexion. The role of the
brachialis as an elbow flexor is widely accepted and unchal¬
lenged [6,22,36,41,44,47,53]. The muscle’s attachment to
the ulna explains why it has no apparent participation in
forearm pronation or supination, since the ulna remains
relatively fixed during pronation and supination. The
brachialis has essentially no moment arm for pronation or
supination regardless of elbow and forearm position and
can generate no moment for either pronation or supination
[15,36]. Thus the sole action of the brachialis muscle is
elbow flexion.

EFFECTS OF WEAKNESS

Weakness of the brachialis results in decreased elbow flexion
strength in all forearm positions.

Figure 12.6: An individual with tightness of the biceps brachii.

The positions of the shoulder and elbow including flexion-exten¬
sion and supination-pronation all affect the length of the biceps
brachii. A. The elbow flexion contracture is assessed with the
shoulder in neutral and the forearm supinated. B. With the
shoulder hyperextended and the forearm still supinated, the
elbow flexion contracture appears greater. C. With the shoulder
flexed and the forearm still supinated, the elbow flexion contrac¬
ture appears decreased.

MUSCLE ATTACHMENT BOX 12.2

ATTACHMENTS AND INNERVATION
OF THE BRACHIALIS

Proximal attachment: Distal one half to two thirds
of the volar surface of the humerus and the medial
and lateral intermuscular septa.

Distal attachment: Ulnar tuberosity and distal aspect
of the coronoid process.

Innervation: Musculocutaneous nerve, C5 and C6. A
branch from the radial nerve innervates a small por¬
tion of the muscle, but this may provide only sensory
input [38].

Palpation: The brachialis lies deep to the biceps
brachii but can be palpated on the medial and
lateral aspects of the biceps tendon as the biceps
brachii tapers toward its insertion. Palpation is facil¬
itated during elbow flexion with the forearm
pronated, decreasing the activity of the biceps
brachii.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

225

EFFECTS OF TIGHTNESS

Unlike tightness of the biceps brachii muscle, tightness of the
brachialis causes diminished elbow extension ROM regard¬
less of shoulder and forearm position.

Clinical Relevance

IDENTIFYING TIGHTNESS OF THE BRACHIALIS

MUSCLE: As a one-joint muscle , tightness of the brachialis
produces a flexion contracture similar to the effects of cap¬
sular tightness at the elbow , that is, unchanged by shoul¬
der or forearm position (Fig. 12.7). Therefore , tightness of
the brachialis can be distinguished from tightness of the

biceps brachii by examining the effects of shoulder posi¬
tion on elbow extension ROM. However ; the clinician must
then distinguish between brachialis and capsular tightness.
The only way to make this distinction is by identifying the
end-feel. End-feel described in Chapter 7 7, is the tactile
sensation the examiner receives when a joint is moved
passively to the end of its available ROM. A joint motion
limited by tight muscular tissue feels rubbery or springy at
the end range. A joint with an abnormally tight capsule
produces a harder ; less springy end-feel. Of course at the
elbow , the restriction could be the result of tightness in
both the brachialis and the capsule; perhaps from chronic
joint inflammation with concomitant elbow flexion posi¬
tioning for comfort.

Figure 12.7: An individual with tightness of the brachialis and elbow joint capsule. Shoulder and forearm positions have no effect on
the elbow flexion contracture. Elbow extension ROM is the same with (A) the shoulder in neutral and the forearm supinated; (B) the
shoulder hyperextended and the forearm supinated; (C) the shoulder flexed and the forearm supinated; and (D) the shoulder in neutral
and the forearm pronated.

226

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Brachioradialis

The brachioradialis is positioned with the superficial extensor
muscles of the wrist and shares an innervation with them
from the radial nerve (Muscle Attachment Box 12.3). Despite
these shared characteristics, the brachioradialis is, indeed, a
muscle of the elbow.

EFFECTS OF TIGHTNESS

Tightness of the brachioradialis muscle results in decreased
elbow extension ROM and may contribute to decreased prona¬
tion and supination ROM. However, these latter consequences
are only conjecture and need to be verified by careful study.

Pronator Teres

ACTIONS

MUSCLE ACTION: BRACHIORADIALIS

Action

Evidence

Elbow flexion

Supporting

Elbow supination

Supporting

Elbow pronation

Supporting

There is virtually complete acceptance of the role of the bra¬
chioradialis as a flexor of the elbow [6,41,44,47,53]. However,
its role in forearm movements is less well understood.
Computer and simulation models of the moment arms of the
brachioradialis muscle reveal that the muscle has a pronation
moment arm when the elbow is supinated and a supination
moment arm when the elbow is pronated [10,36]. Yet
anatomical studies reveal only very small moment arms and
also demonstrate significant variability among individuals
[15,33]. EMG data suggest that the brachioradialis muscle
may contribute to pronation and supination to the neutral
position against heavy resistance but not during unresisted
movement [6].

EFFECTS OF WEAKNESS

Weakness of the brachioradialis contributes to decreased
elbow flexion strength. It may also result in decreased resis¬
ted pronation and supination force output as the forearm
moves toward the neutral position.

MUSCLE ATTACHMENT BOX 12.3

ATTACHMENTS AND INNERVATION
OF THE BRACHIORADIALIS

Proximal attachment: Proximal two thirds of the
lateral supracondylar ridge of the humerus and the
anterior surface of the lateral intermuscular septum.

Distal attachment: Lateral aspect of the distal radius
just proximal to the radial styloid process.

Innervation: Radial nerve, C5 and C6.

Palpation: The brachioradialis is easily palpated on
the lateral aspect of the volar surface of the proxi¬
mal forearm, particularly during resisted elbow
flexion with the forearm in neutral.

The pronator teres is situated with the superficial flexors of
the wrist (Muscle Attachment Box 12.4). However, it func¬
tions solely at the elbow and superior radioulnar joint.

ACTIONS

MUSCLE ACTION: PRONATOR TERES

Action

Evidence

Elbow flexion

Supporting

Elbow pronation

Supporting

There appears to be general agreement that the pronator
teres flexes the elbow [6,25,36,41,44,53]. It has a significant
flexion moment arm, consistent with the concept that the
pronator teres muscle is capable of contributing to elbow flex¬
ion [15,35,36]. However, the conditions under which the
muscle contributes to elbow flexion are less clear. Rasmajian
and De Luca report that the pronator teres contributes to
elbow flexion only against resistance [6]. Other studies reveal
electrical activity in the pronator teres during elbow flexion
with minimal resistance, even when the forearm is supinated
[43,47]. These differences may reflect normal variations
among subjects or differences in data collection and analysis. It
is clear however that at least under some conditions, the prona¬
tor teres contributes to elbow flexion in healthy individuals.

V(5^

MUSCLE ATTACHMENT BOX 12.4

ATTACHMENTS AND INNERVATION
OF THE PRONATOR TERES

Proximal attachment: The humeral head, the larger
of the two heads, arises just proximal to the medial
epicondyle of the humerus from the common ten¬
don of the superficial flexor muscles of the forearm.
It also attaches to the medial intermuscular septum.
The ulnar head attaches to the coronoid process of
the ulna.

Distal attachment: The two heads attach together
on the lateral aspect of the radius midway along
the shaft.

Innervation: Median nerve, C6 and C7.

Palpation: The pronator teres is palpable on the volar
surface of the forearm as it traverses diagonally from
the medial epicondyle to the mid shaft of the radius.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

227

It also is well accepted that the pronator teres participates in
pronation of the forearm. However, as in elbow flexion, there
are other muscles that also pronate the forearm, particularly
the pronator quadratus, which is presented in Chapter 15.
Thus careful analysis is required to identify the exact role the
pronator teres plays in elbow flexion. Unfortunately few con¬
trolled studies of the pronator muscles exist. Bremer et al.
report that the moment arm of the pronator teres is greatest
with the elbow in neutral pronation and supination and is
almost zero with the forearm maximally supinated [10]. The
crank shape of the radius described in Chapter 11 explains
the variation in moment arms. Basmajian and De Luca report
that the pronator teres muscle is held in reserve during
pronation, being recruited only with resistance or during
rapid active pronation [6]. They also note that the position of
the elbow has no effect on the recruitment of the pronator
teres muscle. This latter finding needs careful consideration
because elbow position is an important variable in the assess¬
ment of strength of the pronator muscles. Basmajian and De
Luca investigated the level of recruitment of the muscle,
which may be quite different from the level of force generated
by a muscle contraction. (A full discussion of the relationship
between electrical activity of a muscle and force output is pre¬
sented in Chapter 4.) However, Kendall uses elbow flexion to
help discern the difference in strength between the pronator
teres and the other pronators [25].

Clinical Relevance

MANUAL MUSCLE TESTING (MMT) THE MUSCLES
THAT PRONATE THE FOREARM: The standard position
to perform a MMT of the pronators of the forearm is with
the elbow partly flexed. However ; to assess the strength of

the pronator quadratus alone; the elbow is flexed maximally
[25] (Fig. 12.8). Although according to Basmajian and De
Luca this elbow position does not alter the recruitment pat¬
tern of the pronator teres muscle, it puts the pronator teres
in a very shortened position and alters the muscle's moment
arm [15]. Thus although it may remain electrically active ,
the elbow position shortens the pronator teres enough that
it can no longer effectively generate a pronation force. The
pronator teres apparently exhibits active insufficiency as
described earlier in the biceps brachii with shoulder and
elbow flexion. The force of pronation with the elbow maxi¬
mally flexed presumably comes from the pronator quadra¬
tus muscle.

EFFECTS OF WEAKNESS

From the discussion above, it is clear that weakness of the
pronator teres may contribute to weakness of both elbow flex¬
ion and forearm pronation. However, the role of the pronator
teres in both motions is to provide additional force against
resistance. Thus functional limitations due to weakness of this
muscle alone may only be apparent during activities requiring
additional force, such as loosening a screw with the right hand.

EFFECTS OF TIGHTNESS

Tightness of the pronator teres may contribute to a loss of
ROM in elbow extension and forearm supination. However,
the pronator teres is essentially a two-joint muscle, crossing
the humeroulnar and humeroradial articulations (the elbow
joint proper) and the superior radioulnar joint. Therefore,
manifestation of tightness of the pronator teres depends on
the relative position of these articulations. Elbow extension

Figure 12.8: Standard manual muscle testing procedures to assess the strength of forearm pronation. A. With the elbow slightly flexed,
both the pronator teres and pronator quadratus contribute to the strength of pronation. B. With the elbow maximally flexed, the
pronator teres is in such a shortened position that it cannot exert an effective pronation force.

228

Part II I KINESIOLOGY OF THE UPER EXTREMITY

and forearm supination together apply a maximum stretch to
the pronator teres. If the muscle is tight, ROM in supination
is most limited when the elbow is extended. Similarly, when
the pronator teres is tight, supination ROM may increase as
the elbow is flexed (Fig. 12. 9). So, like the biceps brachii, the
pronator teres demonstrates that understanding the interplay
of joint position and muscle length allows the clinician to
identify the contributions of specific tissues to joint ROM
restrictions.

Figure 12.9: The use of elbow flexion position to distinguish
between tightness of the pronator teres muscle and other struc¬
tures. A. ROM of elbow supination measured with the pronator
teres stretched by elbow extension. B. ROM of elbow supination
measured with the pronator teres slackened by elbow flexion.
Supination ROM is greater in (B) than in (A) if the pronator teres
is limiting supination ROM.

Comparisons among the Elbow Flexors

Chapter 4 presents the parameters of muscle performance,
force output, and the production of movement. It also dis¬
cusses the factors that influence a muscles performance,
including a muscles size, angle of application, and level of
recruitment. It is these factors that distinguish the elbow
flexors from one another. Although there are four primary
elbow flexors, each appears to make its own unique contri¬
bution to elbow function. In this section, data are presented
comparing the structural characteristics of these muscles,
specifically their physiological cross-sectional areas (PCSAs),
moment arms, and muscle lengths. Then EMG data are dis¬
cussed to provide an understanding of the role each muscle
appears to play in the function of the elbow.

STRUCTURAL COMPARISONS
OF THE ELBOW FLEXORS

PCS A is a measure of the number and size of the muscle
fibers available in a muscle and thus is an indicator of a mus¬
cles potential for force production. The larger the PCS A,
the greater the potential for force production. The
brachialis has the largest PCS A (approximately 5.5-8.0
cm 2 ). The biceps brachii is next (approximately 4.5 cm 2 ), fol¬
lowed closely by the pronator teres (approximately 4.0 cm 2 ).
The brachioradialis has the smallest PCS A (approximately
1.3 cm 2 ) [2,11,30,32,35].

In addition to the force of contraction, a muscle s mechan¬
ical output depends on its moment arm and its muscle length.
The moment arm (M = r X F) a muscle applies to the joint
is a function of its force of contraction (F) and its moment
arm (r). The brachioradialis, attaching distally on the radius,
has the largest moment arm, followed by the biceps brachii
and then the brachialis [36,51] (Fig. 12.10). The pronator
teres has the smallest moment arm [35,36].

Thus while the brachialis is the largest elbow flexor, it is at
a mechanical disadvantage because of its moment arm. It is
impossible to measure directly the contribution each muscle
makes to the total flexion moment applied to the elbow.
However biomechanical models suggest that the brachialis
and the biceps brachii make the largest contributions to the
elbow flexion torque during elbow flexion with the forearm in
neutral, although their relative contributions remain debat¬
able [3,35]. Models also suggest that the relative contribution
of the elbow flexor muscles varies through the range of elbow
flexion [3,11].

The varying contributions to the total moment made by
these muscles can be explained partially by their moment
arms, which vary through the joint excursion. Anatomical
studies and computer models demonstrate that the moment
arms of the elbow flexors change significantly through the
range of flexion and extension as well as through pronation
and supination [36,39,51]. The angles of application reach
their maxima in the second half of the range of elbow flexion.
The biceps brachii reaches a maximum moment arm between
about 90° and 110° of elbow flexion. The brachialis moment

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

229

Figure 12.10: The moment arms of the primary flexor muscles of
the elbow. A muscle's moment arm is measured as the perpendi¬
cular distance from the point of rotation to the muscle pull. In
order of longest to shortest moment arms, the muscles are 7,
brachioradialis; 2, biceps brachii; 3 , brachialis; and 4, pronator
teres.

Figure 12.11: A comparison of the moment arms of the four pri¬
mary elbow flexors as they change through the flexion ROM of
the elbow. The moment arms of the elbow flexors peak in the
midrange of flexion. The pronator teres reaches its peak first, at
approximately 75°. The brachioradialis reaches its peak last, at
between 100 and 120°of flexion.

flexor muscles through the ROM, based on data reported by
Pigeon et al. [39]. However, in the extended position, the
moment arms for the flexors are quite small thereby decreas¬
ing the muscles’ capacity to generate a torque. Thus the effect
of elbow joint position on the length of the elbow flexors is
quite different from its effect on muscle moment arm.

arm also peaks at about 90° of flexion. The pronator teres
appears to peak earlier, at about 75°, and the brachioradialis
reaches a maximum moment arm between 100° and 120° of
elbow flexion. Figure 12.11 graphs the approximate moment
arms of these four muscles as the elbow moves through its
ROM. The data in Figure 12.11 are based on data from Pigeon
et al. [39] and Murray et al. [36]. Estimates of the change in
moment arm from complete extension to complete flexion vary
from 30 to 85%. The moment generated by a muscle contrac¬
tion is proportional to the muscles moment arm. Thus for a
given level of contraction, a muscle generates a larger moment
when its moment arm is large. Therefore, the capacity of the
elbow flexors to generate a moment varies dramatically
through the range by virtue of their changing moment arms.

The length of each elbow flexor also changes significantly
through the ROM, increasing in length as the elbow is extended
and decreasing in length as the elbow is flexed. According to
the length-tension relationship, a muscles ability to produce
force improves as the muscle is lengthened and diminishes as
the muscle is shortened. Thus when the elbow is extended, the
elbow flexors are lengthened, facilitating force production.
Figure 12.12 presents estimates of muscle lengths of the elbow

Figure 12.12: A comparison of the muscle lengths of the brachio-
radialis, biceps brachii, and brachialis as they change through the
flexion ROM of the elbow. Muscle length decreases steadily
through most of the range from elbow extension to complete
elbow flexion. No data are available for the pronator teres.

230

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Because these two important factors influencing muscle
performance, moment arm and muscle length, vary in signifi¬
cantly different ways across the range of elbow motion, the
position in which the elbow flexors generate the greatest flex¬
ion torque is in the midrange of flexion [5,28,38]. Figure 12.13
plots the general relationship between elbow flexion strength
and elbow flexion position, based on data presented by
Williams and Stutzman [52] and by Knapik et al. [27]. In the
midposition, neither the moment arm nor the muscle length
is optimal (Fig. 12.14). Rather, the position of greatest elbow
flexion torque output is one of compromise between the mus¬
cle length and the moment arm. Studies report that peak
elbow flexion isometric strength occurs with the elbow flexed
to 90° [11,14,52]. However, this conclusion is based on data
collected in 25-30° increments through the elbow ROM.
Another study reports that peak force output occurs at 70° of
elbow flexion in women during isometric and isokinetic con¬
tractions and in men during isokinetic contractions [27]. This
study reports that isometric contraction force peaks at 90° in
men and that the actual location of the peak force varies
considerably among subjects. Despite these disagreements,
the central point is clear: elbow flexion strength peaks some¬
where in the middle of elbow flexion ROM where neither
the muscles’ moment arms nor their lengths are optimal for
force production.

The length of a muscle s moment arm and its fiber length
also influence the amount of excursion caused by a contrac¬
tion. As noted in Chapter 4, a muscle with a short moment
arm can cause a large joint excursion for a given amount of
shortening, while a muscle with a large moment arm pro¬
duces less joint excursion for the same amount of shortening.
Thus the brachialis with the shortest moment arm is well
designed to move the elbow through a large excursion. The
biceps and brachioradialis possess long muscle fibers that also
contribute to the ability to move the elbow actively through
its full ROM [35].

In conclusion, the architecture of the elbow flexors sug¬
gests that the brachialis and biceps brachii are best suited to

Maximum

Elbow flexion angle (degrees)

Figure 12.13: Maximum isometric elbow flexion force as it
changes through the flexion ROM of the elbow. Elbow flexion
force peaks in midrange between 75 and 90° of flexion.

Moment Muscle

arm length length

Brachioradialis m. — — — -
Biceps brachii m. — — —-
Brachialis m. — — —-
Figure 12.14: A comparison of the changes in muscle lengths
and moment arms of the brachioradialis, biceps brachii, and
brachialis through the flexion ROM of the elbow. As the elbow
moves from complete extension to maximum flexion, the length
of the elbow flexors decreases. However, their moment arms
increase as flexion increases until midrange flexion, at least. Thus
the advantage of increasing moment arms is counteracted by the
disadvantage of decreasing muscle length.

generate large flexor moments at the elbow. The elbow flexors
also exhibit architectural patterns that facilitate active elbow
movement through its full arc. Although understanding the
output potential of each muscle is helpful, a thorough under¬
standing of the role of the elbow flexors requires an apprecia¬
tion of the available data describing their recruitment patterns
during elbow motion. The following reports the available data
regarding activity of these muscles during movement of
the elbow.

COMPARISONS OF FLEXOR MUSCLE ACTIVITY
DURING ELBOW MOTION USING EMG DATA

Few controlled EMG studies have been carried out to deter¬
mine the individual contributions to motion of each of the
elbow flexors. The classic studies are those described by
Basmajian and De Luca [6]. The following is a brief synopsis
of their conclusions. The data they report suggest that the
biceps brachii, brachialis, and brachioradialis function in a
finely coordinated manner with each muscle having its
unique role. However, the authors also emphasize that there
is significant variety in the behavior of these muscles among
the healthy population. Despite this caveat, the data suggest
functional patterns for each muscle.

The biceps brachii is activated in most individuals during
elbow flexion with the forearm supinated, regardless of flexion

■ Region of greatest
elbow flexion strength

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

231

speed or resistance. When the forearm is partially pronated,
the biceps brachii appears to be recruited only with resistance.
When the forearm is fully pronated, the muscle is recruited
only with resistance, and even then it appears to be only par¬
tially activated. Similarly, the biceps is active during resisted
forearm supination when the elbow is flexed. However, when
the elbow is extended, only vigorously resisted supination acti¬
vates the biceps brachii and is often accompanied by slight
elbow flexion.

These data suggest that the biceps brachii is most active
when the elbow is moving in both flexion and supination.
This finding is quite logical, since these are the motions
directly attributed to the biceps brachii. Since a muscles
function is to shorten and the resulting motion is a function
of the line of pull of the muscle, it is to be expected that a
muscle is most active performing the motions for which it is
aligned. When one of those motions is undesirable, the
choice is to recruit another muscle whose precise action is
desired or to inhibit from full activity the muscle with multi¬
ple actions while using another muscle to stabilize the joint
in the desired direction. This latter choice appears to operate
during heavily resisted elbow flexion with the forearm
pronated as well as in resisted supination of the forearm with
the elbow extended. In each case the biceps contributes to
the desired motion but also adds a pull in an undesired direc¬
tion. In resisted elbow flexion with pronation, the biceps
brachii, while active, is inhibited from full activity so that its
supination role does not interfere with the pronation posi¬
tion. Resisted supination with elbow extension similarly
inhibits the activity of the biceps brachii to avoid interfer¬
ence with elbow extension. In both examples, the biceps
brachii is recruited, but only partially.

In contrast, the brachialis is active whenever the elbow is
flexed, regardless of forearm position, resistance, or speed of
movement. The brachialis attaches to the ulna, which moves
very little during pronation or supination of the forearm. Thus
the brachialis is unaffected by forearm position. The constancy
of its activity is efficient, since it has the largest PCS A and
therefore has a potentially large contractile force. It also is
able to move the elbow through its full excursion because of
the muscle s short moment arm. The consistent activity of the
brachialis during any elbow flexion has led to its nickname,
the “workhorse” of elbow flexion.

The brachioradialis appears to be activated with resisted
elbow flexion, particularly in the partially pronated and fully

Figure 12.15: The hypothetical role of the brachioradialis. The
hypothetical explanation for the activity of the brachioradialis
during rapid elbow flexion is to exert a stabilizing force ( F) on
the elbow joint against the forces resulting from the radial accel¬
eration (a r ) of the forearm during high-velocity elbow flexion.

pronated positions but also even when the forearm is
supinated. It is also active during rapid elbow flexion. This lat¬
ter activity is somewhat surprising because of the muscle s
long moment arm. However, it is hypothesized that the mus¬
cle s alignment along the length of the forearm provides an
important stabilizing force for the elbow against the centrifu¬
gal force tending to distract the elbow during rapid move¬
ments (Fig. 12.15). Finally, as noted earlier, the pronator teres
appears to be recruited for either elbow flexion or pronation
only when resistance is added.

These findings are summarized in Table 12.1. Resisted
elbow flexion with the forearm slightly pronated elicits the
greatest electrical activity from the biceps brachii, brachialis,
and brachioradialis. Although the authors do not report data
collected during elbow flexion with a neutral forearm posi¬
tion, their report does not contradict the finding that elbow
flexion is strongest with the forearm in neutral [5,52].
Resisted elbow flexion with the forearm pronated vigorously
activates the brachialis and the brachioradialis but elicits only
partial activity from the biceps, supporting the observation

TABLE 12.1: Summary of EMG Data for the Elbow Flexor Muscles

Elbow Flexion
in Supination

Elbow Flexion in
Semipronated Position

Elbow Flexion with

Pronation

Biceps brachii

With resistance

Slightly active with resistance

Brachialis

Brachioradialis

With resistance

Pronator teres

Not active

With resistance

Data from Basmajian JV, De Luca CJ: Muscles Alive. Their Function Revealed by Electromyography. Baltimore: Williams & Wilkins, 1985.

232

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 12.16: Chin-up exercises. The difficulty of a chin-up exercise changes with the position of the forearms. A. Forearms are supinated,
allowing recruitment of the three large elbow flexor muscles. B. Forearms are pronated, partially inhibiting the biceps brachii, thus
reducing the strength of elbow flexion.

that elbow flexion with the forearm pronated is the weakest.
To test this observation one need only compare the difficulty
of a chin-up exercise with the forearm supinated with one
with the forearm pronated (Fig. 12.16).

There are no known studies that fully replicate the studies
reviewed above. However, some studies provide partial cor¬
roboration. Stewart et al. also demonstrate brachialis activity
in elbow flexion, regardless of forearm position [47]. Studies
show lower biceps brachii activity and higher brachioradialis
activity during elbow flexion with forearm pronation than in
supination [43,47]. However, in one of these studies, brachio¬
radialis activity occurs without resistance [43]. Another study
suggests that the brachioradialis is not fully activated when
the biceps brachii is fully recruited during a maximal volun¬
tary contraction, suggesting that the brachioradialis functions
as a reserve for additional force, as purported by Basmajian
and De Luca [1,6]. Finally, in a study of well-trained compet¬
itive rowers, the partially pronated grip generates a larger
force than the traditional fully pronated position [9]. While
this study provides no direct EMG data, it offers important
functional evidence supporting the belief that the elbow flex¬
ors are more fully recruited when the forearm is only partially
pronated than when it is fully pronated.

These data support the original contention by Basmajian
and De Luca that there is very precise coordination among the
elbow flexors during movement of the elbow and forearm.
An understanding of the interplay among these muscles allows

the clinician to perform a precise evaluation of the muscles
flexing the elbow to develop a specific intervention strategy
that can focus the treatment most effectively on the specific
impairment.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

233

Figure 12.17: Assessing the strength of the three elbow flexors,
biceps brachii, brachial is, and brachioradialis. Three forearm posi¬
tions are needed to fully assess the contribution of the three
large elbow flexor muscles to isometric elbow flexion strength.

A. With the forearm supinated, the biceps and brachialis are
recruited. Additional resistance recruits the brachioradialis. B.
With the forearm in neutral, all three muscles are active. The
brachioradialis is readily seen and palpated. C. With the forearm
pronated, the biceps is partially inhibited and the brachialis is
more easily palpated.

(Continued)

weakness. Measurement of supination strength with the
elbow flexed and extended also helps to identify weakness
of the biceps brachii.

ELBOW EXTENSORS

The primary extensors of the elbow are the triceps brachii and
the anconeus muscles (Fig. 12.18). As in elbow flexion, mus¬
cles of the forearm may also contribute to elbow extension.

Figure 12.18: The primary extensor muscles of the elbow. The
elbow extensors include the triceps brachii and the anconeus.

234

Part II I KINESIOLOGY OF THE UPER EXTREMITY

However, these muscles exert their primary function at the
wrist and hand and therefore are discussed in Chapter 15.

Triceps Brachii

The triceps brachii is the large muscle that constitutes the
entire muscle mass on the posterior aspect of the arm (Muscle
Attachment Box 12.5).

ACTIONS

MUSCLE ACTION: TRICEPS BRACHII

Action

Evidence

Elbow extension

Supporting

Shoulder extension

Inadequate

Shoulder adduction

Inadequate

The role of the triceps brachii as an extensor of the elbow is
uncontested. The muscle attaches to the ulna. Consequently,

MUSCLE ATTACHMENT BOX 12.5

ATTACHMENTS AND INNERVATION
OF THE TRICEPS BRACHII

Proximal attachment: The long head of the triceps
brachii attaches to the infraglenoid tubercle of the
scapula. The lateral head arises from the posterior
aspect of the humerus, proximal and lateral to the
radial groove. The medial head, which is the
largest of the three heads, arises on the posterior
aspect of the humerus, distal and medial to the
radial groove. The lateral and medial heads also
attach to the lateral and medial intermuscular
septa, respectively.

Distal attachment: The three heads attach to the
olecranon process of the ulna and to the deep
fascia of the medial and lateral forearm. The medial
head also sends fibers to the posterior aspect of
the elbow joint capsule.

Innervation: Radial nerve, C6, C7, and C8.

Palpation: The triceps brachii constitutes the entire
muscle mass of the posterior arm distal to the del¬
toid and therefore is easily palpated. The lateral
head is parallel to the posterior border of the del¬
toid muscle and is prominent on the lateral aspect
of the arm. Just medial to the lateral head is the
belly of the long head. It can also be identified as it
enters the axilla inferior and anterior to the posterior
deltoid. The medial head is palpated distally on the
arm close to the medial epicondyle.

unlike its antagonist the biceps brachii, it contributes only to
elbow extension, with no influence on pronation and supina¬
tion. EMG data from surface electrodes suggest that the
three heads of the triceps muscle are recruited individually,
although individual variation in recruitment patterns exists
among the healthy population [18]. Some studies suggest that
the medial head is more frequently recruited first [6,18].
These reports suggest that increased resistance appears to
activate the muscles long and lateral heads. However, another
study, using both surface and indwelling electrodes, demon¬
strates similar recruitment of all three heads, even during
contractions with little resistance [31]. Like that of the
brachialis, the EMG activity of the triceps brachii is unaffected
by the position of the forearm.

Biomechanical analysis suggests that the medial head of the
triceps brachii contributes more to the extensor moment at the
elbow than the lateral head and that the long head contributes
no more than 25% of the total extensor moment [54]. The
physiological cross-sectional area of the long head of the
triceps is less than one-third the total physiological cross-
sectional area of the whole triceps brachii [35].

Like elbow flexion strength, the greatest extension strength
is in the joints midrange. Some authors report that peak
extension torque occurs at 90° of elbow flexion [5,13], while
another study reports peaks at 70° of flexion [27]. The differ¬
ences among these studies are likely the result of differences
in measurement techniques. Normal variation may also con¬
tribute to the differences, and further research is needed to
resolve them. Like the elbow flexors, midrange of elbow
excursion is neither a position of greatest muscle length nor
of greatest angle of application for the triceps brachii. The
length of the triceps brachii increases as the angle of elbow
flexion increases [31]. However, the moment arm of the mus¬
cle appears to reach a peak in early elbow flexion then
decreases steadily as the elbow flexion angle continues to
increase [17,36]. The moment arm of the triceps brachii
changes less than that of any of the elbow flexors [36]. This
may be the result of the expanded attachment of the triceps
around the olecranon process maximizing the moment arm of
at least some of the triceps brachii throughout the range of
elbow flexion.

Although shoulder extension is generally accepted as a
function of the long head of the triceps brachii, little litera¬
ture is available verifying such a role [23,25,41,44]. In a
mathematical model of the shoulder during rapid flexion and
extension, Happee and van der Helm provide indirect evi¬
dence of triceps participation in active shoulder extension
and in the braking action to control rapid shoulder flexion
[20]. However, there are no known studies that provide
EMG data to identify the conditions under which the triceps
brachii is activated during shoulder motion. Similarly, there
are no known studies that confirm the ability of the triceps
brachii to adduct the shoulder, although several references
report adduction as an action of the muscles long head
[25,41,46]. It is clear that further study is needed to clarify
the action of the triceps brachii at the shoulder.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

235

Figure 12.19: Functional activities requiring contraction of the extensor muscles of the elbow. Pushing activities recruit the triceps
brachii. A. The individual pushes the door with active elbow extension. B. Active elbow extension is used to assist an individual in
rising from a chair.

EFFECTS OF WEAKNESS

Weakness of the triceps brachii has a profound effect on the
strength of elbow extension. Although other muscles con¬
tribute slightly to elbow extension, no other muscle has the
capacity to generate as much force in elbow extension. Loss of
the triceps muscle results in almost complete loss of extension
strength. The functional implications of zero elbow extension
strength must be considered carefully. In the upright posture,
the weight of the forearm and hand cause the elbow to extend.
Picking up and putting down an object require concentric and
eccentric contractions of the elbow flexors. However, pushing
an object or using the upper extremity to assist in ris¬
ing from a chair requires the active contraction of the
triceps brachii (Fig. 12.19).

Clinical Relevance

TRICEPS WEAKNESS IN INDIVIDUALS WITH
TETRAPLEGIA: Individuals with tetraplegia at the level of
C6 lack active control of the triceps brachii (mmT <3),
innervated at the level of C7 and C8. Yet these individuals
generally have control of the elbow flexors and the shoulder
muscles. This remaining motor control allows most to per¬
form independent sliding transfers such as to and from

wheelchairs. Despite the absence of elbow extension
strength , the individual is able to bear weight on the upper
extremity by locking the elbow in extension. The elbow can
be maintained in extension passively by placing the elbow
in hyperextension and supporting it by the bones and liga¬
ments of the joint or by keeping the weight of the head and
trunk posterior to the elbow joint , thereby creating an exten¬
sion moment at the joint (Fig. 12.20). However ; the presence
of a flexion contracture prevents the individual from sup¬
porting the elbow passively by locking the elbow and may
compromise function [21]. Grover et al. demonstrate that an
elbow flexion contraction of approximately 25° prevents a
patient with C6 tetraplegia and complete loss of triceps
brachii strength from performing a sliding transfer [19]. Thus
the prevention of elbow flexion contractures is an essential
element in the goal of independent function for individuals
with C6 tetraplegia.

EFFECTS OF TIGHTNESS

Tightness of the triceps brachii limits elbow flexion ROM and
may contribute to diminished shoulder elevation ROM.
Chapter 11 notes that most activities of daily living can be
performed with a total elbow flexion excursion of about 100°
[34]. Thus significant tightness of the triceps could result in

236

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 12.20: Locking the elbow allows stable elbow extension
during weight bearing on the upper extremity, even in the
absence of elbow extension strength.

Figure 12.21: Asymmetrical tonic reflex. The infant demonstrates
the typical posture assumed by a healthy child exhibiting an
asymmetrical tonic reflex (ATNR).

serious functional impairments, especially in personal care
activities such as feeding and hygiene.

Clinical Relevance

ASYMMETRICAL TONIC NECK REFLEX (ATNR) IN A
CHILD WITH A DEVELOPMENTAL DISORDER: The

ATNR is a normally occurring motor reflex in infants. The
reflex is manifested in the upper extremities by a change in
muscle tone in each upper extremity, determined by the
rotation of the head and neck. As the head is turned to one
side; there is an increase in motor tone in the extensor mus¬
cles of the upper extremity to which the head is turned.

There is a concomitant increase in flexor tone in the oppo¬
site extremity (Fig. 12.21). Increased muscle tone in the
extensor muscles creates an increased resistance to flexion.
This reflex usually is integrated as normal motor develop¬
ment unfolds during the first year, before the child can per¬
form many independent activities of daily living. However ; in
some children with developmental delays and impaired
motor control the reflex may continue to be evident even as
the child becomes ready for some functional independence.
In this case ; the abnormal presence of an ATNR may inter¬
fere with the child's ability to gain independence in activities

such as self-feeding. As the child looks at the hand with the
food in it, the extensor tone increases in that limb, increas¬
ing the difficulty of flexing the elbow and bringing the food
to the mouth.

Anconeus

The anconeus is a short, small muscle that lies slightly distal
to the triceps brachii (Muscle Attachment Box 12.6).

ACTIONS

The actions and functional significance of the anconeus
remain elusive.

MUSCLE ACTION: ANCONEUS

Action Evidence

Elbow extension Supporting

Lateral deviation of the ulna Inadequate

Authors consistently report that the anconeus assists the
triceps brachii in elbow extension [23,25,41,44,53]. Bozec
et al. use EMG data to conclude that the anconeus is a “true
extensor muscle,” particularly when a subject generates
small levels of extension torque [31]. However, inspection

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

237

MUSCLE ATTACHMENT BOX 12.6

ATTACHMENTS AND INNERVATION
OF THE ANCONEUS

Proximal attachment: Posterior surface of the lateral
epicondyle of the humerus and adjacent joint cap¬
sule.

Distal attachment: Lateral aspect of the olecranon
process and posterior surface of the proximal ulna.

Innervation: Radial nerve, C6, C7, and C8.

Palpation: The anconeus can be palpated distal to
the triceps brachii on the lateral aspect of the
elbow. It must be carefully distinguished from the
nearby extensor carpi ulnaris [41].

of the anconeus reveals that the muscle is considerably
smaller in PCS A than the triceps. Therefore, its potential
strength is much less. In addition, its moment arm is small¬
er than that of the triceps brachii, which attaches to the
olecranon process of the ulna. Consequently, while the
anconeus may contribute to elbow extension, it is unlikely
to add significant strength to the movement or to be able to
compensate for the loss of the triceps brachii. It is estimated
that it may contribute no more than 10-15% to a total
extensor muscle moment [31,54]. As the extensor muscle
activity increases, the relative contribution of the anconeus
decreases [54].

However, the attachment of the anconeus to the posteri¬
or joint capsule of the elbow may explain another role for
the anconeus in active elbow extension. The capsule is lax
posteriorly, allowing the joint its large flexion ROM.
However, this laxity could permit a portion of the capsule to
be caught between the olecranon process and olecranon
fossa during extension. Such impingement would be very
painful, since joint capsules possess large sensory innerva¬
tions. The function of the anconeus may be to assist in
pulling the posterior joint capsule of the elbow out of harms
way during active extension.

The role of the anconeus in moving the ulna laterally
continues to be debated. Pronation of the forearm is classi¬
cally described as the radius rotating around a fixed ulna.
However, to keep the hand fixed in space, pronation occurs
with slight lateral deviation of the ulna as the radius turns
around it. Some authors suggest that the anconeus con¬
tributes to the lateral deviation, or abduction of the ulna,
during pronation of the forearm when the hand must
remain fixed in space, such as when turning a screwdriver
[24,53]. EMG studies report activity in the anconeus during
pronation. However, the muscle appears active during
supination as well [6]. Therefore, its role in ulnar abduction
has been neither verified nor refuted, requiring further
study to resolve the issue.

EFFECTS OF WEAKNESS

Reports of isolated weakness of the anconeus have not been
found in the literature. As noted above, the triceps brachii
remains the primary extensor of the elbow joint. Therefore,
weakness of the anconeus may have little effect on extension
strength. However, weakness may impair the ability to pre¬
vent impingement of the posterior joint capsule during elbow
extension. In addition, perhaps weakness of the anconeus
impairs the ability to pronate the forearm while maintaining
the hand in the same location. These latter two functional
deficits are merely conjecture. Additional study and clinical
evidence are needed to clarify the impact of weakness of the
anconeus muscle.

EFFECTS OF TIGHTNESS

Isolated tightness of the anconeus is unrealistic, since the fac¬
tors leading to its tightness would also affect the triceps
brachii as well as other nearby structures. Therefore, the
effects of the tightness in the other structures would override
impairments from anconeus tightness.

SUPINATOR MUSCLES

The primary supinator muscles of the forearm are the biceps
brachii and the supinator. Other forearm muscles that are dis¬
cussed in Chapter 15 may contribute to supination, but their
primary actions are at the wrist and hand. The biceps brachii
is discussed earlier in the current chapter. Only the supinator
is presented below.

Supinator

The supinator muscle is usually presented with the other
forearm muscles innervated by the radial nerve. It is pre¬
sented here because its action is focused at the superior
radioulnar joint (Muscle Attachment Box 12.7). It lies deep to
the wrist and finger extensor muscles in the proximal fore¬
arm (Fig. 12.22).

ACTIONS

MUSCLE ACTION: SUPINATOR

Action

Evidence

Elbow supination

Supporting

The reported action of the supinator is, as its name sug¬
gests, supination. There appears to be no doubt among
authors regarding the muscle’s role as a supinator
[23,25,41,44,53]. However, fewer data are available regard¬
ing its coordination with the biceps brachii, the other essen¬
tial supinator of the forearm. The most conclusive EMG
data available examining the functions of these two muscles
are presented by Basmajian and De Luca [6]. EMG data
suggest that the supinator is responsible for slow unresisted

238

Part II I KINESIOLOGY OF THE UPER EXTREMITY

MUSCLE ATTACHMENT BOX 12.7

ATTACHMENTS AND INNERVATION
OF THE SUPINATOR

Proximal attachment: The lateral epicondyle of the
humerus with attachments on the lateral aspect of
the joint capsule, the annular ligament and supina¬
tor fossa and crest of the ulna.

Distal attachment: Lateral, anterior and posterior
surfaces of the proximal one third of the radius.

Innervation: Nerve supply to the supinator muscle
may arise from the posterior interosseus nerve or
from the main trunk of the radial nerve [50]. Thus
authors report spinal innervation from C6 and C7
[50] or from C5 and C6 [31,38].

Palpation: The muscle lies deep to the superficial
extensor muscles of the forearm and is difficult to
palpate. Palpation may be possible just posterior
to the extensor muscle mass if the latter remains
relaxed [41].

supination, regardless of elbow position. The biceps brachii
appears to be held in reserve until supination is resisted.
The supinator remains particularly important during
supination when the elbow is extended, even in the pres¬
ence of resistance, since, as noted above in the chapter, the
biceps brachii is inhibited during supination when the
elbow is extended. Additionally, elbow flexion or extension
position appears to have little effect on the supinators
supination moment arm [10]. In contrast, elbow extension
appears to significantly reduce the supination moment arm
of the biceps brachii.

Clinical Relevance

MANUAL MUSCLE TESTING (MMT) OF THE
SUPINATOR MUSCLE: Standard MMT procedures for the
supinator muscle are described with the elbow extended and
with the elbow and shoulder flexed [25] (Fig. 12.23). EMG data
and an understanding of the mechanics of muscle action
demonstrate the theories underlying these two positions. EMG
data suggest that with the elbow extended\ the biceps brachii is
inhibited. Its contribution to supination strength also may be
reduced because of its decreased supination moment arm in
elbow extension. Therefore>, supination in that position presum¬
ably results from activity of the supinator muscle. However ;
aggressively resisted supination with the elbow extended does
appear to elicit biceps brachii activity ; usually resulting in some

Figure 12.22: Primary supinator muscles of the elbow. The
primary supinator muscles of the elbow are the biceps brachii
and the supinator muscles.

elbow flexion. Since MMT is designed to resist the patient maxi¬
mally ; it is likely that the biceps is recruited in this test at least
in the phase when maximum resistance is applied.

On the other hand ' the MMT position with the elbow
and shoulder flexed is likely to recruit both the supinator
and biceps brachii muscles. However ; in this position the
biceps brachii is shortened almost maximally. As noted

(< continued )

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

239

(Continued)

earlier in this chapter and in detail in Chapter 4, shortening
a muscle reduces its contractile force. Therefore , although
the biceps brachii is most likely active during this MMT of
supination , the force that it can generate in supination is
greatly reduced ' and the force of supination measured is pri¬
marily the force of the supinator muscle. It is important for
the clinician to recognize that either test may be suitable
under certain circumstances. However ; it is essential to keep
in mind the factors influencing the results of each test.

Figure 12.23: Standard manual muscle testing (MMT) positions to
assess supination strength without the contribution of the biceps
brachii. A. Resisted supination with the elbow extended inhibits the
biceps brachii. B. Although the biceps brachii muscle is active during
resisted supination with the elbow maximally flexed, the muscle is
so short that it is unable to generate significant supination strength.

EFFECTS OF WEAKNESS

Weakness of the supinator weakens the strength of supina¬
tion. Of course if the biceps brachii remains intact, the indi¬
vidual continues to have considerable supination strength.
However, forceful supination and the ability to supinate with
the elbow extended are impaired.

Clinical Relevance

A CASE REPORT: A 20-year-old male received multiple
injuries after being hit by a truck. Included in the injuries
was a midhumeral fracture with radial nerve palsy distal
to the innervation of the triceps brachii. Treatment
focused on restoring strength to the wrist and finger
extensor muscles. The subject was reassessed 1 year after
the accident. At the time of reassessment the subject
noted considerable recovery in hand strength and no
apparent difficulty at the elbow. Examination of the elbow
revealed normal ROM and strength of the elbow flexors
and extensors. Examination also revealed that supination
strength with the elbow flexed to 90° was slightly reduced.
However ; supination with the elbow extended was
markedly reduced. The subject was unable to supinate
through the full supination ROM. Any additional resist¬
ance against supination with the elbow extended resulted
in elbow flexion. These results revealed weakness of the
supinator muscle that had gone undetected in previous
examinations.

EFFECTS OF TIGHTNESS

Isolated tightness of the supinator muscle is unlikely.
However, it may be tight along with the biceps brachii.
Tightness of the supinator can be distinguished from tight¬
ness of the biceps brachii by applying an understanding of
two joint muscles. As described in the discussion of tightness
of the biceps brachii, shoulder position affects the length of
the biceps and thus can alter the manifestations of biceps
tightness at the elbow. Shoulder position has no effect on the
supinator muscle s length.

COMPARISONS OF THE STRENGTH
OF ELBOW FLEXION AND EXTENSION

Determining the functional significance of weakness of elbow
muscles is an essential element of a valid evaluation. An
understanding of the relative strength of muscle groups can
help provide a perspective on the extent of weakness that a
patient exhibits. Several studies report the strength of either
the flexor muscle group or the extensor muscle group
[13,42,48,52]. However, only a few studies directly compare

240

Part II I KINESIOLOGY OF THE UPER EXTREMITY

TABLE 12.2: Strength of Elbow Flexor and Extensor Muscle Groups Reported in the Literature

Gallagher et al. [16] a

Askew et al. [5] b

Sale et al. [42] c

Currier [13] d

Williams and
Stutzman [52] e

Flexion, males dominant side

40.0 Nm

725 ± 154 kg-cm

60 Nm

NT f

80 lb

Flexion, males nondominant

708 ± 1 56 kg-cm

Flexion, females dominant side

336 ± 80 kg-cm

30 Nm

Flexion, females nondominant

323 ± 78 kg-cm

Extension, males dominant side

27.1 Nm

421 ±109 kg-cm

48.8 lb

Extension, males nondominant

406 ± 106 kg-cm

Extension, females dominant side

210 ± 61 kg-cm

Extension, females nondominant

194 ± 50 kg-cm

a N = 30 males, 2-30 years old.

b N = 50 males, 41.0 ± 12.3 years; 54 females, 45.1 ± 16.1 years.

c Torque values estimated from graphs. N = 13 males, 22.5 ± 1.6 years; 8 females, 21.0 ± 0.6 years.
d N = 41 males, 20-40 years old

e Torque values estimated from graphs. N = 10 "college men."
f NT, authors did not test that factor.

the strength of the elbow flexors and extensors in individuals
without elbow pathology [5,16,26]. Absolute measures of
strength reported in the literature are difficult to compare
because they are reported in different units of force and
torque. Values of isometric strength collected at 90° of elbow
flexion are presented in Table 12.2. Askew et al. report that
mean isometric extension strength is 61% of mean isometric
flexion strength in 104 male and female subjects [5].
However, Knapik and Ramos report isometric elbow exten¬
sion strength is approximately 82% of isometric elbow flexion
strength in 352 males [26]. Although no direct comparison is
presented, Gallagher et al. consistently report greater flexion
strength than extension strength during concentric isokinetic
contractions at various speeds [16]. Comparison of data across
studies supports the conclusion that flexion strength is greater
than extension strength at the elbow [30].

Men demonstrate significantly greater flexion and exten¬
sion strength than women, regardless of contraction mode
[5,16,27,42]. Flexion and extension strengths also appear to
decrease with age. A small but significant increase in elbow
flexion strength is reported on the dominant side [5,16].
However, there is disagreement about the effect of hand dom¬
inance on elbow extension strength, with one study reporting
small but significant increases on the dominant side, similar to
that found in flexion strength [5]. Another study finds no effect
of dominance on extension strength [16].

Although the data are scanty and direct comparisons are
not easily made, certain principles begin to emerge from
these studies:

• Mean elbow flexion strength is greater than mean exten¬
sion strength.

• Mean elbow strength is greater in men than in women.

• Elbow strength appears to decrease with age.

• Hand dominance may explain only a small difference in
strength between the left and right elbows.

Additional research is needed to confirm these apparent rules
in larger populations as well as to examine other factors
affecting strength. However, clinicians can use these prelimi¬
nary concepts to provide additional perspective as they
attempt to interpret strength measures acquired from
patients in the clinic.

SUMMARY

This chapter reviews the role of the primary muscles of the
elbow in function and in dysfunction. The muscles included
in this chapter are those that insert in the forearm with no
effect distally at the wrist. Each muscle is presented sepa¬
rately to examine its specific contribution to elbow motion
and to pathological motor behavior. EMG data are used to
examine the coordination among the muscles of the elbow
and the way in which function is distributed among the mus¬
cles. The principles of muscle mechanics presented in
Chapter 4 are used to explain the individual roles of each
muscle and the methods used to isolate each muscle during a
clinical examination.

The brachialis and biceps brachii generate the largest flex¬
ion moments at the elbow, and all three primary elbow flex¬
ors exhibit architectural designs to provide active movements
through the elbows full excursion. The medial and lateral
heads of the triceps brachii generate the largest elbow exten¬
sion moments. The pronator teres and supinator muscles are
tested by positioning the subjects elbow so that the other
muscles are rendered ineffective. Several wrist muscles also
affect the elbow. These muscles and their effects on the wrist
and elbow are presented in Chapter 15. In the following
chapter, the issues of joint structure and muscle performance
presented in the previous and current chapters are examined
in combination by studying their effects on the forces at the
elbow joint and loads in the elbow muscles during activity.

Chapter 12 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ELBOW

241

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19. Grover J: The effect of a flexion contracture of the elbow on the
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20. Happee R, van der Helm FCT: The control of shoulder muscles
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CHAPTER

Analysis of the Forces
at the Elbow during Activity

CHAPTER CONTENTS

ANALYSIS OF THE FORCES EXERTED AT THE ELBOW .243

Forces on the Elbow during Simple Upper Extremity Lifting Techniques.243

Forces on the Elbow during Upper Extremity Weight Bearing.247

STRESSES APPLIED TO THE ARTICULAR SURFACES OF THE ELBOW.250

SUMMARY .251

f n the preceding two chapters the structure of the elbow joint and its supporting elements is described and
the roles of the primary muscles of the elbow are presented. The purpose of the present chapter is to discuss
the loads to which the elbow joint and surrounding structures are subjected. Specifically the aims of this
chapter are to

■ Present a two-dimensional analysis of the loads on the elbow during a simple lifting task
■ Present a two-dimensional analysis of the loads on the elbow during upper extremity weight bearing
■ Review the loads the elbow sustains during a variety of activities

■ Discuss the stresses (load/area) applied to the humeroulnar and humeroradial articulations

ANALYSIS OF THE FORCES EXERTED
AT THE ELBOW

Forces on the Elbow during Simple
Upper Extremity Lifting Techniques

Elbow flexion is a basic element of countless activities of daily
living as diverse as eating an apple and lifting a bag of groceries
or an anatomy textbook. Examining the Forces Box 13.1
presents the free-body diagram depicting the task of holding
a 5-lb bag of sugar [5,18]. It also provides the simple two-
dimensional analysis of the required flexor force and resulting

joint reaction force at the elbow generated during the activity.
By lumping the flexor force all into the brachialis muscle, the
calculation reveals that the flexor force needed to hold a 5-lb
load in an outstretched limb is greater than 1 times body
weight! Calculation of the joint reaction force suggests com¬
pressive loads of almost 1.2 times body weight.

The solution presented in Examining the Forces Box 13.1
requires the rather large and inaccurate assumption that all of
the flexor forces can be ascribed to the brachialis muscle,
whose moment arm is known. In reality, it is most likely that
the brachialis and biceps are both participating, and under
enough resistance, the brachioradialis and pronator teres may

243

244

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

EXAMINING THE FORCES BOX 13.1

TWO-DIMENSIONAL ANALYSIS OF THE
FORCES IN THE ELBOW WHILE HOLDING
A FIVE-POUND LOAD WITH THE ELBOW
FLEXED TO 30°

The following dimensions are based on a well-
conditioned male who is 6 feet tall (1.83 m) and
weighs 180 lb (800 N). The limb segment parameters
are extrapolated from the anthropometric data of
Braune and Fischer [5]. The flexion force is assumed
to be provided entirely by the brachialis muscle (B).

Length of forearm and hand (L): 0.4 m
Center of gravity (c.g.) of forearm and hand is 47% of
length of forearm and hand from proximal end (I)
Weight of forearm and hand (W): 3% of body
weight (BW)

Weight in the hand: 5 lb (3% BW)

Moment arm of brachialis (ma): 0.015 m

Solve for brachialis force (B):

SM = 0

(B X 0.015 m) - (0.03 BW X 0.47 X 0.4 m)

- (0.03 BW X 0.4 m) = 0

(B X 0.015 m) = (0.03 BW X 0.47 X 0.4 m)

+ (0.03 BW X 0.4 m)

B = 1.18 BW

Calculate the joint reaction forces (J) on the ulna.
Assume that the brachialis is applied to the ulna at an
angle of 25°.

SF x : J x - B x = 0

J x = B x , where B x = B (cos 25°)

J x = B (cos 25°)

J x = 1.07 BW

£F y : J Y + B y - 0.03 BW - 0.03 BW = 0, where the
weight of the forearm and hand is 0.03 BW and the
5-lb weight = 0.03 BW

J Y = 0.06 BW - B y/ where B Y = B (sin 25°)

J Y = - 0.45 BW

Using the Pythagorean theorem:

j 2 = v+v

J « 1.16 BW

Using trigonometry, the direction of J can be determined:

■ J x
sin 0 = —

0 ~ 67° from the vertical

Free-body diagram of the elbow for the task of lifting a 5-lb load identifies all of the forces acting
on the forearm.

Chapter 13 I ANALYSIS OF THE FORCES AT THE ELBOW DURING ACTIVITY

245

Figure 13.1: Free-body diagram of the elbow during flexion.
The free-body diagram shows the forces from all of the elbow
flexors, demonstrating that there are more unknown forces
than can be determined using the static equilibrium equa¬
tions. The system is described as statically indeterminate.

Bi, biceps brachii; Br, brachialis; Pt, pronator teres; Brd,
brachioradialis.

also contribute to the flexor force. However, the real situation
of multiple muscles contracting simultaneously has more
unknowns than equations to solve and is known as statically
indeterminate [4] (Fig. 13.1).

A problem that is indeterminate has an infinite number
of possible solutions. Chapter 1 briefly describes methods
used to solve the indeterminate problem by choosing the
best solution on the basis of some rather arbitrary optimiz¬
ing criterion. Another approach to managing the case of
more unknowns than equations is to solve only for the
internal moment that must be exerted by the surrounding
muscles and ligaments. The internal moment is the
moment generated by muscles and ligaments to resist the
external moment generated by the weight of the limb and
any additional weights or forces applied from the environ¬
ment. Such forces include additional weights such as the
bag of sugar, resistance applied by a therapist, and ground
reaction forces. By solving only for the internal moment,
there is no attempt to distribute that moment to the mus¬
cles or other surrounding tissue. Therefore, there is no
need for erroneous simplifying assumptions such as lump¬
ing the force into a single muscle. However, the solution
provides no estimates of muscle or joint forces. Examining
the Forces Box 13.2 presents the free-body diagram and the
calculation of the internal moment using the same case
described in Box 13.1.

The internal moment needed from the flexor muscles
to hold a 5-lb (22.24 N) bag is 13.4 Nm. In a published
biomechanical analysis of lifting, the reported peak internal

moment needed to lift a 38-lb (170 N) dumbbell through
the range of elbow flexion is approximately 45 Nm [7].
These results offer enough similarity between the model
presented in Examining the Forces Box 13.2 and the pub¬
lished model to provide at least face validity to the model in
Box 13.2.

Although the solution in Examining the Forces Box 13.1
is based on a clearly erroneous simplification, it does offer
the clinician a useful perspective on the magnitude of the
loads required of the muscles of the elbow during such a
simple task as lifting a relatively small load. The reason for
such large muscular loads is the mechanical disadvantage
of the muscles compared with the advantage of the 5-lb
load and the weight of the forearm and hand. The moment
arm of the brachialis is approximately 8% of the moment
arm of the weight of the forearm and hand and less than
4% of the moment arm of the 5-lb load (Fig. 13.2).
Consequently, the muscle must generate large forces to
create a moment to balance the moments exerted by the
weight of the limb and the 5-lb load.

A muscle s contraction force has a direct impact on the
joint reaction force. The solution seen in Examining the
Forces Box 13.1, albeit only a rough estimate, demon¬
strates that the elbow joint sustains large loads even during
simple tasks such as lifting small loads. Several more elab¬
orate biomechanical models examine the elbow joint
forces during elbow function. Loads of up to 1,600 N (360
lb) and 800 N (180 lb) are reported at the humeroulnar
and radiohumeral articulations, respectively, during simple
lifting tasks, lifting loads of only 2-4 kg (4.4-8.8 lb) [6].
The elbow forces vary significantly with the type of hand
grip used. High-speed flexion and extension movements
generate even larger loads at the humeroulnar and radio-
humeral joints, 1,910 N (approximately 430 lb) and 2,680
N (approximately 602 lb), respectively [2]. Maximum iso¬
metric flexion efforts generate still larger loads, over 3,000
N (675 lb) at each joint [1]. Peak compressive forces par¬
allel to the forearm and directed into the humerus are esti¬
mated to be approximately 45% of body weight during
two-handed push-ups and approximately 65% of body
weight in a one-handed push-up [9,10].

Similar loads on the elbow are reported in falls on an
outstretched limb from a low height (3-6 cm) [8]. It
seems surprising that lifting activities could generate larg¬
er elbow joint forces than push-ups. Differences in mod¬
els may explain some of the differences in solutions, but
differences in the moment arms of the external loads on
the elbow may also contribute to the larger elbow loads
during lifting activities. The effects of moment arms are
discussed in more detail in the next section. These data,
although only estimates, reveal that the elbow articula¬
tions sustain very large loads during activity. A clinician
must remain aware of these loads when establishing an
intervention strategy for an individual with joint disease
and modify the intervention in ways to reduce the loads
on the joint.

246

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

CALCULATION OF THE INTERNAL MOMENT
(Mj) GENERATED IN THE EXAMPLE
PRESENTED IN EXAMINING THE FORCES
BOX 13.1

Dimensions presented in Examining the Forces
Box 13.1 remain the same.

Length of forearm and hand (L): 0.4 m
c.g. of forearm and hand is 47% of length of
forearm and hand from proximal end, (I)

Weight of forearm and hand (W): 3% of body
weight

Body weight: 180 lb (800 N)

Weight in the hand: 5 lb (22.24 N)

SM = 0

M internal + M external = °- where M internal ' S the

moment created by the muscles and ligaments of
the elbow and M externa| is the moment generated
by the weights of the forearm and hand and the
5-lb load

^internal ^external

M internal = (° 03 X 800 N X 0.47 X 0.4 m)

+ (22.24 N X 0.4 m) = 0

M int emal = 13.4 Nm

Free-body diagram of the elbow for the task of lifting a 5-lb load, indicating the external loads and the internal
moments. The free-body diagram reduces the muscle forces and their moment arms to a single internal moment (M).

Clinical Relevance

ELBOW LOADS IN BASEBALL PITCHERS: External valgus
moments (frontal plane moments tending to rotate the elbow
into valgus) of approximately 18 Nm are reported in 77- and
i2-year-o1d male baseball pitchers [23]. Professional baseball
pitchers reportedly sustain valgus moments of approximately

65 Nm [15]. Contrast these moments that are balanced by the
elbow's medial collateral ligament with perhaps additional sup¬
port from forearm muscles with the flexion moment of 28 Nm
balanced by the elbow extensor muscles during crutch walking.
It should not be a surprise that elbow injuries are common in
baseball pitchers. These data support the need to limit the
number of pitches thrown by skeletally immature athletes.

Chapter 13 I ANALYSIS OF THE FORCES AT THE ELBOW DURING ACTIVITY

247

Figure 13.2: Comparison of the brachialis muscle's moment arm with the moment arms of the external loads. The moment arms of the
external loads are much greater than the moment arm of the brachialis ( B), increasing the mechanical advantage of the weight of the
forearm and hand (1/1/) and the weight in the hand (/.).

Forces on the Elbow during Upper
Extremity Weight Bearing

Although the upper extremity is typically described as
having non-weight-bearing joints, discussions regarding
the shoulder in Chapters 9 and 10 clearly indicate that even
in healthy individuals, the upper extremity frequently
participates in weight-bearing activities such as rising from a
chair. When an individual has a lower extremity impairment,
the upper extremities may become even more involved in
weight bearing by actual participation in locomotion, as in
crutch walking and wheelchair propulsion. Increased bone
mineral density is reported in the shaft of the radius in 10
subjects with a mean duration of 8.7 years of crutch use
[25]. These data provide an indirect indication of the
increased stress applied to the elbow region as a result of the
additional weight-bearing responsibility of the upper
extremity in individuals who ambulate with assistive devices.
This report also demonstrates the application of Wolff s law
as the structures of the bones of the elbow respond to their
altered function. Examining the Forces Box 13.3 presents
the basic two-dimensional analysis of the loads on the elbow
joint during the swing phase of crutch ambulation.

The solution in Examining the Forces Box 13.3 is
presented as the estimated force of the triceps brachii
(1.4 times body weight) and as a net internal moment

(28 Nm). It is useful to compare the estimated force of the
triceps brachii in the crutch walking example to the flexor
force in the lifting example in Examining the Forces Box
13.1. Although the resistance in the crutch walking task
(one half body weight) is several times the resistance in the
lifting task, the required force of the triceps brachii is only
30% greater than that of the flexors. The reason for the dif¬
ference in the mechanical requirements of these two tasks
is the difference in the length of the moment arms between
the resistances and between the muscles (Fig. 13.3). In the
crutch walking example, the moment arm of the body
weight is 0.07 m, but in the lifting task, the moment arm of
the 5-lb weight is 0.4 m. Similarly, the moment arm of the
brachialis is 0.015 m, while the moment arm of the triceps
brachii is 0.025 m.

Clinical Relevance

THE IMPACT OF CRUTCH HEIGHT ON ELBOW JOINT
MOMENTS: Although the clinician can rarely alter a
muscle's moment arm , the moment arm of the resistance is
easily manipulated. Reisman et al. report an almost W0%
increase in internal moment in subjects ambulating with

( continued )

248

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

EXAMINING THE FORCES BOX 13.3

CALCULATION OF THE EXTENSION FORCE OF
THE TRICEPS BRACHII (T) AND THE INTERNAL
EXTENSION MOMENT (MJ GENERATED AT
THE ELBOW DURING CRUTCH WALKING

The subject's anthropometric measures are the same as
those used in Examining the Forces Boxes 13.1 and 13.2.
Assume that the elbow is flexed to 10° [7]. Note that
the subject pushes down on the crutch and therefore
the crutch exerts a reaction force (FJ on the hand
and forearm. That force (F c ) is assumed to be equal
to 1/2 BW, based on the presumption that the weight
is borne equally on two crutches. The joint reaction
force on the ulna is (J).

Moment arm of the triceps brachii (T) at 10° elbow
flexion is 0.025 m [17]

Moment arm (I) of the crutch force (F c ) = 0.4 m
(sin 10°) = 0.07 m

SM = 0

F c X 0.07 m - T X 0.025 m = 0
0.5 BW X 0.07 m = T X 0.025 m

T = 1.4 BW

M interna I + M external = 0

M interna I = (°' 5 BW X °- 07 m ) = 0 035 BW m '

where BW = 800 N

^internal = 28 Nm

Normalizing for body weight:

M intemal = 034 N m/k 9' where BW = 81.6 kg

Free-body diagram of the elbow
during crutch walking. A. The external
load and the internal forces exerted
by the triceps muscle and the joint
reaction force. B. The external load
and the internal moment to balance it.

Chapter 13 I ANALYSIS OF THE FORCES AT THE ELBOW DURING ACTIVITY

249

Figure 13.3: Comparisons of the moment arms of the muscles and the external loads on the lifting task (A) and in crutch walking
(B). Although the resistances are smaller in the lifting task, their moment arms are much larger than the moment arm of the crutch
force.

(Continued)

axillary crutches when the crutch handle is raised 1-2 in
from its optimal height [21]. This remarkable increase in the
muscles' requirements is the result of increased elbow flexion
and ' consequentlyan increase in the moment arm of the
resistance (Fig. 13.4). Similar changes in joint loads are
reported in exercises such as the push-up and bench press
[10,14]. These examples provide a powerful example of how
an understanding of joint moments can guide the clinician
in altering the requirements of a joint's muscles and
the load on a patient's joint by altering the moment
applied by the external load.

The net moment of 28 Nm in the crutch walking exam¬
ple also can be compared with other reports in the litera¬
ture. Robertson et al. calculate mean peak internal

moments at the elbow of 12.3 Nm during wheelchair
propulsion by regular wheelchair users and even larger
mean peak moments in those who are not wheelchair
users [22]. In Examining the Forces Box 13.3, the internal
moment is normalized for body weight to compare the
results of this model with additional results from pub¬
lished studies [19,20]. Nordau et al. report normalized
internal moments of about 0.2 Nm/kg in a careful and
thorough analysis of individuals, with and without para¬
plegia, ambulating with forearm crutches. Although the
agreement is not perfect, the solution from the current
example is close enough to be a reasonable reflection of
the biomechanical task. An analysis of the forces sustained
by muscles and joints during a task such as wheelchair
propulsion or crutch walking helps the clinician
appreciate the wear and tear that the upper
extremity sustains during activity

250

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 13.4: Comparison of the moment arm
of the crutch force with different elbow
positions. Increased elbow flexion increases
the moment arm of the crutch force, creat¬
ing a larger flexion moment on the elbow.
A. The elbow is flexed to 10°. B. The elbow
is flexed to 30°.

STRESSES APPLIED TO THE ARTICULAR
SURFACES OF THE ELBOW

The studies described above reveal that the elbow sustains
very large loads during everyday activities. However, how
these loads are applied to the articular surfaces is also an
important factor in understanding the mechanics and patho-
mechanics of the elbow. The shape and relative fit of the
articular surfaces of the humerus, ulna, and radius are
described in Chapter 11. That presentation reveals that
in many elbows, contact between the humerus and ulna
occurs only at the proximal and distal extents of the articular
surface of the trochlear notch [12,17,24]. In fact, measure¬
ments of the joint space reveal that there is greater space

between the humerus and ulna in the center of the trochlear
notch than at the proximal and distal aspects [11].

The joint reaction forces at the elbow are dispersed
across only the contacting surfaces of the humerus and
ulna. Stress is the measure of how a force is distributed over
an area (stress = force/area). Thus the stress at the elbow is
concentrated at the proximal and distal extremes of the
humeroulnar articulation. A study using both mathematical
analysis and experimental tests on a single cadaver speci¬
men assessed the stresses on the humeroulnar joint result¬
ing from simulated isometric extension forces of up to 500
N. (approximately 112 lb) [16]. Stresses of up to 3.6 MPa
(MN/m 2 ) (approximately 522 lb/in 2 , psi) are reported at the
proximal aspect of the trochlear notch and up to 2.3 MPa

Chapter 13 I ANALYSIS OF THE FORCES AT THE ELBOW DURING ACTIVITY

251

Figure 13.5: Compressive forces on the trochlea. Compressive
forces (F c ) on the trochlea are exerted on the proximal and distal
aspects of the trochlear notch.

(approximately 334 psi) in the distal aspect of the notch,
with only 0.45 MPa (approximately 65 psi) at the deepest
part of the notch (Fig. 13.5). This same study suggests that
the subchondral bone in the deepest portion of the notch
sustains tensile forces, while the proximal and distal aspects
undergo compressive loading (Fig. 13.6). Since Wolffs law
states that the structure of bone responds to the loads
applied to it, an understanding of the loading pattern of the
joint helps explain the bony architecture of the ulna.
Studies demonstrate increased mineralization in the
subchondral bone in the proximal and distal aspects of
the trochlear notch [13]. These data provide evidence
that Wolffs law is applicable to the architecture of the
elbow.

Figure 13.6: Tensile forces on the trochlear notch. Tensile forces
(f) are exerted in the deepest aspect of the trochlear notch.

Figure 13.7: Radiograph of a fracture of the olecranon. An X-ray
film of a fracture of the olecranon reveals that it occurred
through the middle of the trochlear notch at its deepest point,
where the bone mineralization is reduced and the notch sustains
tensile forces. (Courtesy of S. Kozin, MD, Shriner's Hospital for
Children, Philadelphia, PA.)

Clinical Relevance

OLECRANON FRACTURES: Olecranon fractures can occur
from an aggressive pull of the triceps causing an avulsion frac¬
ture or from a direct blow to the tip of the olecranon. In vitro
experiments with 40 cadaver limbs reveal that olecranon frac¬
tures through the deepest part of the trochlear notch are easily
produced by direct impacts to the proximal olecranon [3] (Fig.
13.7). The average impact producing the fracture is 4100 N
(approximately 920 lb). Such an impact simulates a fall onto
the tip of the elbow. It is significant that the fractures occur in
the deepest part of the trochlear notch where there is less min¬
eralization in the subchondral bone. Thus an awareness of the
bony architecture of the elbow helps explain a common injury.

Summary

This chapter examines the forces that are likely to be sus¬
tained by the elbow musculature during activity. Simple
examples used to determine muscle and joint forces demon¬
strate that the elbow sustains loads equal to at least body
weight during simple lifting tasks and may sustain loads sev¬
eral times body weight during more vigorous activities. The
examples demonstrated that the forces required of the mus¬
cles, and consequently the loads on the joints, can be altered
readily by manipulation of the magnitude or location of the
external loads.

The joint reaction force at the elbow is spread unevenly
across the joint surface of the humeroulnar articulation,

252

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

resulting in two areas of concentrated stress in the trochlear
notch. Data are presented demonstrating uneven bone min¬
eralization consistent with Wolffs law. The clinician can use
the understanding of joint forces gained from this chapter to
guide interventions to optimize the positive outcomes while
minimizing the deleterious effects of joint loading. This
understanding will also heighten the clinician s appreciation
of the hurdles to overcome in the design of prosthetic devices
for even one of the simplest joints, the elbow.

References

1. Amis AA: The derivation of elbow joint forces, and their relation
to prosthesis design. J Med Eng Technol 1979; 3: 229-234.

2. Amis AA, Dowson D, Wright V: Analysis of elbow forces due
to high-speed forearm movements. J Biomech 1980; 13:
825-831.

3. Amis AA, Miller JH: The mechanisms of elbow fractures: an
investigation using impact tests in vitro. Injury 1995; 26:163-168.

4. Andrews JR: Glenoid labrum tears related to the long head of
the biceps. Am J Sports Med 1985; 13: 337-341.

5. Braune W, Fischer O: Center of gravity of the human body. In:
Human Mechanics; Four Monographs Abridged AMRL-TDR-
63-123. Krogman WM, Johnston FE, eds. Wright-Patterson Air
Force Base, Ohio: Behavioral Sciences Laboratory, 6570th
Aerospace Medical Research Laboratories, Aerospace Medical
Division, Air Force Systems Command, 1963; 1-57.

6. Chadwick EKJ, Nicol AC: Elbow and wrist joint contact forces
during occupational pick and place activities. J Biomech 2000;
33: 591-600.

7. Challis JH, Kerwin DG: Quantification of the uncertainties in
resultant joint moments computed in a dynamic activity. J
Sports Sci 1996; 14: 219-231.

8. Chou P-H, Chou Y-L, Lin C-J, et al.: Effect of elbow flexion on
upper extremity impact forces during a fall. Clin Biomech 2001;
16:888-894.

9. Chou PH, Lin CJ, Chou YL, et al.: Elbow load with various fore¬
arm positions during one-handed pushup exercise. Int J Sports
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10. Donkers MJ, An K, Chao EYS, Morrey BF: Hand position
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11. Eckstein F, Lohe F, Hillebrand S, et al.: Morphomechanics of
the humero-ulnar joint: I. Joint space width and contact areas

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318-326.

12. Eckstein F, Lohe F, Schulte E, et al.: Physiological incongruity
of the humero-ulnar joint: a functional principle of optimized
stress distribution acting upon articulating surfaces? Anat
Embryol 1993; 188: 449^55.

13. Eckstein F, Muller-Gerbl M, Steinlechner M, et al.: Subchondral
bone density in the human elbow assessed by computed tomog¬
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the joint surfaces. J Orthop Res 1995; 13: 268-278.

14. Elliott BC, Wilson GJ, Kerr GK: A biomechanical analysis of
the sticking region in the bench press. Med Sci Sports Exerc
1989; 21: 450-462.

15. Fleisig GS, Barrentine SW, Zheng N, et al.: Kinematic and kinetic
comparison of baseball pitching among various levels of devel¬
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16. Merz B, Eckstein F, Hillebrand S, Putz R: Mechanical implica¬
tions of humero-ulnar incongruity-finite element analysis and
experiment. J Biomech 1997; 30: 713-721.

17. Milz S, Eckstein F, Putz R: Thickness distribution of the sub¬
chondral mineralization zone of the trochlear notch and its cor¬
relation with the cartilage thickness: an expression of functional
adaptation to mechanical stress acting on the humeroulnar
joint? Anat Rec 1997; 248: 189-197.

18. Murray WM, Delp SL, Buchanan TS: Variation of muscle
moment arms with elbow and forearm position. J Biomech
1995; 28: 513-525.

19. Noreau L, Comeau F, Tardif D, Richards CL: Biomechanical
analysis of swing-through gait in paraplegic and non-disabled
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21. Reisman M, Burdett RG, Simon SR, Norkin C: Elbow moment
and forces at the hands during swing-through axillary crutch
gait. Phys Ther 1985; 65: 601-605.

22. Robertson RN, Boninger ML, Cooper RA, Shimada SD:
Pushrim forces and joint kinetics during wheelchair propulsion.
Arch Phys Med Rehabil 1996; 77: 856-864.

23. Sabick MB, Torry MR, Lawton RL, Hawkins RJ: Valgus torque
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24. Tillmann B: A contribution to the functional morphology of
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25. Wing PC, Tredwell SJ: The weightbearing shoulder. Paraplegia
1983; 21: 107-113.

UNIT 3

WRIST AND HAND UNIT

T he preceding chapters discuss the structure and function of the shoulder and elbow. The shoulder is remark¬
ably mobile, and that mobility creates a huge potential space through which the hand can be moved (Figure),
In contrast, the elbow is much less mobile but allows the hand to approach and move away from the body.
The ultimate functional application for both of these joint complexes is to position the hand. The hand is responsible
for carrying out the work of the upper extremity.

The functions that can be accomplished by the hand are as varied as kneading bread dough and sculpting a master¬
piece, as diverse as meat cutting and neurosurgery. The hand is a manipulator and a communicator. Hands are thrown
up in disgust or laid tenderly on a baby's cheek. The hand is powerful enough to be a weapon and gentle enough to
be a tool of art and love.

Such diversity requires a wide range of positions and forces, as well as remarkable sensitivity. This broad spectrum
of performances demands structural complexity with relative ease of operation. The hand represents a significant
increase in architectural complexity compared with the more proximal joints of the upper extremity. Yet the organiza¬
tion of the hand offers remarkable synergy among its structures, which allows efficient completion of a task.

The approximate space through which the hand can move. Motion
of the shoulder and elbow can position the hand anywhere in a
huge volume in front of, beside, and behind the body.

253

UNIT 3

WRIST AND HAND UNIT

The focus of the next six chapters is the structure and function of the hand and all of its components. These chapters
demonstrate how the components function individually and together, so that the clinician can appreciate how pathol¬
ogy in one element affects the entire complex. These six chapters are divided into two interrelated groups. Chapters
14, 15, and 16 present the linkage between the hand and the rest of the upper extremity, focusing on the wrist and
the muscles of the forearm. Since many of the muscles of the forearm affect the hand, this section includes the bones
and joints of the hand. As in the preceding units on the shoulder and elbow, the first chapter in this unit (Chapter 14)
presents the bones and joints of the region. The second chapter (Chapter 15) provides a discussion of the mechanics
and pathomechanics of the muscles, and the third chapter (Chapter 16) furnishes an analysis of the forces sustained
by the region. The specific purposes of Chapters 14-16 on the wrist and forearm are to

■ Present the structure and function of the bones and joints of the wrist and hand

■ Discuss the muscles of the forearm and their contribution to the function and dysfunction of the hand

■ Analyze the forces that are transmitted through the wrist

Chapters 17, 18, and 19 present the morphology and function of the structures that are specific to the hand, including
the intrinsic muscles of the hand. The purposes of Chapters 17 through 19 are to discuss the soft tissue structures
that are intrinsic to the hand and to relate their function to the function of the joints and extrinsic muscles already
discussed. Chapter 17 presents the special connective tissue structures of the hand and discusses their participation
in the function and dysfunction of the hand. Chapter 18 presents the structure and function of the intrinsic muscles
of the hand. Chapter 19 examines the mechanics of pinch and grasp and then explores the forces applied to the digits.
The specific goals of these chapters are to

■ Review the morphology and function of the special connective tissue structures found within the hand

■ Discuss the mechanics and pathomechanics of the intrinsic muscles of the hand

■ Present the functional interplay between the intrinsic and extrinsic muscles of the hand

■ Discuss the mechanics and pathomechanics of grasp and pinch

■ Analyze the forces sustained by the fingers and thumb during activity

254

Structure and Function of the
Bones and Joints of the Wrist
and Hand

CHAPTER

CHAPTER CONTENTS

STRUCTURE OF THE BONES OF THE WRIST AND HAND.256

Distal Radius and Shaft.256

Distal Ulna and Shaft .257

Carpal Bones .259

Metacarpals.263

Phalanges.264

Sesamoid Bones.265

Bony Landmarks .265

ARTICULATIONS AND SUPPORTING STRUCTURES OF THE JOINTS OF THE WRIST AND HAND .265

Distal Radioulnar Joint.265

Joints of the Wrist.268

Movements of the Wrist.273

Global Wrist Motions .275

Carpometacarpal Joints .278

MCP Joints of the Digits.283

Interphalangeal Joints of the Fingers and Thumb.286

SUMMARY .289

T his chapter focuses on the skeleton and joints of the wrist and hand. All of these structures are considered
together, since many of the muscles of the forearm extend into the fingers. Understanding the role of these
muscles in the fingers requires a thorough knowledge of the joints and movements of the fingers. The specific
purposes of this chapter are to

■ Describe the structure of the bones of the wrist and hand to understand how they contribute to move¬
ments of the hand

■ Discuss the ligaments and supporting structures of the joints of the wrist and hand and their contribu¬
tion to the stability of the hand

■ Demonstrate the clinical relevance of some of the specific anatomical details of the bones and ligaments
of the region

■ Review the normal ranges of motion in the wrist and hand

255

256

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Details of the bony structures in the wrist and hand are presented first to demonstrate how the shapes of the bones
influence the mechanics and pathomechanics of the wrist and hand. The structure of the joints and their supporting
structures are discussed next. An understanding of the joints and surrounding structures forms the basis for the pres¬
entation of the motions that occur at the wrist and within the hand.

STRUCTURE OF THE BONES
OF THE WRIST AND HAND

A partial explanation for the precision movements available in
the hand is the presence of so many bones and joints that can
move in concert with, or independently of, one another. The
shoulder and elbow complexes each consist of three bones,
although the shoulder complex also involves a fourth bone,
the sternum. The hand, however, contains 27 primary bones
with an important, albeit indirect, association with a twenty-
eighth, the ulna, and a variable number of sesamoid bones!
The relevant characteristics of the bones that influence the
mechanics and pathomechanics of the wrist and hand are pre¬
sented below.

Distal Radius and Shaft

The proximal radius is described in Chapter 11 with the
elbow. The shaft of the radius is somewhat triangular in the
transverse plane, with a sharp medial edge that provides
attachment for the interosseous membrane, also known as the
interosseous ligament. The radial shaft is largely covered by
muscles of the forearm, and only the proximal and distal ends
of the radius are readily palpated. The radius widens distally
in the medial and lateral directions so that the distal end is the
widest part of the radius.

The distal end of the radius has five important surfaces:
dorsal (posterior), volar (anterior), radial (lateral), ulnar
(medial), and distal (Fig. 14.1). The dorsal surface is charac¬
terized by a palpable prominence known as the dorsal tuber¬
cle, with grooves on either side of it for the extensor pollicis
longus tendon on its ulnar side and for the extensor digitorum
and the extensor indicis tendons radially The dorsal tubercle
serves as a pulley to redirect the pull of the extensor pollicis
longus.

The radial surface of the radius is roughened and termi¬
nates in the distal projection, the styloid process, which is eas¬
ily palpated on the radial aspect of the wrist joint in the
anatomical snuffbox. The volar surface of the radius is slightly
concave in the radioulnar direction and terminates distally in
a distinct ridge to which the capsule of the wrist attaches. This
ridge is palpable about 2.5 cm proximal to the thenar emi¬
nence. It serves as a reliable identifying landmark of the
radiocarpal joint.

The ulnar surface of the distal radius is composed of the
ulnar notch, providing an articular surface for the distal
radioulnar joint. This notch is generally described as concave
from its volar to its dorsal borders [77,141]; however, its shape

is quite variable. It may be concave, flat, and even S-shaped,
or sigmoid [38]. Consequently, clinical literature frequently
refers to the ulnar notch as the sigmoid notch [34,77].
Because of its use in the clinical literature, this text employs
the term sigmoid notch rather than the anatomical term,
ulnar notch. The notch is variable in its proximal-to-distal ori¬
entation [34,77] and appears to be influenced by the relative
length of the ulna [29,65] (Fig. 14.2). Like all joints, the
mobility and stability of the distal radioulnar joint are influ¬
enced significantly by the shape of the articular surfaces,
including the sigmoid notch.

The distal surface of the radius is the proximal articular
surface of the wrist. It articulates with the scaphoid and
lunate. It is biconcave, concave in both the volar-dorsal and
the ulnar-radial directions (Fig. 14.3). Although the articular
surface is continuous, there is a ridge that separates the sur¬
face into distinct surfaces for the scaphoid radially and the
lunate on the ulnar side of the ridge. The distal articular sur¬
face is tilted in a volar direction approximately 10-15° and
faces in an ulnar direction approximately 15-25° [12,127]
(Fig. 14.4). Karnezis suggests that the volar tilt decreases the
shear forces on the distal radius during lifting tasks and is pos¬
itively correlated with the wrist joint reaction force [64]. The
tilt and inclination of the distal radius also helps explain the
direction of carpal subluxation, ulnar and volar, in the unsta¬
ble wrists of patients with rheumatoid arthritis.

tubercle

A B

Figure 14.1: Distal end of the radius. The distal radius widens
and displays five distinct surfaces: dorsal, volar, radial, ulnar,
and distal. A. Volar view. B. Dorsal view.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

257

Figure 14.2: Tilt of the sigmoid notch. The proximal-distal tilt
of the sigmoid notch is influenced by the relative length of the
articulating ulna. When the ulna is relatively short, the notch is
often tilted proximally as demonstrated in this figure.

Clinical Relevance

DISTAL RADIUS FRACTURE: Fractures of the distal radius
are the most common fractures in adults over 50 years of
age, more than three times more common in women than
in men [74,78,79,107]. The most common type of distal
radius fracture is the Coties' fracture, an extraarticular frac¬
ture in which the distal fragment of the radius undergoes
dorsal displacement accompanied by dorsal tilting [99]. A
close approximation of the original alignment of the radius
is essential to restore normal movement and load distribu¬
tion across the wrist [64,94,103,126,145]. Malalignment of
the fragment can lead to significant reductions in the result¬
ing range of motion (ROM) at the wrist and at the distal
radioulnar joint. Limited ROM secondary to bony malalign¬
ment does not respond to exercise. The clinician must be
able to distinguish between range limitations resulting from
bony blocks and those caused by soft tissue restrictions,
which can respond to conservative treatment.

Ulnar

Figure 14.3: The surface of the distal radius is biconcave, concave
in the radioulnar and dorsal-volar directions.

Figure 14.4: Tilt of the articular surface of the distal radius. The
articular surface of the distal radius is tilted (A) volarly (anteriorly)
and (B) in an ulnar direction.

Distal Ulna and Shaft

Although the ulna is separated from the wrist proper by a
fibrocartilaginous disc, the ulna is an important functional
element of the wrist, integral to the normal function of the
forearm and hand [38]. The proximal aspect of the ulna is
described in Chapter 11. The shaft of the ulna is triangular
for most of its length and narrows from proximal to distal.
The posterior border of the shaft of the ulna is subcutaneous
and palpable along its entire length. The distal end of the
ulnar shaft expands slightly into the head of the ulna that
articulates with the distal radius and with the triangular
fibrocartilage between the ulna and the carpal bones. The
rounded head of the ulna is easily palpated dorsally when
the forearm is pronated.

The head of the ulna has two articular surfaces (Fig. 14.5).
The articular surface for articulation with the radius is known
as the seat of the ulna and lies on the circumference of the
head of the ulna. The seat of the ulna encompasses two
thirds to three quarters of the perimeter of the ulnar head
and is covered with articular cartilage [38,77]. Like the
radius’s articular surface for the ulna, the ulna’s articular sur¬
face for the radius varies in shape and curvature [65,77]. The
ulna is generally flatter than the reciprocating surface of the
radius in the anterior-posterior direction, allowing gliding
motions between the two bones and providing little inherent
stability [34,77].

The distal aspect of the ulna consists of three parts, the
ulnar styloid process, the fovea, and the pole (Fig. 14.5).
The ulnar styloid process is a medial bony projection, easily
palpated on the ulnar aspect of the wrist with the forearm
supinated. The fovea is a roughened depression at the base
of the styloid process on its radial aspect. It provides attach¬
ment for the apex of the fibrocartilaginous disc. The pole is

258

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.5: Head of the ulna. The circumference of the head is
known as the seat of the ulna and is the articular surface for the
distal radioulnar joint. The distal end of the ulna consists of the
styloid process, the fovea, and the pole, which articulates with
the fibrocartilaginous disc.

Figure 14.6: Ulnar variance is a greater than 1.0-mm difference
between the articular surfaces of the distal radius and distal
ulna.

a U-shaped articular surface for articulation with the fibro¬
cartilaginous disc. It lies radial to both the styloid process
and fovea.

The relative lengths of the radius and ulna are variable
[29]. The difference between the lengths of these bones is
called ulnar variance and is described as abnormal when
there is more than a 1-mm difference in lengths of the distal
radius and ulna measured at their distal articular surfaces
(Fig. 14.6). Ulnar variance is determined by an individuals
age and genetic traits as well as by external forces at the wrist
or pathology at the elbow. Although ulnar variance is
described as a static alignment, the relative lengths of the
radius and ulna also change with forearm position. Pronation
results in a functional shortening of the radius as the ulna
moves distally, and supination causes a functional lengthening
of the radius as the ulna moves proximally [38,61]. This
change in relative lengths of the radius and ulna affects the
tension in the interosseous membrane during pronation and
supination [38]. Supination generates greater tension in the
interosseous membrane than pronation as the radius moves
distally [30]. Individuals with a negative ulnar variance (a
shorter ulna) exhibit increased ulnar deviation range of
motion (ROM) at the wrist compared with those with a radius
and ulna of equal length [128].

Clinical Relevance

ULNAR VARIANCE: A positive ulnar variance in which
the distal articular surface of the ulna extends more than
1 mm beyond the radius is associated with degenerative
changes of the ulna, the fibrocartilaginous disc , and some
carpal bones [29]. The increased relative length of the ulna
may produce abnormal loading of the ulnar aspect of the
wrist joint (ulnocarpal impaction) since the ulna projects
distally beyond the radius. Conversely, a decrease in the
relative length of the ulna (negative ulnar variance) is likely
to decrease the stability of the lunate, leading to an
increase in shear forces, microtrauma, and perhaps
eventually to avascular necrosis of the lunate (Kienbock's
disease) (Fig. 14.7). Negative ulnar variance may also
result in increased loading on the radial side of the wrist
[113,127].

Positive and negative ulnar variance alignments in indi¬
viduals with no history of wrist trauma are associated with
differences in mineralization of the subchondral bone of the
distal radius, supporting the notion of altered loading pat¬
terns with ulnar variance deformities [42].

Positive ulnar variance is reported in young female
gymnasts who subject their wrists to repeated loading,
apparently inducing microtrauma leading to premature
closure of the radial growth plate. Similarly, patients with
distal radial fractures or who have undergone radial head

(continued)

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

259

(Continued)

resections may exhibit positive ulnar variance as the
radius migrates proximaiiy [29]. These patients may be
prone to more degenerative changes in the ulna , the
medial carpal bones , and the intervening disc. In cases
of ulnar variance; the patient may need to learn joint
protection strategies to reduce the magnitude of the loads
sustained at the wrist.

Carpal Bones

The carpus is composed of eight bones that are arranged
roughly into two rows, proximal and distal (Fig. 14.8). The
proximal row contains the scaphoid, lunate, triquetrum, and
pisiform. The distal row consists of the trapezium, trapezoid,
capitate, and hamate. The scaphoid appears to extend across
both rows, giving the appearance of the proximal row curving
around the capitate. As a whole, the carpal bones form an
arch, convex dorsally and concave volarly (Fig. 14.9). The arch
is transformed to an enclosed carpal tunnel by the transverse
carpal ligament, also known as the flexor retinaculum. This
ligament spans the carpal arch, attaching to the scaphoid and
trapezium on the radial side and to the pisiform and hamate
on its ulnar aspect.

The proximal surface of the carpus is biconvex, articulating
with the reciprocal biconcavity of the radius and triangular
fibrocartilage (Fig. 14.10). The distal surface of the carpus is
much more irregular, forming multiple articular surfaces for

Figure 14.8: The eight carpal bones. An exploded view of the
volar aspect of the eight carpal bones reveals their positions
in the proximal and distal rows of the carpus.

Figure 14.9: View of the carpus from its proximal end. The carpal
bones form an arch with an anterior concavity. This arch is the
carpal arch of the hand.

260

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.10: The proximal and distal articular surfaces of the
carpus exhibit two very different types of surfaces. The proximal
surface is biconvex, and the distal surface is irregular with many
different articular facets.

the proximal surfaces of the articulating metacarpal bones.
These variations in articular surfaces result in considerable
variety in the motion and stability available throughout the
carpus. The articular surfaces and surrounding ligaments pro¬
vide the essential support of the carpal arch and individual
articulations. The tendons of the forearm muscles crossing
the wrist also provide substantial indirect support to the car¬
pus [82,136].

Although the eight bones of the carpus may function
together as units, each bone possesses unique characteristics
that help explain the normal mechanics of the wrist and hand
as well as some of the abnormal mechanics that result from
trauma or disease. The articulations among the carpal bones
are depicted in Fig. 14.11. Each bone has a unique position
within the carpus, with articulations that contribute to the sta¬
bility and mobility of each bone.

SCAPHOID (ALSO KNOWN AS THE NAVICULAR)

The scaphoid is the largest carpal bone of the proximal
row. It is somewhat elongated, with its long axis projecting
radially, distally, and volarly. The scaphoid articulates with
five other bones, although the shapes of its articular surfaces

Figure 14.11: The articular surfaces between adjacent carpal bones
vary considerably among the carpal bones and influence the direc¬
tion and magnitude of movement between adjacent carpal bones.

are varied. Its articular surface for the radius is convex,
while its articular surface for the capitate is concave. These
articulations allow significant mobility between the articu¬
lating surfaces. However, the surfaces for articulation with
the lunate, trapezium, and trapezoid are generally flat.
These surfaces provide considerably less mobility, since they
allow mostly translation between adjacent surfaces. The
scaphoid tubercle lies on the radial side of the volar surface
of the scaphoid and provides attachment for the transverse
carpal ligament as well as for some of the intrinsic muscles
of the thumb. It is palpable just proximal to the thenar emi¬
nence when the wrist is extended.

Clinical Relevance

SCAPHOID FRACTURE AND AVASCULAR NECROSIS:

The proximal and distal portions of the scaphoid', known as
the proximal and distal poles are joined together by a
slightly narrowed region known as the waist of the scaphoid.
This is the site of most scaphoid fractures, the most common
fracture of the carpal bones [40]. Fractures of the scaphoid
usually occur as a result of impact between the dorsal surface
of the scaphoid and the dorsal border of the distal radius.
Such impacts result from forceful hyperextension of the wrist.
As with distal radius fractures, the typical mechanism for a
scaphoid fracture is a fall on an outstretched hand [142].
Reports suggest that impacts with the wrist in more than 95°
of hyperextension result in scaphoid fractures; while impacts
with less wrist hyperextension are more likely to result in distal
radial fractures [137,138].

The dorsal nonarticular aspect of the scaphoid is also the
primary location of the nutrient foramina through which the
scaphoid receives its blood supply. However, in approximately
13-14% of individuals the nutrient foramina are located distal
to the waist of the scaphoid [40,141]. A scaphoid fracture at

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

261

Figure 14.12: Avascular necrosis of the scaphoid following frac¬
ture. Coronal magnetic resonance imagery (MRI) of the wrist
(Tl-weighted) shows a dark signal replacing the normal bright
marrow signal of the proximal pole of the scaphoid, indicating
necrosis of the proximal pole. (Reprinted with permission from
Chew FS, Maldjian C, Leffler SG: Musculoskeletal Imaging, A
Teaching File. Philadelphia: Lippincott Williams & Wilkins, 1999.)

the waist of the scaphoid in an individual whose nutrient
foramina are located only in the distal pole of the scaphoid
leaves the proximal fragment without a blood supply. Avascu¬
lar necrosis of the proximal fragment is a common
complication of scaphoid fractures; delaying or preventing
union of the fracture (Fig. 14.12). The scaphoid is palpable in
the floor of the anatomical snuffbox (Fig. 14.13). Tenderness
on palpation here in the presence of a history of trauma to the
thumb or wrist indicates the need for further assessment to
rule out a scaphoid fracture or avascular necrosis from a pre¬
viously undiagnosed fracture [138].

Figure 14.13: Palpation of the scaphoid. The scaphoid is palpable
in the floor of the snuff box.

LUNATE

The lunate receives its name from its crescent shape, con¬
vex proximally to articulate with the radius and triangular
fibrocartilage and concave distally to articulate with the
head of the capitate. Its other articular surfaces are rela¬
tively flat, allowing mostly translation between the articular
surfaces. The lunate is located in the center of the proximal
row of carpal bones and plays an important role in stabiliz¬
ing the entire carpus. Palpation is possible just distal and
slightly ulnar to the dorsal tubercle of the radius with the
wrist slightly flexed [55,127]. The dorsal tubercle is just
proximal to the scapholunate joint line, or interval. Ten¬
derness in the interval suggests an injury to the scapholu¬
nate ligament.

Since no muscles attach to it, stability of the lunate within
the carpal arch depends primarily upon the shape of the artic¬
ular surfaces and upon the surrounding ligamentous struc¬
tures [123]. Unlike most of the carpal bones, the volar surface
of the lunate is broader than its dorsal surface.

Clinical Relevance

LUNATE DISLOCATION: The shape of the lunate may
explain why dislocations of the lunate typically occur in
the volar direction. The narrower dorsal surface can slip
volarly with little obstruction , while the broader volar
surface is less likely to protrude dorsally. The lunate is the
second most frequently injured carpal bone [40]. The
lunate is particularly susceptible to avascular necrosis
(Kienbock's disease) (Fig. 14.14). One study reports that
about 8% of the 75 cadaver limbs examined had a lunate
that received its blood supply only from the volar surface
[40]. The authors suggest that such a supply can be dis¬
rupted easily by injury.

TRIQUETRUM

The triquetrum is small, and much of its surface is covered
by ligaments [12]. It articulates with the fibrocartilaginous
disc on its proximal and ulnar surface during ulnar deviation
of the wrist. It attaches to the hamate by a concave-convex
surface, allowing significant movement between the two
bones. The triquetrum is palpable on the ulnar side of the
wrist during radial deviation.

PISIFORM

The pisiform is named for its pealike shape. It sits anteriorly
on the triquetrum and provides attachment for the tendon
of the flexor carpi ulnaris muscle, improving the mechanical
advantage of this muscle. The pisiform also provides
attachment for the distal continuation of the flexor carpi

262

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.15: Position of the thumb. The thumb is positioned
slightly anterior to the palm because the trapezium articulates
anteriorly on the scaphoid.

ulnaris, the pisohamate ligament [100,141]. It provides
attachments for many other important ligamentous and mus¬
cular structures of the wrist and hand, including the trans¬
verse carpal ligament. The pisiform is easily palpated in the
heel of the hand just distal to the distal wrist crease.

TRAPEZIUM (FORMERLY KNOWN AS THE GREATER
MULTANGULAR)

The trapezium boasts a saddle-shaped facet for articulation
with the base of the metacarpal bone of the thumb. How¬
ever, its remaining articular surfaces are flat or only slightly
curved. The tubercle of the trapezium is located proximally
on the anterior surface and provides attachment for the
transverse carpal ligament. It is palpable at the base of the
thenar eminence, distal to the distal wrist crease. It is impor¬
tant to note that the trapezium articulates volarly with the
scaphoid. This articulation places the trapezium out of the
plane of the other carpal bones of the distal row. Conse¬
quently, the thumb lies at about a 45° angle with the index
finger [25,127] (Fig. 14.15). The trapezium is palpated radi¬
ally and dorsally at the articulation of the trapezium and the
metacarpal of the thumb.

TRAPEZOID (FORMERLY KNOWN AS THE LESSER
MULTANGULAR)

The trapezoid is one of the smallest carpal bones and is cov¬
ered almost entirely by flat articular surfaces. It provides the
primary articulation for the metacarpal bone of the index
finger, and it is surrounded by bones on all sides. Therefore,
it contributes to the stable base of the index finger [138].
This stable base is critical to the role of the index finger

during powerful pinch, which is discussed in greater detail
in Chapter 19. The trapezoid is not palpable.

CAPITATE

The capitate is the largest carpal bone and is located in the
center of the carpus, acting as a keystone of the carpal arch,
with many of the ligaments supporting the wrist attaching to
it. The capitate is divided into a proximal head and a distal
body, joined by a neck. The head is approximately half a
sph ere and projects into the concavity created by the lunate
and scaphoid. The other articular surfaces for the carpal and
metacarpal bones are flat [12,104] or slightly curved [141].
The capitate is in line with the dorsal tubercle of the radius,
the lunate, and the base of the metacarpal of the long finger.
It is palpable proximal to the metacarpal bone, with the
wrist flexed slightly.

HAMATE

The hamate also is a large carpal bone and is characterized
by a large projection or hook on its distal anterior surface.
The hook, or hamulus, gives this carpal bone its name. The
hook projects volarly and radially so that its tip points toward
the radial side of the hand. The tip is palpated easily by plac¬
ing the interphalangeal joint (IP) of the palpating thumb on
the pisiform, pointing toward the subjects thumb web
space. The hook of the hamate lies directly under the tip of
the palpating thumb [55] (Fig. 14.16). This hook provides
the fourth and final attachment of the transverse carpal lig¬
ament (Fig. 14.17). The proximal segment of the hamate is
convex for articulation with the triquetrum and, in ulnar
deviation, with the ulnar side of the lunate. The distal facets
are flatter, allowing translation between adjacent surfaces.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

263

Figure 14.16: Palpation of the hook of the hamate. To palpate
the hook of the hamate, place the interphalangeal joint of the
palpating thumb on the subject's pisiform, pointing toward
the subject's thumb web space. The hook of the hamate lies
directly under the tip of the thumb.

Metacarpals

The metacarpal bones are miniature long bones with com¬
mon characteristics among all of the digits. Each metacarpal
consists of a proximal base, a shaft, and a distal head. The
bases are the most varied of the metacarpals’ characteristics
among the five digits (Fig. 14.18). Their shapes reflect the

Figure 14.18: The shapes of the fingers' metacarpal bases are
quite varied and influence the amount of motion that is avail¬
able at each articulation.

Figure 14.17: The transverse carpal ligament attaches on the four
"pillars" of the carpus: the scaphoid, trapezium, pisiform, and
hamate.

articulations between each metacarpal and corresponding
carpal bone(s). The base of the metacarpal of the thumb is
characterized by its saddle-shaped articular surface allowing
the distinctive opposition motion available in the human
thumb [138]. The base of the metacarpals of the index and
long fingers have rather flattened facets for articulation with
their respective carpal bones. However, the metacarpal of
the ring finger has a slightly more curved facet for the
hamate, and the base of the metacarpal of the little finger has
a somewhat saddle-shaped facet for articulation with the
hamate. This variation in the bases of the metacarpal bones of
the fingers produces distinct differences in the mobility of
their carpometacarpal (CMC) articulations. The CMC artic¬
ulations of the index and long fingers exhibit almost no
motion, while the articulation between the metacarpal bone
of the little finger and hamate is quite mobile.

The heads of the metacarpals of the fingers are
almost perfectly round from volar to dorsal and are more
curved than the bases of phalanges to which they attach [9]
(Fig. 14.19). The articular cartilage on the heads of the
metacarpals covers the volar and distal surfaces and extends
slightly onto the dorsal surface, providing an articular surface
for a small amount of hyperextension at the metacarpopha¬
langeal (MCP) joints.

In the ulnar and radial directions, the articular surfaces of
the metacarpal heads are convex but somewhat asymmetrical

264

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Base of proximal phalanx

Figure 14.19: A sagittal view of a metacarpal head and articulat¬
ing phalangeal base. The metacarpal head is more curved than
its respective phalangeal base.

and more variable [45,48,86,127] (Fig. 14.20). This asymmetry
influences the amount of ulnar and radial deviation that can
occur at each of the MCP joints of the digits. The metacarpal
heads also are broader in the radioulnar direction on their
volar surfaces than on their dorsal surfaces [138]. This varia¬
tion in width contributes to the decrease in radioulnar mobil¬
ity of the MCP joints when the joints are flexed.

The head of the thumb s metacarpal is flatter and broader
in the radioulnar direction than those of the other metacarpals
[63] (Fig. 14.21). This flattening reduces the radioulnar
motion available at the thumbs MCP joint. However, the
shape of the head of the thumb s metacarpal is quite variable,
which may help to explain the wide variation of available
motion at this joint reported in the literature [17,138,141].

Phalanges

The phalanges, like the metacarpals, possess bases, heads,
and shafts (Fig. 14.22). There are 14 phalanges in each hand,
3 in each finger (proximal, middle, and distal), and 2 in the

Figure 14.20: A frontal view of the metacarpal heads of the
fingers reveals asymmetry among the fingers. The view also
demonstrates that the width of the metacarpal heads is
narrower at their dorsal borders than at their volar surfaces.
This difference contributes to the decrease in mobility in radial
and ulnar deviation when the MCP joints are flexed.

Figure 14.21: The head of the thumb's metacarpal is broader and
flatter in the radial and ulnar direction than the metacarpal
heads of the fingers.

thumb (proximal and distal). The phalanges decrease in size
from proximal to distal. The bases of the proximal phalanges
are biconcave, although the base of the proximal phalanx of
the thumb is flatter in the radioulnar direction than those of
the fingers. The articulating surface of each base of the mid¬
dle and distal phalanges is almost a mirror image of the
articulating head of the respective phalanx. However, as in
the metacarpal bones, the bases of the phalanges are slightly
flatter than the articular surfaces of the heads of the adja¬
cent phalanx [1]. The heads of the proximal and middle pha¬
langes are convex in a dorsal-volar direction, with a
prominent central groove so that each head takes on a pul¬
ley, or trochlear, shape similar to that of the elbow [17]. The
ulnar and radial aspects of the trochlea are also known as
condyles. This trochlear shape limits the radioulnar move¬
ment at the joints. The two condyles of the proximal phalanx
are slightly asymmetrical; the condyles of the heads of the
middle phalanges are more symmetrical [76]. The shapes of
the heads of the phalanges affect the precise direction of
flexion and extension that occurs at each finger [72,138]. As
a result, the fingers converge toward the base of the thumb
during finger flexion. The heads of the distal phalanges nar¬
row to a nonarticulating point distally and provide an anchor
for the fingernails.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

265

Figure 14.22: Dorsal view of the phalanges. Like the metacarpal
bones, the phalanges possess a base, a shaft, and a head. The
heads of the proximal and middle phalanges have a distinct
trochlear shape.

Sesamoid Bones

Typically there are two sesamoid bones, one in each of the
tendons of the flexor pollicis brevis and the adductor pollicis.
Similar sesamoid bones are frequently found in the tendons
anterior to the MCP joint of the little finger, less frequently at
the index finger, and only infrequently at the other fingers
[141]. These bones alter the lines of pull of the tendons in
which they insert, thus improving the muscles’ mechanics.

Bony Landmarks

The wrist and hand are composed of more than 27 bones,
each with its own unique characteristics. The bony landmarks
that can be palpated in the forearm and hand are listed below:

• Radial styloid process

• Volar ridge of distal radius

• Dorsal tubercle of radius

• Ulnar crest

• Ulnar head

• Styloid process of ulna

• Dorsal surface of the scaphoid

• Scaphoid tubercle

• Dorsal surface of lunate

• Triquetrum

• Trapezium

• Dorsal surface of capitate

• Pisiform bone

• Hook of the hamate

• Shafts and the margins of the bases and heads of the
metacarpal bones and phalanges

As demonstrated already in the preceding chapters on the
shoulder and elbow, the shape of each bone and its articular
surfaces directly affects the motion and stability of
the joints of the hand. The following section presents
the joints, their supporting structures, and the avail¬
able motion of the wrist and hand.

ARTICULATIONS AND SUPPORTING
STRUCTURES OF THE JOINTS OF THE
WRIST AND HAND

The wrist and hand possess multiple articulations that
together provide the dexterity of movement exhibited by the
hand. Each of the joints is presented below with a full discus¬
sion of the articulations, supporting mechanism, and motion
available. Although the carpus is involved in four distinct
joints, the radiocarpal, midcarpal, and the two CMC joints, it
represents a distinct functional unit as well. A discussion is
presented on the stability and mobility of the carpus as a
whole and those structures that contribute to its support.

Distal Radioulnar Joint

The distal radioulnar joint has been described as part of a
compound joint with the proximal radioulnar joint [46].
Together these two joints are the source of pronation and
supination of the forearm [38,46,65]. The distal radioulnar
joint [33,65] allows the large excursions of forearm prona¬
tion and supination as well as ulnar deviation of the wrist
that enhance the manipulating skills of the hand [2]. The
distal radioulnar joint also allows transmission of loads from
the hand and radius onto the ulna [115]. Resection of the
head of the ulna eliminates most of the load transmission so
that most of the axial load is borne through the radius.
Although the distal radioulnar joint is not part of the wrist
joint proper, it is important to the normal function of the
wrist and frequently is implicated in wrist pathology
[33,34,38,61,68]. A thorough awareness of the joint and sur¬
rounding tissues is essential for a complete understanding of
both the elbow and the wrist.

266

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

The distal radioulnar joint is a synovial joint, and the con¬
tributing surfaces of the radius and ulna are both covered by
articular cartilage. It is classified as a pivot joint. However,
there also is considerable gliding between the head of the
ulna and the sigmoid notch on the radius [61,77]. One rea¬
son for the extensive gliding that occurs between the radius
and ulna at the distal radioulnar joint is the differences in
the curvatures of each articular surface. The radius of cur¬
vature of the ulnar head is shorter than that of the sigmoid
notch, indicating that the ulnar head is more curved than
the sigmoid notch [34,68,77] (Fig. 14.23). The difference in
curvatures between these two surfaces encourages gliding
motion at the joint.

Clinical Relevance

ULNAR HEAD RESECTION AND ARTHROPLASTY:

Patients with rheumatoid arthritis and wrist involvement
sometimes develop pain and even instability at the distal
radioulnar joint , making hand function difficult and painful.
In a painful but stable wrist resection of the ulnar head
(a Darrach procedure) may reduce the pain and restore
function. However ; in unstable wrists the patient may have
better pain relief and functional improvement with an ulnar
head arthroplasty.

SUPPORTING STRUCTURES OF THE DISTAL
RADIOULNAR JOINT

The shapes of the articular surfaces of the distal radioulnar
joint contribute little to the stability of the distal radioulnar
joint [112]. The stability of the joint comes from the sur¬
rounding soft tissue structures. The nonmuscular supporting
structures of the distal radioulnar joint are the joint capsule
and the triangular fibrocartilage complex (TFCC), which con¬
sists of the triangular fibrocartilage, dorsal radioulnar liga¬
ment, volar radioulnar ligament, ulnar collateral ligament
complex, and the meniscus homologue. Although the ulnar
collateral ligament plays an important role in stabilizing the
distal radioulnar joint, it also contributes to the support of the
wrist [33,61]. Therefore, it is discussed later in this chapter as
part of the supporting structure for the radiocarpal joint and
carpus as a whole. The interosseous membrane and annular
ligaments also provide support to the distal radioulnar joint
and are described in detail in Chapter 11 with the bones and
joints of the elbow. They bind the radius and ulna together
proximally and along their shafts, supporting both the supe¬
rior and inferior radioulnar joints [135].

The capsule of the distal radioulnar joint attaches to the
periphery of the sigmoid notch of the radius, the proximal and
lateral borders of the seat of the ulna, and the borders of the
triangular fibrocartilage (Fig. 14.24). The capsule projects a
small pocket proximally between the radius and the ulna, cre¬
ating an L-shaped joint space [104]. The distal aspect of the
capsule is more robust and may help stabilize the radioulnar

Figure 14.23: Curvature of the sigmoid notch. A view of the
distal surface of the radius and ulna reveals that the radius of
curvature of the sigmoid notch is longer than that of the ulnar
head. In other words, the ulnar head is more curved than its
articular surface on the radius.

Figure 14.24: The capsule of the distal radioulnar joint attaches
to the periphery of the sigmoid notch, the seat of the ulna, and
the borders of the fibrocartilaginous disc.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

267

articulation in an axial direction [68]. Isolated sectioning of
the joint capsule produces significant distal radioulnar joint
instability in cadaver models [134]. The capsule is quite thin
anteriorly but has folds that unfold during supination to allow
full ROM. Although the capsule is thicker posteriorly, it has
fewer folds than its anterior counterpart.

Clinical Relevance

TIGHTNESS OF THE DISTAL RADIOULNAR JOINT
CAPSULE AND LIMITED PRONATION AND SUPINA¬
TION ROM: The capsule of the distal radioulnar joint is fre¬
quently described as weak, providing little resistance to the
normal limits of pronation and supination ROM [34]. How¬
ever, a clinical study of nine patients suggests that an
abnormal joint capsule may contribute to pathological limi¬
tations of ROM [68]. The study reports that capsulectomies
in these patients with limited pronation and supination with¬
out any pathology of the TFCC resulted in increased prona¬
tion and supination ROM. The authors note that scarring of
the capsule and adhesions in the volar (anterior) folds were
visible in the capsules of these patients and may have con¬
tributed to the limited ROM. The adhesions are probably
similar to those that can form in the folds of the gleno¬
humeral joint in adhesive capsulitis (Chapter 8). These data
suggest that the capsule of the distal radioulnar joint may
play a part in decreased pronation and supination ROM in
some patients. Treatments to stretch or release the capsule
may prove beneficial in some patients.

The TFCC is critical to the function of the distal radioulnar
joint [112]. It performs several functions including

• Stabilizing the distal radioulnar joint

• Cushioning the ulna on the carpus

• Allowing axial loading of the ulnar aspect of the forearm

• Increasing the articular surface for the carpus

• Stabilizing the ulnar side of the carpus itself

The dorsal and volar radioulnar ligaments of the TFCC
blend with the joint capsule but are histologically distinct
from it [68]. They attach to the dorsal and volar surfaces of
the sigmoid notch, respectively, and both have some attach¬
ment to the triangular fibrocartilage (Fig. 14.25). These two
ligaments provide critical support to the distal radioulnar
joint, but the role each plays in limiting normal pronation
and supination remains controversial [34,46,61,77,112].
Some authors report that the volar radioulnar ligament is
tight in pronation and the dorsal radioulnar ligament is tight
in supination [34,46]. Others report exactly the opposite;
that is, the dorsal radioulnar ligament is tight in pronation,
and the volar radioulnar ligament is tight in supination
[77,112,133]. The complex nature of the movement between
the radius and the ulna during pronation and supination may

Figure 14.25: The volar radioulnar ligament blends with the
capsule of the distal radioulnar joint on the volar surface of
the joint. The dorsal radioulnar ligament does the same on
the dorsal surface.

actually justify both conclusions. As explained in Chapter 11,
during pronation with the hand fixed in space, the ulna
glides radially as the radius rolls into pronation [30]. It
appears that both ligaments participate together to stabilize
the distal radioulnar joint in both pronation and supination
by limiting both the rotation and the translation of the radius
and ulna [32,38].

The triangular fibrocartilaginous disc fills the space
between the ulna and the carpus. As its name indicates, the
disc is shaped like a triangle, with its base attached to the dis¬
tal border of the sigmoid notch of the radius (Fig. 14.26). The
apex of the disc is attached by loose connective tissue to the
base of the ulna’s styloid process and fovea. The disc is con¬
cave on both its proximal and distal surfaces for articulation
with the pole of the ulna proximally and with the lunate and
triquetrum distally. The central portion of the disc is quite
thin and actually may be perforated in older adults, creating a
communication between the distal radioulnar and the radio¬
carpal joints. The triangular fibrocartilage serves as a shock
absorber between the ulna and the carpus. It helps distribute
any load transmitted by the hand to the ulna and may con¬
tribute to the axial and medial-lateral stability of the distal
radioulnar joint [116]. However, excision of up to two thirds
of the disc in cadavers seems to have little effect on the sta¬
bility of the distal radioulnar joint [87].

The meniscus homologue is the soft tissue that runs from
the dorsal border of the radius medially to the volar surface of
the medial aspect of the triquetrum [61]. Histological exami¬
nation reveals that the meniscus homologue is vascularized
loose connective tissue rather than fibrocartilage or ligamen¬
tous tissue [38]. Its functional significance is unclear.

268

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Ulnar

styloid

process

Figure 14.26: The fibrocartilaginous disc between the ulna and
the carpus. A distal view of the radius and ulna reveals that
the fibrocartilaginous disc between the ulna and the carpus
is triangular and attaches to the distal border of the sigmoid
notch of the radius and to the base of the ulna's styloid
process and fovea.

In conclusion, there are several supporting structures that
are essential to the stability of the distal radioulnar joint
[67,112]. Despite the relative absence of stability imparted by
the articular surfaces, the connective tissue surrounding the
joint confers considerable stability to the distal radioulnar
joint. Dislocation of the distal radioulnar joint appears to
occur only with complete disruption of the TFCC [112,133].

MOTIONS OF THE DISTAL RADIOULNAR JOINT

The motion of the distal radioulnar joint is intimately tied to
the motion of the proximal radioulnar joint, the two joints
essentially acting as a single compound joint. Thus pronation
and supination occur simultaneously at the two joints. Values
of normal pronation and supination ROM found in the liter¬
ature are presented in Table 11.1 in Chapter 11. However,
the motion at the distal radioulnar joint is more complex than
a simple pivot around a fixed point. As noted in Chapter 11,
pronation can occur around a hand fixed in space or around a
hand that moves to a new location in space (see Fig. 11.28).
In the latter case, the radius rotates about the ulna, with an
axis close to the fovea of the ulna. However, when the hand is
fixed in space as when the hand is grasping a doorknob or
turning a screwdriver, both the ulna and radius move during
pronation. In this instance, the axis of rotation is located more
laterally in the ulna. As the radius rotates about the ulna dur¬
ing pronation with the hand fixed, the ulna moves dorsally
and radially about the radius [30,127,139] (see video in Chap¬
ter 11). This gliding motion is allowed by the incongruities of
the joint surfaces of the distal radioulnar joint.

Figure 14.27: The capsule of the radiocarpal joint attaches to the
distal radius, the fibrocartilaginous disc, and the proximal row of
carpal bones.

Joints of the Wrist

The wrist is the junction of the hand and forearm. Although
the radiocarpal joint is the most familiar joint of the wrist,
motion of the wrist also comes from the midcarpal and
intercarpal joints. Each of the joints is described below,
including a discussion of the articular surfaces and individ¬
ual joint capsules. The stability and mobility of the joints are
so interdependent that discussion of the extracapsular sup¬
ports and the motions of the joints can occur together. The
joint surfaces and the individual joint capsules are presented
first. The extracapsular supports for the region are discussed
as a unit. Individual joint contributions to overall wrist
motions are discussed after all of the joints are reviewed.

RADIOCARPAL JOINT

The radiocarpal joint is the articulation between the radius
and the proximal row of carpal bones, but only the scaphoid
and the lunate articulate directly with the radius. The tri¬
quetrum articulates with the distal surface of the triangular
fibrocartilage. The capsule of the radiocarpal joint encloses all
of these surfaces.

The distal surface of the radius with the adjoining triangu¬
lar fibrocartilage is biconcave; the proximal surface of the

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

269

proximal row of carpal bones is biconvex. The bony articulat¬
ing surfaces are covered by articular cartilage. Although the
reciprocal articulating surfaces appear congruent, contact
between the scaphoid and lunate and the radius is neither
constant nor uniform [94]. Contact involves approximately
20% of the surface area when a load of less than 25 lb is
applied. A load of almost 50 lb across the wrist increases the
contact area to a maximum of 40% by causing deformation of
the articular cartilage. In addition, the area of contact is
greater between the scaphoid and radius than between the
lunate and radius. Thus the radiocarpal joint demonstrates
less congruency between the articulating surfaces than is
expected of a classic biaxial joint. While the overall shape of
the surfaces of the radiocarpal joint are consistent with a sim¬
ple biaxial unit, there is significant evidence that the individ¬
ual carpal bones play unique individual roles in the mechanics
of the radiocarpal joint [28,60,69,96].

The radiocarpal joint has little inherent stability imparted
by the shapes of the articular surfaces. Instead it is supported
on all four sides of the joint by a fibrous capsule and liga¬
ments. The capsule of the radiocarpal joint surrounds and
attaches to the dorsal, radial, and volar borders of the articu¬
lar surface of the radius and to the dorsal, ulnar, and volar
edges of the triangular fibrocartilage. Thus the triangular
fibrocartilage forms the floor of the distal radioulnar joint
and the roof of the ulnar aspect of the radiocarpal joint. The
capsule attaches distally to the periphery of the proximal
articular surfaces of the scaphoid, lunate, and triquetrum
(Fig. 14.27).

MIDCARPAL JOINT

The midcarpal joint is the junction between the proximal and
distal rows of carpal bones. All of the articular surfaces are
covered with typical articular cartilage. The joint has two dis¬
tinct regions encased by a single joint capsule (Fig. 14.28).
The medial compartment is composed mainly of a proximal
concavity formed by the scaphoid, lunate, and triquetrum
and a distal convexity formed by the capitate and hamate.
This portion of the joint functions as a biaxial or condyloid
structure [62]. The lateral portion of the joint is composed of
the flatter articular facets of the distal scaphoid and the tra¬
pezium and trapezoid bones. This lateral portion is described
as either a saddle joint [141] or a plane joint [62]. Regardless
of the classification of the joint, the motions between adja¬
cent carpal bones of the two rows are complex and are dis¬
cussed later in this chapter.

The irregular surface of the midcarpal joint leads to an
uneven distribution of the loads across the joint surface [94].
Data collected on cadaver specimens suggest that the great¬
est load is transmitted through the scaphoid-capitate articu¬
lation, and the least is through the triquetrum-hamate
connection. As in the radiocarpal joint, light loads appear to
be distributed through only a small area of the articular sur¬
face (slightly more than 25%). The area of contact increases
to approximately 35% with heavy loads.

Figure 14.28: The midcarpal joint has two distinct regions, the
medial and lateral compartments.

The irregularity of the articular surfaces of the midcarpal
joint provides some inherent stability to the joint, but liga¬
mentous support remains the primary source of stability. The
midcarpal joint is supported by its capsule as well as by the
extrinsic and intrinsic ligaments of the wrist and carpus. It is
important to recognize that although the midcarpal joint is
anatomically distinct from the radiocarpal joint, these two
joints are structurally and functionally interdependent, so
that if one structure fails, the effects are felt throughout the
wrist. The capsule of the midcarpal joint is irregular because
it encloses the joint space between the proximal and distal
rows of carpal bones and also sends projections proximally
and distally between the adjacent carpal bones of each row
(Fig. 14.29). Therefore, the capsule of the midcarpal joint
creates joint spaces for each of the intercarpal articulations
except the triquetropisiform articulation, which usually has
its own joint capsule and joint space [141].

INTERCARPAL JOINTS

The intercarpal joints are the articulations between adjacent
carpal bones within each row. These articulations are synovial
and are encapsulated by extensions of the capsule of the mid¬
carpal joint. They are regarded as plane joints. These articu¬
lations are stabilized by the joint capsule and by the extrinsic
and intrinsic ligaments described in the next section. The

270

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.29: The capsule of the midcarpal joint is very irregular,
extending proximally and distally into the intercarpal articulations.

transverse carpal ligament, or flexor retinaculum, is an acces¬
sory ligament that supports the entire carpal arch. It attaches
medially to the pisiform bone and hook of the hamate. The
lateral portion of the transverse carpal ligament attaches to the
tubercles of the trapezium and scaphoid. The entire trans¬
verse carpal ligament bridges the carpal arch, creating the
carpal tunnel helping to stabilize the arch and the contents of
the tunnel. It is pulled taut in both maximum pronation and
maximum supination [37]. Surgical release of the transverse
carpal ligament is a common treatment for carpal tunnel
syndrome (CTS), in which the contents of the carpal tunnel
including the median nerve are compressed by swelling
within the carpal tunnel.

Clinical Relevance

CARPAL TUNNEL RELEASE (CTR): One treatment to
relieve the pressure on the median nerve is a carpal tunnel
release performed by cutting the transverse carpal ligament.
Surgical approaches include complete transection of the liga¬
ment, Z-plasties that retain some continuity, and complete
transection with ligamentous reconstruction [18]. Authors
report an increase in the width of the carpal arch at rest fol¬
lowing CTR and also an increase in maximum width of the
arch when a supination torque is applied to the hand

[37,39]. Changes in the dimensions of the carpal tunnel after
CTR are particularly apparent during activities against a load
[39]. The clinical importance of these changes in the carpal
tunnel is unknown. Although CTR often provides substantial
relief of pain in patients with CTS, the altered transverse
carpal ligament may contribute to instability of the hand
and pain, creating functional difficulties. The clinician needs
to appreciate the mechanical and functional implications of
this surgical intervention.

EXTRACAPSULAR SUPPORTING STRUCTURES
OF THE WRIST

Although there is considerable variation in the literature in
the names used to identify the extracapsular supporting struc¬
tures of the wrist, there is a broad acceptance of their general
organization. The ligaments of the wrist can be divided
into two large categories: extrinsic and intrinsic. The extrinsic

Figure 14.30: General organization of the ligaments of the wrist.
Most of the proximal ligaments of the wrist converge on the
lunate, and most of the distal ligaments of the wrist converge
on the capitate.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

271

ligaments have attachments to the radius, ulna, or the TFCC,
as well as to the carpal bones. The intrinsic ligaments are con¬
tained entirely within the carpus. Most of the ligaments of the
wrist are found on either the palmar (volar) or dorsal surfaces,
although the radial and ulnar collateral ligaments lie slightly
more laterally and medially, respectively. The palmar liga¬
ments are thicker, stronger, and more critical to the stability
of the wrist than are the dorsal ligaments [19,125].

Examination of the overall system of ligaments of the wrist
reveals that the radius and four of the carpal bones—the
scaphoid, lunate, triquetrum, and capitate—have extensive
ligamentous attachments. The ligaments also form a pattern of
convergence in the midline of the hand, with the proximal lig¬
aments converging on the lunate and the distal ligaments con¬
verging on the capitate (Fig. 14.30). The following presents a
closer examination of the individual ligaments of the wrist to
illuminate the role each plays in stabilizing the wrist complex.

The extrinsic ligaments of the wrist reinforce the radio¬
carpal joint capsule on the palmar, dorsal, radial, and ulnar

surfaces (Fig. 14.31). These ligaments serve to support
both the radiocarpal and midcarpal joints. The palmar are
the thickest and most extensive of the extrinsic ligaments
[19,125]. Anatomical texts describe five large extrinsic lig¬
aments, the palmar radiocarpal ligament, the ulnocarpal
complex (which is also on the palmar side), the dorsal
radiocarpal ligament, and the radial and ulnar collateral
ligaments [104,141].

The extrinsic ligaments also can be described as more dis¬
crete bundles of fibers to individual carpal bones, with dis¬
tinct functional roles [102,118,130]. The names of the
individual ligamentous bundles, based on their bony attach¬
ments, vary slightly among authors (Table 14.1). Despite the
slight variations among the names applied to the wrist liga¬
ments, the basic organization remains the same throughout
the literature [12,102,118,125,130].

The collateral ligaments are reportedly weaker than the
other extrinsic ligaments of the wrist [141]. A comparison of
the cross-sectional area of the radial collateral ligament with

Figure 14.31: Extrinsic ligaments of the wrist. A. Dorsal extrinsic ligaments of the wrist. The dorsal radiocarpal ligament arises from
the dorsal surface of the distal border of the radius. It projects distally onto the lunate and triquetrum, although attachments to the
scaphoid are also described. B. Palmar extrinsic ligaments of the wrist. The palmar radiocarpal ligament extends from the radius to
the proximal row of carpal bones including the scaphoid, lunate, and triquetrum and onto the capitate in the distal row. The radial
and ulnar collateral ligaments project from the radial and ulnar styloid processes, respectively. The radial collateral ligament extends
to the radial aspect of the scaphoid and onto the capitate as part of the radioscaphocapitate ligament. It lies more on the palmar sur¬
face and sometimes is described as part of the palmar radiocarpal complex. The ulnar collateral ligament projects to the triquetrum
and the metacarpal of the little finger and sends a slip to the pisiform.

272

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

TABLE 14.1: Extrinsic Ligaments of the Wrist

Surface

Ligament

Individual Fiber
Bundles

Palmar

Palmar radiocarpal

Radioscaphocapitate

Radioscapholunate

Short radiolunate

Long radiolunate

Palmar

Ulnocarpal complex

Ulnocapitate

Ulnotriquetral

Ulnolunate

Palmar and radial

Radial collateral

Palmar and ulnar

Ulnar collateral

Dorsal

Dorsal radiocarpal

other dorsal and palmar radiocarpal ligaments in seven
cadaver specimens reveals that the collateral ligament has no
more than half the cross-sectional area of the other extrinsic
ligaments of the wrist [109].

Although the role of the extrinsic wrist ligaments has been
extensively studied [100,110,125,136], questions remain
regarding their individual roles in stabilizing and guiding the
movement of the wrist complex. In general, the palmar liga¬
ments limit excessive extension ROM, while the dorsal liga¬
ments resist excessive flexion ROM. The radial ligaments
assist in limiting ulnar deviation, and the ulnar ligaments help
limit radial deviation. The extrinsic ligaments function with
the intrinsic ligaments to stabilize the wrist complex and limit
its mobility.

The intrinsic ligaments of the wrist have attachments
solely in the carpus. They are classified as dorsal and palmar
midcarpal ligaments and interosseous ligaments [12,19,141]
(Table 14.2). Like the extrinsic ligaments, the palmar mid-
carpal ligaments are thicker and stronger than the dorsal
midcarpal ligaments [123]. The interosseous ligaments are

TABLE 14.2: Intrinsic Ligaments of the Wrist

Ligament Individual Fiber Bundles

Palmar midcarpal Palmar scaphotrapezium-

trapezoid

Scaphocapitate

Triquetrocapitate

Triquetrohamate

Dorsal midcarpal Dorsal scaphotriquetral

Dorsal intercarpal

Interosseous Scapholunate

Lunotriquetral

Pisotriquetral

Trapezium-trapezoid

Trapeziocapitate

Capitohamate

thick, strong, horseshoe-shaped ligaments that run between
adjacent carpal bones of each row. They are named accord¬
ing to their attachments. For example, the scapholunate lig¬
ament attaches to the scaphoid and lunate bones of the
proximal row. The proximal portions of the interosseous lig¬
aments of the proximal row consist mainly of fibrocartilage
rather than fibrous material, giving them unique mechanical
properties [12,102].

The mechanical properties of some of the ligaments of
the wrist have been examined. There is general agreement
that the interosseous ligaments are much stronger than the
other intrinsic and extrinsic ligaments of the wrist
[12,102,112]. Loads to failure of more than 300 N (67 lb)
are reported for the interosseous ligaments and of less than
200 N (45 lb) for extrinsic ligaments, although individual
ligaments vary [12,102]. In contrast, the interosseous liga¬
ments [92] are significantly less stiff than other wrist liga¬
ments sustaining more elongation before failure [109].
However, all of the wrist ligaments tested sustain more
elongation before failure than ligaments elsewhere in the
body, such as the anterior cruciate ligament of the knee
[92,93]. Wrist ligaments also behave viscoelastically,
exhibiting higher loads to failure with increasing loading
rate. As noted in Chapter 2, viscoelasticity may provide
protection, allowing ligaments to sustain the high, rapidly
applied loads generated by accidents such as falling on an
outstretched arm.

These mechanical properties suggest that the individ¬
ual ligaments of the wrist are highly specialized. At least
two classes of ligaments seem to be present in the wrist,
one that is stiff but able to sustain only moderate loads or
large deformations and another that is less stiff but
stronger in both load and deformation to failure. Despite
the unique mechanical characteristics of these wrist
ligaments, however, patients commonly sprain the liga¬
ments of the wrist resulting in considerable impairment
and disability.

Clinical Relevance

LUNATE INSTABILITY-A CASE REPORT: A 30+-year-old
mechanic for a car dealership visited a therapist complaining
of wrist pain, particularly with motion. He reported that he
had been changing a tire when the tire unexpectedly
bounced and forcefully flexed his wrist. Evaluation revealed
that the patient had limited and painful wrist flexion. Flexion
combined with ulnar deviation was particularly painful. ROM
was 0-30° of flexion, with pain at the end of the range. How¬
ever, in a slightly reduced range the patient was able to func¬
tion without pain and exhibited strength within normal limits.
The patient was unable to return to his job, which requires
full wrist mobility Evaluation revealed that the patient had
torn the scapholunate interosseous ligament, producing
malalignment of the lunate. Radiographs revealed that

(continued)

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

273

(Continued)

the lunate was tilted dorsally with respect to the scaphoid
(fig. 14.32| The patient's wrist was treated by pinning the
lunate to the capitate and scaphoid and immobilizing it for
4 weeks. After the cast was removed ' the patient resumed
active and passive exercise. Pain-free mobility was restored[
although passive ROM remained slightly diminished. He was
able to return to full-time work as a car mechanic. This case
study demonstrates that a tear in a single interosseous liga¬
ment can produce serious impairments and functional deficits.

Movements of the Wrist

The wrist as a whole is a condyloid or biaxial joint allowing
flexion, extension, radial deviation (abduction), and ulnar
deviation (adduction) (Fig. 14.33). Like any biaxial joint, the
wrist also combines these motions to perform circumduction,
a circular movement of the hand on the forearm. These
motions often are referred to as global motions of the wrist.
However, the motion at the wrist is much more complex than
these movements suggest. To understand the movements
available at the wrist, the movements of the individual com¬
ponents must be appreciated. The following reviews the indi¬
vidual movements of the bones of the wrist. Then their
contribution to overall wrist motion is presented.

MOVEMENT IN THE PROXIMAL ROW
OF CARPAL BONES

Considerable effort has been exerted to define the relative
motion of the carpal bones [22,28,60,69,96,105,108,110,124].

Despite over 20 years of study, however, disagreements
continue about the direction and magnitude of relative motion
among the carpal bones during wrist movements. There are at
least two important reasons for the continued confusion. First,
accurate methods to assess small, three-dimensional motions
have become available only recently [22,59,60,69,96,110].

Figure 14.33: Total, or global, motions of the wrist include flex¬
ion, extension, radial and ulnar deviation, and circumduction.

274

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Second, many studies report on such a small number of spec¬
imens that the results cannot be generalized [60,110].

Despite the limitations of the studies of carpal move¬
ment, there are some accepted concepts. There is now a
consensus that each of the carpal bones is capable of three-
dimensional motion, flexion, extension, radial and ulnar
deviation, and even pronation and supination [12,22,60,69].
Most studies suggest that the distal row moves almost as a
single unit, and its motion reflects the motion of the hand,
specifically the motion of the metacarpal of the long finger
[12,95,110,140]. In contrast, the scaphoid, lunate, and
triquetrum appear to move more independently of one
another. They flex on the radius during wrist flexion and
extend during wrist extension [12,69,140], with the scaphoid
moving through the greatest excursion in both directions

[22.108] . These findings reveal that there is relative flexion
between the scaphoid and lunate during wrist flexion and
relative extension during wrist extension [12,69]. There are
also reports that during wrist flexion, the scaphoid and tri¬
quetrum undergo pronation with respect to the lunate, and
supination during wrist extension [69,117]. The motions
that occur in a plane different from the global wrist motion
are known as out-of-plane motions.

The contribution to global wrist motion made by each row
of carpal bones also is important in understanding the
mechanics of wrist movement. In wrist flexion and extension,
the distal row of carpal bones moves as a unit and undergoes
simple flexion and extension on the proximal row of bones
[12,110]. Several studies using the capitate to represent the
motion of the distal row report that in flexion, the capitate is
more mobile than any of the bones of the proximal row

[12.22.69.108] . Estimates of the capitate’s (and thus the distal
row’s) contribution to total wrist flexion range from 60 to 70%

[69.108] . Yet others report that the radiocarpal joint con¬
tributes more to flexion than the midcarpal joint [96] or that
the contributions from radiocarpal and midcarpal joints are
equal [13,124]. Similarly, there are differences in the reports
of the distal row’s relative contributions to extension, some
indicating that the midcarpal joint contributes most of the
extension [22,95,124] and others saying that most comes from
the radiocarpal joint [108]. Finally, some studies suggest that
the capitate (and therefore, the midcarpal joint) contributes
more motion to both flexion and extension of the wrist than
the lunate, representing the radiocarpal joint [12,22,69,140].
The differences in these reported data may be the result of
differences in measurement techniques or subject variation
[120]. Jackson et al. report that their two specimens exhibited
virtually opposite results from one another [60]. Another
study suggests that the relative contributions made by the
radiocarpal and midcarpal joints to total wrist motion vary
depending upon where in the range the measurements are
taken [95]. Although there is no consensus about the relative
contributions of the two rows to wrist flexion and extension,
there is clear recognition that normal global motion of the
wrist requires substantial movement at both the radiocarpal
joint and the midcarpal joint (Fig. 14.34).

Figure 14.34: Source of total wrist motion. Total wrist motion is
the combined result of motion from both the radiocarpal and
midcarpal joints.

Radial and ulnar deviations of the wrist appear to include
more complex, out-of-plane motions of the carpal bones.
Radial deviation of the wrist appears to be accompanied by
flexion of the proximal row of carpal bones, which helps the
scaphoid avoid impingement on the radial styloid process
[22,123,127]. At the same time, the distal row of carpal
bones undergoes extension [12,22,69,70]. The reverse
appears true in ulnar deviation of the wrist. Similarly, radial
deviation is reported to occur with relative pronation of the
proximal row, while ulnar deviation reportedly is accompa¬
nied by supination of the proximal row [12,22,70]. Reports
suggest that the distal row also demonstrates out-of-plane
motions that are nearly the reverse of the proximal row dur¬
ing radial and ulnar deviation: pronation with ulnar devia¬
tion, supination with radial deviation [22,70]. Most studies
agree that the distal row contributes most of the ROM in

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

275

ulnar and radial deviation [12,69,105,140,144]. Finally,
carpal bone motions may be altered by forearm position
[13,28] and by wrist movements that combine flexion,
extension, and radial or ulnar deviation [110,140].

Clinical Relevance

MOBILIZATION TECHNIQUES FOR THE WRIST:

An understanding of the individual contributions to wrist
motion made by the carpal bones provides the biomechani¬
cal rationale for the many manual techniques developed to
evaluate and restore motion to individual bones in the car¬
pus. Appropriate assessments and interventions depend on
a clear understanding of how carpal movement accompa¬
nies each global wrist motion.

It is clear from the results presented above that continued
research is needed to clarify the precise movements of the
individual motions of the carpal bones as well as the factors
that affect those motions. However, certain conclusions can
be drawn:

• The carpus functions primarily as two separate rows.

• The distal row functions more as a unit.

• The bones of the proximal row demonstrate significant
independent movement.

• The motions of the carpal bones are three-dimensional
even when the wrist movement occurs in a single plane.

• All wrist motions are composed of significant contributions
from both the radiocarpal and midcarpal joints.

The movement of the carpal bones is primarily the result of
ligamentous pull and/or the push and pull of adjacent carpal
bones [12,27,114]. It is likely that patients who have even
small instabilities or subluxations of a single carpal bone may
exhibit significant dysfunction of the wrist as a whole [81,118].

Global Wrist Motions

While an understanding of the relative motions of the indi¬
vidual bones of the carpus is essential to understanding the
mechanics of the wrist, measurement of total, or global, wrist
motion remains a standard clinical assessment tool. Such an
assessment describes the motion of the wrist as the relative
orientation of the hand with respect to the forearm, typically
represented by the metacarpal of the long finger and the long
axis of the radius or ulna [91,108] (Fig. 14.35). This measure¬
ment assumes that the motion of the wrist can be described
by motion around a single fixed axis. The data describing the
individual contributions of the carpal bones to wrist motion
demonstrate that this assumption is untrue. However, studies
show that such an assumption allows a reasonable approxima¬
tion of the total wrist movement [3,144]. Several studies have
attempted to identify the axis or axes of rotation of the wrist
[3,81,110,144], but there is little agreement on the precise
location [108]. Many authors suggest that the theoretical

Figure 14.35: Assessment of total wrist motion. Total wrist
motion is approximated typically by assessment of the position
of the metacarpal of the long finger with respect to the long
axis of the forearm.

axis or axes, if existent, lie within or very close to the proximal
capitate [3,12,60,96,144]. There is some evidence that the
location of the theoretical axis of wrist motion changes with
wrist position, suggesting that wrist motion includes consider¬
able translation of the carpal bones in addition to rotation [95].

Clinical Relevance

WRIST ROM: Because global wrist motion is a composite
of motions between the radius and proximal row of carpal
bones; between the proximal and distal rows , and among
the individual carpal bones, a thorough clinical assessment
of global motion includes assessment of global motion
through the use of a goniometric device as well as evalua¬
tion of the component motions performed by palpation and
discrete passive movements. Similarly , treatment includes
mobilization techniques to restore the discrete carpal move¬
ments and active and passive ROM exercises to increase the
total wrist motion.

Total wrist motion consists of flexion, extension, radial and
ulnar deviation, and the combined motion of circumduction.
The wrist also allows a limited and generally unquantified
amount of pronation and supination. When the wrist is
relaxed, the wrist can be passively rotated on the forearm. To
transmit forearm pronation and supination to the hand during
tasks such as turning a light bulb or a doorknob, the wrist is
stabilized by muscles, allowing little pronation and supination
between the hand and forearm [12]. Pronation and supina¬
tion available at the wrist can amplify forearm pronation and
supination in tasks in which increased ROM is required.
Gupta and Moosawi report an average of approximately
15° pronation and supination at the wrist in healthy male
subjects [44].

276

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Clinical Relevance

WRIST INSTABILITY DURING PRONATION AND
SUPINATION: The wrist is frequently involved in patients
with juvenile rheumatoid arthritis (JRA). A patient with wrist
involvement may exhibit swelling and instability at the wrist
and hand with no involvement of the elbow. Clinical inspec¬
tion may reveal little difficulty with forearm movement when
the hand is free to move in space. Yet this patient may have
severe functional deficits associated with an inability to
pronate and supinate. For examplethe patient may be
unable to turn a doorknob. The difficulty arises as the
patient fixes the hand on the doorknob and then attempts
to transmit forearm motion to the hand through the wrist.
This transmission requires the wrist to be stabilized on the
forearm by muscle activation. This forearm motion puts
stress on the wrist and produces pain.

WRIST ROM REPORTED IN THE LITERATURE

Normal passive ranges of motions reported for the wrist in
the literature are presented in Table 14.3. Some of the stud¬
ies cited provide descriptions of the populations on which the
data are based [14,15,106,111,121,131]. There are differ¬
ences in the reported data, but general trends in the values
are apparent. All of the sources report that ulnar deviation
ROM is greater than radial deviation ROM. Similarly, the
reports suggest that wrist flexion ROM is equal to, or greater
than, wrist extension ROM. Age and gender seem to have
only slight effects on ROM [14,91,106].

Clinical Relevance

WRIST POSITIONS DURING FUNCTION: Because there
is such diversity in the wrist ROM reported ' the clinician must
use additional criteria to determine the adequacy of a
patient's ROM. Comparison with the uninvolved limb, when
possibleis critical in determining whether an individual's
ROM is normal. Another standard useful in judging a
patient's ROM is the ROM needed for pain-free, efficient func¬
tion. Studies of the wrist positions and motions of healthy
individuals during functional activities reveal that personal
hygiene activities are accomplished with the wrist positioned
between 50° of flexion and 40° of extension [20] (Fig. 14.36).
Other activities studied-using a fork , holding a newspaper,
opening a jar, pouring from a pitcher-typically use up to
35-40° of extension. Typing at a standard computer terminal
uses approximately W° of wrist extension [119]. Rising from a
chair with upper extremity assistance uses 50 to 60° of
extension ROM [4,20,88,106]. In individuals with spinal cord
injuries, the level of spinal cord injury influences upper
extremity joint positions used during wheelchair propulsion;
those with higher injuries use more wrist extension
(>40°) [90]. Such data can assist the clinician in
judging the adequacy of a patient's ROM as well as in
establishing appropriate treatment goals.

Although wrist motions are typically assessed in the sagit¬
tal (flexion-extension) and frontal (ulnar and radial devia¬
tion) planes, observation of common daily activities
demonstrates that the wrist normally functions in a diagonal
plane, combining extension with radial deviation and flexion

TABLE 14.3: Normal ROM Values for Wrist Movement from the Literature

Flexion (°)

Extension (°)

Radial Deviation (°)

Ulnar Deviation (°)

Steindler [122]

30-50

US Army/Air Force [31]

Boone and Azin [15] a

74.8 ± 6.6

74.0 ± 6.6

21.1 ± 4.0

35.3 ± 3.8

Walker etal. [131] fc

64 ± 10

63 ± 9

19 ± 6

26 ± 7

Schoenmarklin and Marras [111 ] c

62 ± 10

57 ± 9

20 ± 7.5

28 ± 7

Gerhardt and Rippstein [41]

Bird and Stowe [14]

96.2 d

60.0 d

31.5"

36.7 d

98.2 e

66.5 e

34.1 e

37.2 e

Ryu et al [106] f

79.1

59.3

21.1

37.2

Spilman and Pinkston [121] g

—

16.7 ± 5.5

32 ± 5.0

—

18.6 ± 5.8

32.4 ± 6.2

—

18.9 ± 6.2

35 ± 5.3

a Data from 56 men over 19 years of age.

d Data from 30 men and 30 women aged 60-84 years.

c Data from 39 industrial workers, 22 men and 17 women. Mean age was 41.7 ± 10.5 years.
d Data from 8 males and 5 females aged 40-49 years.
e Data from 5 males and 6 females aged 50-80+ years.
f Data from 20 males and 20 females.

9 Data from 63 males and 37 females aged 18-28 years in three test positions.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

277

Figure 14.36: Wrist positions in various activities of daily living. Activities of daily living demonstrate the varied positions of the wrist.
Buttoning a shirt (A) f holding a telephone (B) f and typing at a computer (C) use less wrist extension than is required when using the
upper extremity for weight bearing, such as with a cane (D).

with ulnar deviation (Fig. 14.37 ) Li et al. show a
strong coupling of ulnar and radial deviation with
flexion and extension, respectively, during active
unlar and radial deviation, with somewhat less coupling dur¬
ing active flexion and extension [76]. These authors also
report that maximum total excursion in flexion/extension

and radial/ulnar deviation occurs when the wrist is in the
neutral position with respect to the other plane of motion.
For example maximum flexion/extension excursion occurs
with the wrist in neutral radial/ulnar deviation. The clinician
must take care to maintain the neutral alignment when tak¬
ing standard ROM measurements. The clinician may also

278

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.37: Wrist function in diagonal patterns. The wrist usu¬
ally functions in diagonal patterns, combining wrist extension
with radial deviation and flexion with ulnar deviation.

need to consider assessing wrist motion in both the cardinal
planes and in the more functional diagonal patterns.

Carpometacarpal Joints

There are two CMC joints of the hand, one for the thumb and
a second for the other four digits of the hand. It is at the CMC
joint that the thumb separates from the rest of the hand. Both
joints are described below.

CMC JOINT OF THE THUMB

This joint has received a great deal of attention in anatomy and
anthropology because it is the primary source of the purport¬
edly unique human movement of opposition of the thumb
[50]. It is a synovial joint whose bony surfaces are covered with
articular cartilage. It is important to note that because the tra¬
pezium is positioned anteriorly out of the plane of the hand,
the metacarpal of the thumb also lies out of the plane of the
hand. The volar surface of the thumb is turned slightly toward
the little finger, and the radial aspect of the thumb is directed
slightly toward the volar surface of the forearm [25] (Fig.
14.38). This position facilitates opposition of the thumb.

The joints proximal articular surface on the trapezium is
saddle-shaped, concave in the plane of the thumb s abduction
and adduction motion and convex in the plane of its flexion and
extension motion [24,47,104,141]. The articulating surface on
the base of the metacarpal is reciprocally convex and concave.

Figure 14.38: The resting position of the thumb is anterior to
the plane of the palm, with the thumb rotated slightly toward
the fingers.

Clinical Relevance

OSTEOARTHRITIS OF THE CIVIC JOINT OF THE
THUMB: Although the articular surfaces of the trapezium and
base of the metacarpal of the thumb are reciprocally convex
and concave\ they are not mirror images of each other
[8,97,98]. The joints in female specimens demonstrate less con¬
gruency that those of males. Less congruent articular surfaces
may lead to areas of high stress (force/area) in the joint surface.
This inherent incongruency at the CMC joint of the thumb may
help explain why the CMC joint is so commonly affected by
osteoarthritis, especially in women [7,10,97].

The supporting structures of the CMC joint of the thumb
include the joint capsule, radial CMC ligament, the dorsal
(posterior) and volar (anterior) oblique CMC ligaments, and
the intraarticular beak ligament [47,57,97,129]. The beak lig¬
ament lies on the radial side of the palmar aspect of the joint,
within the joint capsule. It provides protection against exces¬
sive dorsal translation of the thumb s metacarpal on the tra¬
pezium during pinch [97]. It is considered by many as the
primary ligamentous stabilizer of the carpometacarpal joint of
the thumb. All of these ligaments support the joint but also
serve an important role in guiding the motion of the CMC
joint of the thumb [47,138]. In addition, there are two
intermetacarpal ligaments connecting the proximal ends of
the metacarpals of the thumb and index fingers.

The joint is described typically as a saddle joint
[24,47,58,104,141] but is described by some as a ball-and-socket
joint [21] or as a condyloid joint [52,53,57]. Despite the various
classifications of the joint, there is little disagreement about the
motions available at this joint. The saddle-shaped articular

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

279

surfaces allow flexion, extension, abduction, and adduction as
well as some rotation about the long axis of the metacarpal.

The classic and most common classification of the CMC
joint of the thumb is as a saddle joint. However, the real issue
for the clinician is to understand the basis for, and quality of,
the movement at this joint. The names of the motions of the
CMC joint of the thumb vary considerably among clinicians
and anatomists. This book uses the widely used terms, flexion.

extension , abduction , and adduction. It is useful for the
reader also to appreciate that many hand specialists use radial
abduction in place of extension and palmar abduction in
place of abduction of the thumbs CMC joint. Flexion and
extension of the CMC joint of the thumb are defined as
movements of the metacarpal of the thumb in the plane of
the palm toward and away from the ulnar side of the hand,
respectively (Fig. 14.39). Abduction and adduction occur as

Figure 14.39: Motion of the CMC joint of the thumb. The CMC joint of the thumb is capable of (A) flexion and extension (radial abduc¬
tion), which occur in the plane of the palm; (B) abduction (palmar abduction) and adduction, which occur perpendicular to the plane
of the palm; (C) medial and lateral rotation about the long axis of the thumb; and (D) opposition, which is a combination of flexion,
abduction, and medial rotation.

I KINESIOLOGY OF THE UPPER EXTREMITY

280

Part II

Figure 14.40: Rotation of the CMC joint of the
thumb produced by the pull of the dorsal and
volar oblique carpometacarpal ligaments. A. The
pull of the dorsal oblique CMC ligament rotates
the metacarpal in the ulnar direction during flex¬
ion and abduction. B. The pull of the volar
oblique CMC ligament rotates the metacarpal
radially during extension and adduction.

the thumb moves away from and toward the palm, respec¬
tively, in a plane perpendicular to the palm. Adduction
beyond the palm is known as retropulsion. Medial rotation
(pronation) of the CMC joint of the thumb is the rotation of
the pulp of the thumb toward the palm, and lateral rotation
(supination) is the rotation of the pulp away from the pulp of
the other fingers. Opposition of the CMC joint is defined as
simultaneous flexion, abduction, and medial rotation.

Medial rotation of the thumbs CMC joint occurs con¬
comitantly with either flexion or abduction, while lateral rota¬
tion occurs with extension or adduction. Once the joint is
flexed or when the muscles crossing the joint contract and
increase the compression between the trapezium and the
head of the metacarpal of the thumb, independent rotation at
the CMC joint is impossible. Under these circumstances, the
thumbs CMC joint behaves as a biaxial, or condyloid joint
[25,47,53]. However, when the joint is in the neutral position,
up to 45° of passive rotation of the joint is available [47].

Because rotation of the thumbs CMC joint depends on
the amount of flexion or abduction present, it appears to

result from ligamentous tension, specifically from the pull of
the oblique CMC ligaments of the thumb. The attachment of
these two ligaments on the ulnar side of the head of the
metacarpal explains their contributions to rotation of the
CMC joint of the thumb [47]. Flexion of the CMC joint pulls
the dorsal oblique CMC ligament taut, which then pulls the
thumbs metacarpal into medial rotation (Fig. 14.40). Abduc¬
tion of the CMC joint produces the same effect on the dorsal
oblique CMC ligament, resulting in medial rotation of the
thumb. Conversely, the volar oblique CMC ligament pulls the
metacarpal back into lateral rotation as it is stretched during
extension or adduction of the thumbs CMC joint. Thus the
oblique ligaments of the CMC joint of the thumb contribute
to the motion of the joint just as the coracoclavicular ligament
contributes to the motion of the sternoclavicular and
acromioclavicular joints of the shoulder complex (Chapter 8).

Ranges of motion of the thumbs CMC joint reported in
the literature are presented in Table 14.4. Reports of
retropulsion ROM are not found in the literature because it
is rarely if ever assessed. There are only a few reports of the

TABLE 14.4: Normal ROM Values from the Literature for Motion of the CMC of the Thumb

Flexion (°)

Extension (°)

Abduction (°)

Steindler [122]

•

American Academy of Orthopaedic Surgeons [43]

Gerhardt and Rippstein [41]

US Army/Air Force [31]

a Reports 35-40° combined flexion and extension excursion.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

281

Figure 14.41: Thumb rotation in a closed fist. The thumb appears
to have a large amount of rotation when positioned in full
opposition, as in a closed fist.

normal ROM of the CMC joint of the thumb and no known
reports in which the methods used to obtain the ranges are
reported. Although rotations of the CMC joint of the thumb
are not measured separately, Haines reports that passive
medial rotation of approximately 30° and passive lateral rota¬
tion of about 15° are available [47]. The rotation ROM avail¬
able at the CMC joint of the thumb does not appear to match
the rotation of the whole thumb that occurs during grasping
activities of the whole hand (Fig. 14.41). Disagreements
remain regarding the source of the total rotation, or circum¬
duction , mobility of the thumb [23]. Perhaps rotation also
occurs at the MCP and IP joints of the thumb. Or, more
likely, the axes of flexion and extension of these joints are
aligned so that flexion turns the distal segments toward the
palm and extension turns them away The data presented in
Table 14.4 reveal considerable variations in reported values
for the ROM of the CMC joint of the thumb. Population
studies are needed to clarify the ranges of motion of the CMC
joint and the normal variability in a healthy population.

Clinical Relevance

ROM OF THE THUMB: In the absence of reliable norma¬
tive values of ROM for the thumb and because ROM meas¬
ures are difficult to perform , clinicians frequently assess the
excursions of the joints of the thumb by examining the abil¬
ity of the thumb to oppose the fingers. This is readily evalu¬
ated by taking a linear measurement of the distance
between the tip of the thumb and the tip of the opposing
finger (Fig. 14.42). Such a measurement may be very useful
to monitor the progress of a single patient. However ; these
measures are affected by the length of the digits and cannot
be used to compare subjects.

Figure 14.42: Linear measurements of the distance between the
tips of the thumb and fingers may be more convenient in the
clinic to assess changes in thumb mobility.

CMC AND INTERMETACARPAL JOINTS
OF THE FINGERS

Although the CMC articulations of the fingers are enclosed
by a single joint capsule creating a single synovial joint space,
the individual articulations between the carpal bones and the
metacarpals of the fingers are each unique, resulting in
important functional differences. The respective articular
surfaces are covered with articular cartilage. The supporting
structures include the capsule that surrounds the entire com¬
mon CMC joint and sends extensions distally between the
adjacent metacarpals of the fingers. These projections create
the intermetacarpal joints of the fingers. The CMC and
intermetacarpal articulations are reinforced by dorsal and
palmar CMC and intermetacarpal ligaments and by
interosseous ligaments. Strong ligaments extend from the
capitate to the metacarpals of the index, long, and ring fingers.
The metacarpal of the little finger receives strong ligamen¬
tous support from the hamate and pisiform bones.

The CMC articulations are typically characterized as a
common gliding joint [104]. The metacarpal of the index fin¬
ger articulates with the trapezium, trapezoid, capitate, and
metacarpal of the long finger. Consequently, it is wedged in
securely and is the least mobile of all CMC articulations [104]
(Fig. 14.43). The mobility of the CMC articulations of the
fingers increases from radial to ulnar sides of the hand as the
articular surfaces become more curved. The metacarpal of
the little finger articulates only with the hamate and the adja¬
cent metacarpal of the ring finger. The articulation between
the little finger and the hamate is characterized by recipro¬
cally concave-convex surfaces and is sometimes described as
a saddle articulation [62,104]. Consequently, the CMC artic¬
ulation of the little finger exhibits considerable mobility, sec¬
ond only to the thumb s CMC joint.

282

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.43: CMC joint of the fingers. The mobility of the CMC
joint of the fingers is least at the index finger because its
metacarpal is wedged in among the trapezium, trapezoid, capi¬
tate, and metacarpal of the long finger. Mobility increases
ulnarly as the articulations become more curved, with fewer
bony attachments.

Clinical Relevance

THE EFFECT OF CIVIC JOINT MOTION ON WRIST
FLEXION AND EXTENSION ROM MEASURES: Texts
describing ROM measurements of the wrist direct the clini¬
cian to align the movable arm of the goniometer along the
metacarpal of the long finger [43] or along the metacarpal
of the little finger [91]. In both cases the goniometer crosses
the radiocarpal midcarpai, and CMC joints; but the specific
CMC articulation varies between the two methods. Use of
the long finger means that the wrist ROM measurement
reflects the wrist motion and the motion between capitate
and the metacarpal of the long finger. Several studies
demonstrate that there is little or no motion between the
metacarpal of the long finger and capitate during wrist
motions [12,95,110,140].

Use of the little finger as the reference for alignment of
the goniometer means that the measurement reflects radio¬
carpal and midcarpai joint motion and the motion between

the metacarpal of the little finger and the hamate. Observa¬
tions reveal considerable motion between the hamate and
the metacarpal bone of the little finger, particularly in the
sagittal plane [138]. Consequently, wrist flexion ROM values
are likely to be greater when using the metacarpal of the
little finger than when using the long finger's metacarpal
(Fig. 14.44). Differences in the location of the reference, or
movable, arm of the goniometer on the metacarpals may
contribute to the differences in wrist ROM measures reported
in the literature and demonstrated in Table 14.3 [43]. When
possible, use of the metacarpal bone of the long finger as
the reference is recommended to assess wrist motion, partic¬
ularly for flexion and extension. Use of the metacarpal of
the little finger does provide information about the mobility
of the CMC articulation that may be clinically useful in some
circumstances. Clinicians must recognize that both measures
are used clinically but are not interchangeable.

Figure 14.44: Wrist flexion ROM measurements using (A) the
metacarpal of the long finger and (B) the little finger. Measured
wrist flexion or extension ROM may vary depending upon
whether the metacarpal to the long or little finger is used.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

283

Figure 14.45: The volar arch of the hand is apparent during
a forceful grasp.

The intermetacarpal articulations are gliding joints, but most
of their movements are so small that they are not measured.
Since there is more mobility at the articulation between the
hamate and the metacarpal bone of the little finger than
between the CMC articulation of the ring finger, there also
is simultaneous gliding between the metacarpal bones of the
little and ring fingers. The motions at these CMC and inter¬
metacarpal articulations allow the formation of the second
transverse arch of the hand, distal to the first transverse arch,
the carpal arch. This second arch, known as the volar arch,
is essential for powerful grasp (Fig. 14.45).

Clinical Relevance

LOSS OF THE VOLAR ARCH: There are several reasons
for an individual to be unable to form the volar arch includ¬
ing weakness in the intrinsic muscles of the hand ' severe
scarring of the skin on the dorsal surface of the hand subse¬
quent to a severe burn , and ligamentous tightening follow¬
ing immobilization with a flattened arch. If the skin or
ligaments are allowed to tighten enough to prevent the for¬
mation of the transverse arch , the patient may be unable to
perform a powerful grasp. The clinician must take care to
maintain the arches of the hand during immobilization and
healing , to preserve function. The mechanics of powerful
grasp are discussed in greater detail in Chapter 19.

MCP Joints of the Digits

The structure and function of the MCP joints are similar
throughout the five digits. However, the joint of the thumb
and those of the fingers are discussed separately because they
exhibit small but important differences in structure that affect
their function.

MCP JOINT OF THE THUMB

The MCP joint is the synovial joint between the head of the
metacarpal of the thumb and the base of the thumb s proximal

phalanx. Although the head of the thumbs metacarpal
bone is convex in the volar-dorsal and radioulnar direc¬
tions, it is flatter in the radioulnar than in volar-dorsal
direction [63]. The degree of curvature in the radioulnar
direction is quite variable and leads to considerable varia¬
tion in available mobility and to differences in joint classi¬
fication [71,138]. When the radioulnar curvature is
notable, abduction and adduction mobility is present, and
the MCP joint of the thumb is described as a biaxial joint,
reflecting its ability to flex and extend and to abduct and
adduct [54,62,141]. As the radioulnar curvature decreases,
the ability to abduct and adduct decreases. Some authors
report very limited abduction mobility, and others describe
the joint as a simple hinge joint allowing only flexion and
extension [11,51].

The supporting structures of the MCP joint of the thumb
include the capsule, the collateral ligaments, and a volar
(palmar) plate (Fig. 14.46). The capsule itself is somewhat
thin, particularly dorsally, where it is reinforced by the ten¬
don of the extensor pollicis longus [138]. The collateral lig¬
aments are thick bands running obliquely from the head of
the metacarpal to the volar aspects of the ulnar and radial

Volar

plate

Figure 14.46: The supporting structures of the MCP joint of the
thumb include the capsule, collateral ligaments, and the volar
plate.

284

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 14.47: Forces on the thumb's MCP joint during pinch. Lateral
pinch applies a valgus stress on the thumb's MCP joint which is
resisted by the ulnar collateral ligament.

surfaces of the base of the phalanx and to the volar plate
[71]. The volar plate consists of fibrous connective tissue
and fibrocartilage, the latter providing additional articular
surface for the head of the metacarpal bone. It covers the
volar surface of the joint and is firmly attached with the cap¬
sule to the proximal border of the head of the metacarpal.
Distally, the plate is attached loosely just distal to the base of
the proximal phalanx.

Clinical Relevance

SKIER S THUMB (ALSO KNOWN AS GAMEKEEPER S
THUMB): Injuries to the ulnar collateral ligament of the
thumb's MCP joint are common and potentially debilitating.
A common mechanism is a fall on the outstretched hand
while skiing. If the skier keeps the strap of the ski pole
wrapped around the thumb, the fall and the pull on the
pole strap can apply an extension and valgus stress to the
thumb's MCP joint, loading the ulnar collateral ligament.

A rupture of the ligament produces valgus laxity, making
it difficult and painful to stabilize the thumb during lateral
pinch (Fig. 14.47). New ski pole designs have emerged to
minimize the risk of such injuries.

Although the motion of the MCP joint is highly variable, it
is generally agreed that the motion is less than that found at the
other MCP joints [63,104,141]. Both the articular surfaces and
the supporting structures influence the direction and quantity
of movement, the collateral ligaments restraining radial and
ulnar movement as well as volar displacement of the proximal
phalanx [71]. The volar plate limits hyperextension excursion.

Flexion ROM of the thumb s MCP joint is reported to be
approximately 0-50° [31,43], although ranges of up to 80° are
also reported [122]. Few studies describe a data collection
method and the population from which ROM measures are
derived [56]. One reports active ROM from 35 men aged
26-28 years, with no history of hand pathology. The average
flexion excursion was 56° in 30 subjects, but the remaining 5
subjects exhibited a mean flexion excursion of less than 30°,
despite the absence of any evidence or history of pathology. A
report on seven cadaver specimens notes that all specimens
exhibited approximately 10° of hyperextension [49]. Maxi¬
mum flexion ranged from 40° to 80°. Although more studies
are required, clinicians must recognize that there may be a
broad range of mobility found in thumb MCP joint flexion,
even in individuals without pathology.

Only one known source offers any magnitudes for abduc¬
tion and adduction. Kapandji suggests that there are only “a
few degrees” of adduction [62]. Abduction is described as
greater than adduction but is not quantified. Clinicians are
cautioned to use these data carefully. Research is needed to
establish normal values based on population studies and to
determine the clinical significance of abnormal joint move¬
ment at the thumb s MCP joint.

MCP JOINTS OF THE FINGERS

The MCP joints of the fingers usually are described as condy¬
loid or biaxial joints but also are known as ellipsoid joints
[62,86]. The term ellipsoid reflects the difference in the dor¬
sal-volar and radioulnar diameters of the articular surfaces.
As noted in Chapter 7, both condyloid and ellipsoid joints are
biaxial, so both terms reflect the available motion at the joints.

The supporting structures of the MCP joints of the fingers
are similar to those of the MCP joint of the thumb, including
the capsule, collateral ligaments, and the volar (palmar) plates
(Fig. 14.48). In addition, the joints are supported by the

Figure 14.48: The supporting structures of the
MCP joints of the fingers include the capsule,
collateral ligaments, the accessory and glenoid
ligaments, and the volar plate.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

285

accessory collateral ligaments and the phalangioglenoid [83]
or metacarpoglenoid [138] ligaments, the transverse inter-
metacarpal ligaments, and the surrounding tendons and asso¬
ciated soft tissue [83,138]. Studies demonstrate that the
collateral, accessory, and glenoid ligaments help support the
MCP joints of the fingers throughout the ROM [73,83,84].
However, the collateral ligaments are the primary support of
the MCP joints of the fingers [84].

Careful analysis of the collateral ligaments reveals that they
are complex structures with deep and superficial parts. The
radial collateral ligaments are thicker and wider than the ulnar
ligaments [83]. The radial and ulnar collateral ligaments have
broad attachments on the sides of the metacarpal heads and
proximal phalanges [73,83]. Their broad attachments on the
metacarpals help to explain why abduction and adduction
mobility is present when the joint is extended but virtually dis¬
appears when MCP joint flexion approaches 90° [83]. As the
joints go from extension to flexion, the ligament is drawn taut,
thus limiting mediolateral displacement. In addition, because
the metacarpal heads are broader volarly than dorsally, the col¬
lateral ligaments are stretched more as they lie over the
expanded head during MCP flexion when the phalanx is in con¬
tact with the volar portion of the metacarpal head (Fig. 14.49).
The stretch is relieved in extension when the proximal phalanx
is in contact with the narrower more dorsal surface [127].

Clinical Relevance

FUNCTIONAL IMPAIRMENT RESULTING FROM
TIGHTNESS IN THE COLLATERAL LIGAMENTS:

Flexion of the MCP joints pulls the collateral ligaments taut.
Full MCP flexion through the normal excursion requires

compliance of the collateral ligaments. If a patient's hand is
immobilized with the MCP joints extended , the collateral lig¬
aments may shorten , thus preventing MCP joint flexion
mobility once immobilization is discontinued. Therefore, a
hand that requires immobilization must be positioned with
the MCP joints flexed to maintain adequate length of the
collateral ligaments.

As in the thumb, the volar plates add articular surface for the
metacarpal head and limit hyperextension mobility of the
MCP joints. The volar plates also serve an important protec¬
tive role by providing a protective fibrocartilaginous covering
for the articular surfaces during grasp. Grasping a large
object such as a baseball or soda can uses only slight flexion
excursion of the MCP joints of the fingers [75,89]. Since
articulation between the proximal phalanx and metacarpal
head moves progressively more distally on the surface of the
metacarpal head as flexion decreases, slight MCP flexion
leaves the volar surfaces of the heads of the metacarpal
bones exposed to the structures being held in the hand. The
volar plates protect this surface of the metacarpal heads from
abrasion by the object in the hand. A more forceful grasp
increases the risk of injury. Activities involving powerful
grasp of a large object such as hammering or chopping wood
could be very painful and damaging to the metacarpal heads
without the protection of the volar plates.

Ranges of motion of the MCP joints of the fingers are
slightly better studied than the MCP joint of the thumb. The
joints are biaxial joints, allowing flexion and extension as well
as abduction and adduction. The shapes of the metacarpal
bones differ among the four fingers, producing differences in
the motion available at each MCP joint. Although these joints

Figure 14.49: Effects of flexion and extension
on the collateral and accessory ligaments of the
MCP joints of the fingers. In extension, the col¬
lateral and accessory ligaments are slightly
slack and allow abduction and adduction at the
MCP joints of the fingers. In flexion, the collat¬
eral and accessory ligaments are stretched and
allow little abduction and adduction at the
MCP joints of the fingers.

286

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

are biaxial, there is some disagreement about whether the axis
of flexion and extension is fixed or moving. Some studies
report that the axis remains fixed in the center of the
metacarpal head [45,143]. Others suggest that the axis moves
in a volar direction during flexion and in a dorsal direction dur¬
ing extension [36,73]. The clinical importance of this contro¬
versy is its relationship to the motion at the MCP joint. Those
who suggest that the axis of flexion and extension moves report
significant volar and dorsal translation of the proximal phalanx
on the metacarpal during flexion and extension of the joint.

Clinical Relevance

JOINT MOBILIZATIONS TO RESTORE MCP FLEXION
AND EXTENSION: A common treatment in therapy to
restore MCP flexion and extension ROM is to glide the proxi¬
mal phalanx passively in an anterior and posterior direction
on the head of the metacarpal. The theoretical basis for this
approach is the need to restore the translatory motion that
purportedly occurs in normal flexion and extension. Yet
there is some evidence that MCP flexion and extension
involve little or no translation. The manual gliding tech¬
niques may still be beneficial in restoring normal motion by
reducing joint stress and improving lubrication , even if sig¬
nificant translation does not occur in the normal movement
of these joints. Additional research is needed to explain the
benefits of manual therapy at the joints of the fingers.

Reports also suggest that in the fingers, MCP flexion, exten¬
sion, abduction, and adduction are accompanied by slight
rotation [9,35,45,73,75,84,127]. The complexity of the
motion of the MCP joints of the fingers can be appreciated
by noting the position of the fingers when the hand is open
and when it is closed (Fig. 14.50). When the hand is open,
the extended fingers are slightly spread, and the difference in
finger lengths is readily apparent. However, when the fingers
flex and the hand closes, the fingers converge toward the
thenar eminence, and the finger lengths appear much more
equal. The convergence of the fingers begins at the MCP
joints by combining flexion with radial deviation and rotation
toward the thumb [26,127]. This combination is particularly
apparent in the little and ring fingers. The combined move¬
ments of flexion, radial deviation, and rotation are facilitated
by the shape of the metacarpal heads and by the pull of the
collateral ligaments [73].

Available radial deviation excursion is less than ulnar devi¬
ation excursion at the MCP joints, particularly in the index
and long fingers. The ring finger exhibits approximately equal
excursions in radial and ulnar deviation, and the little finger
may have slightly more radial than ulnar deviation [127].

Ranges of flexion and extension motions of the MCP
joints of the fingers found in the literature are reported in
Table 14.5. There is only one known study reporting passive
ROM data collected from a described population of healthy
individuals [80]. This study, based on 60 men and 60 women

Figure 14.50: Comparison of the position of the fingers when the
hand is open and fingers extended and the closed hand with
flexed fingers. A. When the hand is open, the fingers are slightly
spread, and the varied length of the fingers is apparent. B. In a
fist, the fingers converge toward the thenar eminence, and the
fingertips are nearly aligned with each other.

aged 18 to 35 years, reveals that women exhibit significantly
greater hyperextension ROM than men at the MCP joints of
the fingers. The data also demonstrate an increase in mobil¬
ity from the radial to the ulnar fingers. This variation in flex¬
ion excursion allows the finger tips to come into line with one
another when the hand is closed. The authors deny any effect
of hand dominance on the mobility of the MCP joints.

Interphalangeal Joints of the Fingers
and Thumb

There are nine interphalangeal joints in the fingers and
thumb, four proximal interphalangeal (PIP) and four distal
interphalangeal (DIP) joints in the fingers and a single inter¬
phalangeal (IP) joint in the thumb. The structure of these
joints is similar. Each is a hinge joint with trochlear articular
surfaces. The joint surfaces are covered by articular cartilage
typical of most synovial joints. The capsules surround the joint
surfaces and attach to the margins of the articular surfaces.

The condyles of the proximal phalanges are slightly asym¬
metrical creating what some describe as a slight carrying

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

287

TABLE 14.5: Normal ROM Values (°) from the Literature for Motion of the MCP of the Fingers

60 Men

Mallon et al. [80] a

60 Women

US Army/Air Force [31]

Hume [56]'’

Flexion

Index

90 c

Long

100

Ring

102

103

Little

107

Extension

Index

45 c

Not measured

Long

Ring

Little

a Mean passive ROM measurements from 60 men and 60 women, aged 18-35 years. No standard deviations reported.
fa Mean active ROM measurement from 35 men, aged 26-28 years. No standard deviations reported.
c No separate values for individual fingers.

angle at the PIP joints of all the fingers except the long fin¬
ger and at the IP joint of the thumb [6,54]. The consequence
of these slight asymmetries is that the axes of motion are
slightly tilted, and the resulting flexion and extension
motions occur at a slight angle with respect to the long axes
of the digits (Fig. 14.50). This slight out-of-plane flexion and
extension motion assists the convergence of the fingers and
thumb toward the thenar eminence during hand closure [54]
(Fig. 14.51). In contrast, the articular surfaces of the DIP
joints are more symmetrical, and motion at these joints
occurs in planes parallel to the long axes of the fingers.

The primary noncontractile supporting structures of the
interphalangeal joints of the thumb and fingers are similar.
They consist of a capsule, collateral ligaments, and a volar (pal¬
mar) plate (Fig. 14.52). The collateral ligaments include a cord¬
like portion that attaches to the proximal and distal segments of
the joint and another fan-shaped section, or accessory liga¬
ment, that attaches to the proximal segment and to the volar
plate. These collateral ligaments provide the primary support
to the joints in a radioulnar direction throughout the range of
flexion and extension excursion [66,85,101,127]. The articular
surfaces and the accessory ligaments also contribute to radioul¬
nar stability, particularly when the joints are extended.

The volar plate at each joint is similar to those at the MCP
joints. The proximal border of the volar plate is attached at
its ulnar and radial margins to the proximal phalanx. The
volar plates of the interphalangeal joints serve purposes sim¬
ilar to those at the MCP joints, limiting hyperextension
excursion and protecting the volar surface of the head of
each phalanx [16]. The interphalangeal joints also receive
considerable support from the surrounding tendons and
from the connective tissue structures related to these ten¬
dons. The dorsal aspects of the joints receive reinforcement
from the extensor digitorum tendons.

Classified as hinge joints, these interphalangeal joints
allow only flexion and extension excursion. However, slight
radioulnar movement and rotation also occur at these joints,
particularly at the IP joint of the thumb [26,85]. These
motions help direct the fingers toward the thenar eminence

Figure 14.51: Axes of flexion and extension of the fingers' MCP,
PIP, and DIP joints. Flexion about the oblique axes of the fingers'
MCP (A) and PIP (B) joints contribute to the convergence of the
fingers toward the thumb during finger flexion. Flexion about
the medial-lateral axis (C) of the DIP joint produces sagittal
plane motion.

288

Part

I KINESIOLOGY OF THE UPPER EXTREMITY

Joint capsule

Figure 14.52: The supporting structures of the IP
joints of the fingers include the capsule, collat¬
eral ligaments, and the volar plate.

and the thumb toward the fingers as the digits flex. The
ranges of motion reported in the literature are presented in
Tables 14.6 and 14.7.

There is considerable variability in the ranges presented,
but certain trends are consistent throughout the literature:

• The IP joint of the thumb exhibits less mobility than the
interphalangeal joints of the fingers [91].

• The PIP joints of the fingers exhibit more flexion mobility
than the MCP or DIP joints of the fingers [80].

• The DIP joints of the fingers allow more extension excur¬
sion than the PIP joints [26,127].

Few authors report extension excursion of the IP joint of the
thumb, and the values vary widely [5,56]. Extension may be

present, particularly in the presence of abnormal mobility or
stability of the more proximal joints of the thumb. The pres¬
ence of extension of the thumb s IP joint affects the mechan¬
ics of pinch, which is discussed in more detail in Chapter 19.

Clinical Relevance

MEASURES OF FINGER MOBILITY IN THE CLINIC:

ROM assessment for all of the joints of the hand is
extremely time consuming. Consequently, linear measure¬
ments of finger motions are frequently performed in the
clinic. These measures are convenient are relatively quick,

TABLE 14.6: Normal ROM Values (°) from the Literature for Motion of the Interphalangeal Joint of the Thumb

and the Proximal

Interphalangeal

Joints of Fingers

Mallon et al. [80] a AArkC b

US Army/Air
Force [31]

Hume et al.
[56]-

Apfel [5] d

Men

Women [43]

Men Women

Flexion

Thumb

78.6 ± 9.5 83.5 ± 10.9

Index

106

107

100*

105 f

Long

110

112

Ring

110

108

Little

111

Extension

Thumb

Not reported

35.2 ± 16.4 25.8 ± 14.4

Index

0 f

Not measured

Long

Ring

Little

a Mean passive ROM measurements from 60 men and 60 women, aged 18-35 years. No standard deviations reported.

^American Academy of Orthopaedic Surgeons

c Mean active ROM measurement from 35 men, aged 26-28 years. No standard deviations reported.

d Mean passive ROM based on the right hands of 19 men, mean age 35.7 ± 13.9 years, and 12 women, mean age 33.7 ± 6.0 years.
Reported data from Mallon et al. [80],
f Not reported for individual fingers.

Chapter 14 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE WRIST AND HAND

289

TABLE 14.7: Normal ROM Values (°) from the Literature for Motion of the Distal Interphalangeal
Joints of Fingers

Mallon et al. [80] a

AAOS"

[43]

US Army/Air
Force [31]

Hume et al.
[56]'

Men

Women

Flexion

Index

90 e

85 e

Long

Ring

Little

Extension

Index

0 e

Not measured

Long

Ring

Little

a Mean passive ROM measurements from 60 men and 60 women, aged 18-35 years. No standard deviations reported.
^American Academy of Orthopaedic Surgeons.

c Mean active ROM measurement from 35 men, aged 26-28 years. No standard deviations reported.

Reported data from Mallon et al. [80],
e Not reported for individual fingers.

and can be reliable, but they reflect the total mobility of a
digit and are a function of the size of the hand. They may
be quite appropriate in some clinical situations; for example,
linear measures may be useful to monitor weekly changes
in hand mobility in a patient with hand burns. Precise ROM
measures of individual joints of the fingers remain useful to
assess changes in individual joints and to compare subjects.
For example, to compare the results of two types of PIP joint
arthroplasties, discrete ROM measures may be necessary.

Summary

This chapter presents a discussion of the bones and joints that
compose the wrist and hand. The architecture of this region
is more intricate than the more proximal elbow and shoulder
complexes by virtue of the large number of bones and joints.
The bony architecture throughout the region allows substan¬
tial mobility but provides minimal stability. Each joint is sta¬
bilized by an intricate system of ligaments.

The distal radioulnar joint is supported primarily by the
TFCC and, with the proximal radioulnar joint, allows prona¬
tion and supination. The wrist, composed of the radiocarpal,
midcarpal, and intercarpal joints, is supported by extrinsic
and intrinsic ligaments. The radiocarpal and midcarpal joints
both contribute to global wrist motion. The CMC joint of the
thumb separates the thumb from the hand and allows the
thumb to oppose the fingers. The CMC joints of the fingers
help form the volar arch that contributes to powerful grasp.
The MCP joints of the thumb and fingers are supported by a
capsule, collateral ligaments, and a volar plate, but the thumb
is less mobile than the fingers. The interphalangeal joints of

the thumb and fingers exhibit similar supportive structures,
but their articular surfaces allow only uniaxial motion.

This complex of joints with its considerable mobility pro¬
vides the hand with the potential for remarkably varied and
precise movements as long as the surrounding muscles pro¬
vide adequate dynamic control. The following chapter pres¬
ents the muscles of the forearm, the primary movers of the
wrist and significant contributors to motion of the digits.

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CHAPTER

Mechanics and Pathomechanics
of the Muscles of the Forearm

CHAPTER CONTENTS

SUPERFICIAL MUSCLES ON THE VOLAR SURFACE OF THE FOREARM .297

Pronator Teres.297

Flexor Carpi Radialis.297

Palmaris Longus.299

Flexor Digitorum Superficialis (Also Known As Flexor Digitorum Sublimis).300

Flexor Carpi Ulnaris.302

SUPERFICIAL MUSCLES ON THE DORSAL SURFACE OF THE FOREARM .304

Extensor Carpi Radialis Longus and Extensor Carpi Radialis Brevis.304

Extensor Digitorum (Also Known As Extensor Digitorum Communis) .306

Extensor Digiti Minimi (Also Known As Extensor Digiti Quinti) .310

Extensor Carpi Ulnaris.310

COMBINED ACTIONS OF THE FIVE PRIMARY WRIST MUSCLES.312

DEEP MUSCLES ON THE VOLAR SURFACE OF THE FOREARM .313

Flexor Digitorum Profundus.313

Flexor Pollicis Longus .315

Pronator Quadratus .316

DEEP MUSCLES ON THE DORSAL SURFACE OF THE FOREARM .317

Supinator.317

Abductor Pollicis Longus .317

Extensor Pollicis Brevis .320

Extensor Pollicis Longus .321

Extensor Indicis (Also Known As the Extensor Indicis Proprius) .322

SYNERGISTIC FUNCTION OF THE FOREARM MUSCLES TO THE WRIST AND HAND .323

Active Coordination of the Dedicated Wrist Muscles and the Finger Muscles.323

Passive Interactions between the Dedicated Wrist Muscles and the Finger Muscles.325

COMPARISONS OF STRENGTHS IN MUSCLES OF THE FOREARM .325

Pronation versus Supination .325

Wrist Flexion versus Extension.326

Radial versus Ulnar Deviation of the Wrist.327

Finger Flexion versus Extension.327

SUMMARY .328

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Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

295

T he preceding chapter presents the bones and joints of the wrist and hand. That chapter discusses the influ¬
ence of the articular surfaces and surrounding ligaments on the available motion and resulting stability at
each joint. The current chapter presents the muscles of the forearm, which not only serve as the motors for
the wrist and hand but also contribute to the stability of the entire complex.

Many of the forearm muscles cross some or all of the joints of the thumb or fingers. These muscles are known as the
extrinsic muscles of the hand. Their effects on the hand are intimately related to the intrinsic muscles of the hand and
to supporting structures that are unique to the hand. Consequently, a full understanding of the influence of these
extrinsic muscles on the mechanics and pathomechanics of the hand can be complete only after presentation of the
special structures of the hand. The special connective tissue structures found in the hand are discussed in Chapter 17.
The intrinsic muscles of the hand are presented in Chapter 18.

Although more than one half of the muscles of the forearm are positioned to have some effect on the elbow, only the
pronator teres and supinator muscles function primarily at the elbow. The other forearm muscles act primarily at the
wrist and hand. Their roles at the elbow are discussed in this chapter, although these actions are inadequately studied
and poorly understood. Since the normal elbow possesses other large muscles dedicated to moving it, these forearm
muscles may have little functional significance at the elbow except in individuals lacking normal elbow musculature.

In these individuals, the forearm muscles crossing the elbow may be functionally important.

Manifestation of weakness in many of the forearm muscles differs somewhat from that of more proximal muscles.
Weakness of a muscle alters the normal balance of forces crossing any joint. As a result of such an imbalance, the
stronger, opposing muscle forces tend to pull the joint in the opposite direction. For example, in the presence of
weakness of the triceps brachii, the elbow flexors tend to pull the elbow into flexion, but in the upright position, the
weight of the forearm and hand helps resist the flexion force of the stronger elbow flexors. The weights of the distal
segments in the wrist and hand are less significant and thus are less able to resist the deforming forces of muscle
imbalances. Consequently, weakness or tightness of muscles of the wrist and hand are more directly associated with
deformities than elsewhere in the upper extremity. In the following presentations of the forearm muscles, discussions
of weakness and tightness of these muscles include discussions of the potential deformities. It is important to recog¬
nize that muscle balance in the hand depends on the balance among the extrinsic muscles presented in this chapter
and also on a balance between these extrinsic muscles and the intrinsic muscles presented in Chapter 18. Additional
details of many of these deformities are found in Chapter 18.

One characteristic common to most of the muscles in the forearm is the proximity of each muscle to the axes of
motion of the joints they cross. The moment arms of muscles at the shoulder and the elbow are typically a few
centimeters or more. In contrast, the moment arms of muscles in the forearm range from approximately 0.1 to 3.0 cm.
Because the moment arms are so small and most of the tendons can migrate slightly around a joint, many of the
forearm muscles have variable actions. Such variability stems from a change in the muscle's moment arm from, for
example, extension to flexion. The flexor carpi radialis provides a good example (Fig. 15.1). This muscle attaches to
the medial epicondyle of the humerus, lying very close to the elbow's axis of flexion and extension. Reports sug¬
gest that the muscle lies anterior to the axis when the elbow is flexed and consequently produces a flexion
moment at the elbow [1]. When the elbow is extended, the muscle apparently slides to the posterior side of
the elbow's axis and produces an extension moment. The presence of small moment arms and the ability of the
tendons to slide from one side of a joint axis to another help explain disagreements presented in this chapter
regarding muscle actions.

The purposes of the present chapter are to

■ Describe the architecture and action of each of the muscles of the forearm
■ Discuss the functional roles of each of the forearm muscles at the elbow, wrist, and hand
■ Begin to examine the contributions to functional deficits in the wrist and hand made by impairments of
individual muscles

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 15.1: Effect of joint position on the moment arm of a
muscle. The flexor carpi radialis (FCR) lies so close to the axis of
flexion and extension at the elbow that the muscle may be anterior
to the axis and thus act to flex the elbow when the elbow is
flexed, and may slide to the posterior side of the axis and act to
extend the elbow when the elbow is extended.

■ Describe the necessary interplay between the wrist and hand muscles of the forearm required for
optimal hand function

■ Compare the relative strengths of opposing muscle groups of the forearm

The muscles of the forearm are readily divided into four distinct groups, two on the volar surface and two on the
dorsal surface of the forearm. There are deep and superficial muscle groups on each surface. The four forearm muscle
groups are

■ Superficial muscles of the volar surface

■ Superficial muscles of the dorsal surface

■ Deep muscles of the volar surface

■ Deep muscles of the dorsal surface

Each muscle group is presented below. After all of the muscles of the forearm are discussed, the synergistic activity
between the wrist and extrinsic hand muscles is presented. Finally, the relative strengths of the forearm muscles are
reviewed.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

297

SUPERFICIAL MUSCLES ON THE VOLAR flexor tendon. These muscles have additional proximal attach-
SURFACE OF THE FOREARM ments that are presented with each muscle.

There are five superficial forearm muscles on the volar
surface: pronator teres, flexor carpi radialis, palmaris longus,
flexor digitorum superficialis, and flexor carpi ulnaris
(Fig. 15.2). Each of these muscles has a common origin on the
medial epicondyle of the humerus by way of the common

Figure 15.2: The five superficial muscles on the volar surface
of the forearm. From radial to ulnar the five superficial muscles
on the volar surface of the forearm are pronator teres (PT),
flexor carpi radialis (FCR), palmaris longus (PL), flexor
digitorum superficialis (FDS), and flexor carpi ulnaris (FCU).

Pronator Teres

The pronator teres is presented in Chapter 12 with the other
flexor muscles of the elbow. Its actions consist of elbow flex¬
ion and pronation [34,75]. Electromyographic (EMG) data
suggest that its role is to provide additional force in these
motions against heavy resistance [5]. It is readily palpated in
the middle of the volar aspect of the forearm. Weakness of
the pronator teres contributes to decreased strength in
elbow flexion and pronation. If the other elbow flexors
remain unaffected, resulting elbow flexion weakness is
minimal. Similarly, if the pronator quadratus (presented later
in this chapter) remains intact, the disability associated with
pronator teres weakness is restricted to activities requiring
forceful pronation.

Tightness of the pronator teres can result in decreased
supination range of motion (ROM). However, the effect of the
muscle s tightness depends on the position of elbow flexion,
since the pronator teres affects both pronation and flexion.
The interrelationship of these two joint positions is discussed
in detail in Chapter 12, but elbow flexion puts the pronator
teres in a slackened position and consequently allows more
supination ROM (see Fig. 12.9). Conversely, increasing the
stretch on the pronator teres by extending the elbow decreases
flexibility in the direction of supination.

Flexor Carpi Radialis

The flexor carpi radialis is fusiform and lies just medial to the
pronator teres (Muscle Attachment Box 15.1). It is one of six
dedicated wrist muscles of the forearm whose distal function

MUSCLE ATTACHMENT BOX 15.1

ATTACHMENTS AND INNERVATION
OF THE FLEXOR CARPI RADIALIS

Proximal attachment: The medial epicondyle of the
humerus by way of the common flexor tendon and
from the surrounding fascia. It lies medial to the
pronator teres.

Distal attachment: The palmar surfaces of the bases
of the metacarpals to the index and long fingers.

Innervation: Median nerve, C6 and C7.

Palpation: The tendon of the flexor carpi radialis is
readily palpated in the distal forearm just medial to
the radial artery.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

is directed solely at the wrist. The other dedicated wrist mus¬
cles are the palmaris longus, flexor carpi ulnaris, extensor
carpi radialis longus and brevis, and extensor carpi ulnaris.
Other muscles of the forearm affect the wrist too but have
important functions at the digits as well.

ACTIONS

MUSCLE ACTION: FLEXOR CARPI RADIALIS

Action

Evidence

Wrist flexion

Supporting

Wrist radial deviation

Supporting

Elbow flexion

Conflicting

Elbow pronation

Conflicting

The role of the flexor carpi radialis in wrist flexion and radial
deviation is supported by EMG data and analysis of the mus¬
cle s moment arms [5,8,50,55,67,75]. It has moment arms of
approximately 1.0 cm for both wrist flexion and radial devia¬
tion [8,12] (Fig. 15.3).

Contraction of the flexor carpi radialis results in simulta¬
neous flexion and radial deviation of the wrist. For the mus¬
cle to participate in only wrist flexion or only radial deviation,
at least one other muscle must contract at the same time to

prevent the undesired movement. For example, the flexor
carpi radialis participates in pure wrist flexion by contracting
with the flexor carpi ulnaris, whose ulnar deviation pull
provides a counterbalance to the radial deviation pull from the
flexor carpi radialis (Fig. 15.4). EMG analysis suggests that
recruitment of the flexor carpi radialis is greatest when
the subject exerts a force in the direction of both flexion and
radial deviation [12]. This finding is similar to the findings
regarding biceps brachii recruitment. The biceps brachiis
participation in elbow flexion is reduced when the forearm
is pronated (Chapter 12).

The role of the flexor carpi radialis at the elbow is con¬
troversial. Most reports suggest it flexes the elbow
[1,8,34,51], although one study reports that it both flexes
and extends the elbow, depending on the elbows position [1].
Most biomechanical studies report that it possesses a pro¬
nation moment arm [1,10,28,38,42], although the limited
EMG data available show no active contraction during
pronation [1,5,46,51]. Biomechanical studies suggest that
the flexor carpi radialis has the mechanical potential to exert
moments at the elbow; however, its functional activity at
the elbow is unverified. The clinician is cautioned to recog¬
nize that the flexor carpi radialis may have little function at
the elbow under normal conditions, but the muscle has
the potential to participate in the absence of other muscle

Figure 15.3: The moment arms of
the flexor carpi radialis for both
flexion (A) and radial deviation
(B) are approximately 1.0 cm.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

299

Figure 15.4: Wrist flexion without radial deviation requires the
combined contraction of the flexor carpi radialis (FCR) and the
flexor carpi ulnaris (FCU).

MUSCLE ATTACHMENT BOX 15.2

ATTACHMENTS AND INNERVATION
OF THE PALMARIS LONGUS

Proximal attachment: The medial epicondyle of the
humerus by way of the common flexor tendon and
from the surrounding fascia. It lies medial to the
flexor carpi radialis.

Distal attachment: The superficial surface of the
flexor retinaculum and to the proximal portion of
the palmar aponeurosis.

Innervation: Median nerve, C8 and perhaps C7.

Palpation: The muscle is readily palpated superficial
to the transverse carpal ligament during wrist flexion
with simultaneous opposition of the thumb and
little finger.

EFFECTS OF TIGHTNESS

Like weakness, isolated tightness of the flexor carpi radialis,
although unusual, upsets the balance of muscles at the wrist.
Tightness results in diminished flexibility in the direction of
extension and ulnar deviation.

Palmaris Longus

The palmaris longus is a small fusiform muscle medial to the
flexor carpi radialis (Muscle Attachment Box 15.2). It has a
long tendon that is particularly prominent at the wrist
because the tendon remains superficial to the transverse
carpal ligament (Fig. 15.5). However, the muscle is absent in
approximately 10% of the population [8].

control at the elbow. Such potential may be realized in
individuals with profound loss of the primary elbow muscu¬
lature, for example, individuals with polio.

EFFECTS OF WEAKNESS

Discrete weakness of the flexor carpi radialis is uncommon.
However, tendinitis of the flexor carpi radialis does occur,
producing pain with contraction. Consequently, the patient
may avoid contraction and function as though there is
weakness. The obvious impairment from reduced activity of
the flexor carpi radialis is weakness in the combined move¬
ments of wrist flexion and radial deviation. The resulting
functional deficit at the wrist stems from the muscular
imbalance that ensues from weakness of this wrist muscle.
This same effect is seen whenever one or a combination of
wrist muscles is impaired. A more detailed discussion of the
role of muscle balance and the effects of muscle imbalances
at the wrist is presented after all of the dedicated wrist
muscles are discussed.

Figure 15.5: The palmaris longus is very prominent because it is
superficial to the flexor retinaculum of the wrist.

300

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

ACTIONS

MUSCLE ACTION: PALMARIS LONGUS

Action

Evidence

Wrist flexion

Supporting

Hand cupping

Inadequate

Skin anchor

Inadequate

The palmaris longus lies in the center of the wrist and con¬
sequently acts as a pure flexor of the wrist. Estimates of its
physiological cross-sectional area suggest that the palmaris
longus is less than half the size of the flexor carpi radialis and
less than one third the size of the flexor carpi ulnaris [1].
Consequently, its contribution to the flexion torque at the
wrist is small in most individuals. There are no known EMG
studies examining its participation in functional activities.

The functions of the palmaris longus to cup the hand
and tighten or support the skin of the hand are the result of
the muscle’s attachment to the palmar aponeurosis. The
functional significance of this aspect of the palmaris longus
muscle s action is not known.

EFFECTS OF WEAKNESS

As already noted, the palmaris longus is absent in many
individuals. There are no known reports of impairments
associated with the absence of the palmaris longus. The
muscles tendon is commonly used by surgeons as graft
material for tendon repairs [8,21,68].

MUSCLE ATTACHMENT BOX 15.3

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITORUM
SUPERFICIALIS

Proximal attachment: The humeroulnar head
attaches to the medial epicondyle of the humerus
by way of the common flexor tendon and from the
surrounding fascia as well as from the coronoid
process of the ulna and the medial collateral liga¬
ment of the elbow. The radial portion arises from
the anterior surface of the radius from the radial
tuberosity to the attachment of the pronator teres.

Distal attachment: A tendon to each finger enters a
flexor sheath proximal to the MCP joint. At the MCP
joint the tendon splits into two strands through
which the tendon of the flexor digitorum profundus
travels. The two slips of the flexor digitorum super-
ficialis reunite at the proximal end of the middle
phalanx and insert on its palmar surface.

Innervation: Median nerve, C8 and T1, perhaps C7.

Palpation: Tendons of the flexor digitorum super-
ficialis (particularly those to the ring and long fingers)
can be palpated in the midline of the wrist as they
cross the radiocarpal joint. The muscle belly is palpable
along the medial border of the proximal ulna.

EFFECTS OF TIGHTNESS

There are no known reports of discrete tightness of the
palmaris longus muscle.

Flexor Digitorum Superficialis (Also
Known As Flexor Digitorum Sublimis)

The flexor digitorum superficialis is the largest of the
superficial muscles on the volar surface (Muscle
Attachment Box 15.3).

ACTIONS

MUSCLE ACTION: FLEXOR DIGITORUM SUPERFICIALIS

Action

Evidence

PIP flexion

Supporting

MCP flexion

Supporting

Wrist flexion

Supporting

Wrist radial deviation

Supporting

Wrist ulnar deviation

Supporting

Elbow flexion

Inadequate

Elbow extension

Inadequate

Elbow pronation

Inadequate

Elbow supination

Inadequate

The flexor digitorum superficialis is the only muscle that flexes
the PIP joints of the fingers without flexing the distal inter-
phalangeal (DIP) joints [5,62,75]. However, contraction of
the flexor digitorum superficialis affects each of the joints that
the muscle crosses. The tendons of the flexor digitorum
superficialis are the most superficial muscles on the volar sur¬
face of the MCP joints of the fingers. Consequently, they have
the largest moment arms for flexion [2,8] (Fig. 15.6). EMG
studies reveal activity of the flexor digitorum superficialis dur¬
ing MCP flexion [5]. The flexor digitorum superficialis to the
long finger is substantially stronger than that to the index and
ring fingers [15], and the flexor digitorum superficialis to the
little finger is the weakest [8]. The muscle appears to have no
contribution to radial or ulnar deviation of the MCP joints of
the fingers under normal conditions [2,7].

The muscle is described as having four separate and dis¬
tinct muscular slips, each going to a different finger [75], but
the tendons to the index, ring, and little finger actually receive
muscle fibers from multiple muscle bundles [8]. Only the ten¬
don to the long finger has completely unique muscle fibers.
Therefore, only the long finger has completely independent
activation of the flexor digitorum superficialis. In contrast, the
index and little fingers share some proximal muscle attach¬
ments but have independent fiber contributions more distally.
As a result, independent activation of the flexor digitorum
superficialis at either of these fingers occurs only in low-load

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

301

Figure 15.6: The moment arm of the flexor digitorum superficialis.
At the MCP joints of the fingers the flexor digitorum superficialis
(FDS) lies anterior to all of the other muscles and, therefore, has
the largest flexor moment arm of all of these muscles.

activities. More-vigorous contractions recruit the shared mus¬
culotendinous units so that the index and little fingers con¬
tract together. The tendon to the little finger also may be defi¬
cient or completely absent [3,13,29].

Clinical Relevance

CLINICAL ASSESSMENT OF THE INTEGRITY OF THE
FLEXOR DIGITORUM SUPERFICIALIS: The muscular
attachments and variations of the flexor digitorum super¬
ficialis are clinically significant for attempting to assess the
viability or strength of the flexor digitorum superficialis to
individual fingers. Inability to perform the unique flexor dig¬
itorum superficialis action of flexion of the PIP joint of the
little finger may lead a clinician to conclude that the mus¬
cle is impaired , when in fact the muscle is inhibited because
it is unable to contract independently of the index finger or
perhaps the muscle is absent entirely. The clinician must
use additional information including the activation avail¬
able at other fingers as well as the strength of more-
proximal joints of the same digit to draw conclusions regard¬
ing the status of the flexor digitorum superficialis. Allowing
the patient to flex both the index and little fingers' PIP joints
simultaneously may be particularly helpful in assessing the
little finger's flexor digitorum superficialis tendon [3j.

The flexor digitorum superficialis crosses the wrist approxi¬
mately 1.5 cm anterior to the axis of rotation for flexion and
extension and has a significant potential for wrist flexion [8].
This role is supported by EMG studies that reveal flexor
digitorum superficialis activity during wrist flexion along with
activity of the dedicated wrist flexors, the flexor carpi radialis,
and flexor carpi ulnaris [5]. At the wrist, the flexor digitorum
superficialis tendons cross over the capitate, the approximate
location of the axis of radial and ulnar deviation (Fig. 15.7). The
tendons of the flexor digitorum superficialis can slide

Figure 15.7: The flexor digitorum superficialis crosses the wrist
approximately over the capitate, the estimated axis of wrist radial
and ulnar deviation. The tendons can slip in a radial and ulnar
direction, contributing to radial and ulnar deviation, respectively.

in either a radial or ulnar direction as they cross the wrist, sup¬
porting reports that suggest that the flexor digitorum superfi¬
cialis is active in both radial and ulnar deviation of the wrist [5,8].
The extent of its contribution to these two motions is not clear.

Clinical Relevance

SUBSTITUTION PATTERNS OBSERVED DURING
MANUAL MUSCLE TESTING (MMT) OF WRIST
MUSCLES: Standard MMT procedures to assess the
strength of the dedicated wrist flexor muscles call for the
fingers to be relaxed [28,34] (Fig. 15.8). Careful observation
is essential throughout the test to detect the gradual recruit¬
ment of the flexor digitorum superficialis as the resistance to
the dedicated wrist flexors muscles overcomes their strength.
By allowing the flexor digitorum superficialis to participate
in the wrist flexion strength test , the clinician may fail to
identify weakness in the dedicated wrist flexor muscles.

302

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 15.8: The standard manual muscle test (MMT) procedure
for wrist flexion. A. The standard MMT procedure for wrist
flexion strength requires that the fingers remain relaxed.

B. Flexion of the PIP joints of the fingers during the MMT of
wrist flexion demonstrates substitution of the flexor digitorum
superficialis during the test.

Few authors mention a role of the flexor digitorum superfi¬
cialis at the elbow joint. An investigation of the moment arms
of muscles that cross the elbow suggests that the flexor digito¬
rum superficialis is similar to the flexor carpi radialis in having
a moment arm that results in an extension moment at the
elbow when the elbow is extended and a moment arm con¬
tributing to a flexion moment when the elbow is flexed [1].
Studies also report that the flexor digitorum superficialis can
generate either a pronation or supination moment depending
upon the position of the forearm and elbow [1,46]. The
moment arms of the flexor digitorum superficialis and flexor
carpi radialis at the elbow are less than 1 cm. Whether either
muscle contributes significantly to elbow motion remains
unclear. As with the flexor carpi radialis, the importance of this
potential effect at the elbow may be more apparent in individ¬
uals who lack the primary muscles of the elbow.

EFFECTS OF WEAKNESS

The unique effect of weakness of the flexor digitorum superfi¬
cialis is weakness in PIP flexion while the DIP remains relaxed.
Weakness of the flexor digitorum superficialis also can have an
impact on all of the joints that it crosses, including the wrist and
the MCP joints. Because the flexor digitorum superficialis adds
force to finger flexion, flexor digitorum superficialis weakness
may result in hyperextension of the PIP and flexion of the DIP
during forceful pinch [8]. Functionally, weakness of the flexor
digitorum superficialis leads to a reduction in grip strength.

EFFECTS OF TIGHTNESS

Although individual tightness of the flexor digitorum superficialis
is unusual, tightness of the flexor digitorum superficialis along
with the flexor digitorum profundus is a common consequence
of a loss of muscle balance between the extrinsic and intrinsic
muscles, resulting in a clawhand deformity. A detailed discussion
of the deformities resulting from imbalance of the intrinsic and
extrinsic muscles of the hand is presented in Chapter 18.

Flexor Carpi Ulnaris

The flexor carpi ulnaris is a large pennate muscle that has the
largest physiological cross-sectional area of the dedicated
wrist muscles [8,40,42] (Muscle Attachment Box 15.4).
Consequently, it is a very powerful muscle, responsible for
stabilizing the wrist during such activities as slicing meat and
using a hammer [56] (Fig. 15.9).

ACTIONS

MUSCLE ACTION: FLEXOR CARPI ULNARIS

Action

Evidence

Wrist flexion

Supporting

Wrist ulnar deviation

Supporting

Elbow flexion

Conflicting

Elbow extension

Conflicting

Elbow pronation

Conflicting

Elbow supination

Conflicting

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

303

MUSCLE ATTACHMENT BOX 15.4

ATTACHMENTS AND INNERVATION
OF THE FLEXOR CARPI ULNARIS

Proximal attachment: The humeral head attaches to
the medial epicondyle of the humerus by way of
the common flexor tendon and from the surround¬
ing fascia. The larger ulnar portion arises from the
medial aspect of the olecranon and from the poste¬
rior surface of the proximal two thirds of the ulna
and from the adjacent intermuscular septum.

Distal attachment: The pisiform bone and ultimately
to the hook of the hamate and the base of the
metacarpal bone of the little finger by way of the
pisohamate and pisometacarpal ligaments.

Innervation: Ulnar nerve, C7, C8, T1.

Palpation: The tendon of the flexor carpi ulnaris
is palpable as it crosses the wrist toward the
pisiform bone.

Wrist flexion and ulnar deviation are actions of the flexor
carpi ulnaris verified by EMG analysis [5,12,34,55,75]. The
moment arm of the flexor carpi ulnaris for wrist flexion
is very similar to that of the flexor carpi radialis, approxi¬
mately 1.0 cm. The moment arm is enhanced by the mus¬
cles attachment on the pisiform bone, effectively lifting the
muscle anteriorly and improving its flexion moment arm
(Fig. 15.10). By the same token, the muscles attachment on
the pisiform prevents much change in the flexion moment
arm as the wrist flexes [8,43].

The effect of the flexor carpi ulnaris at the elbow is con¬
troversial. Studies support its participation in both elbow
flexion [51] and elbow extension [1]. Similarly, studies sug¬
gest that the flexor carpi ulnaris contributes to pronation
[1,46] and supination of the forearm [46], while other stud¬
ies suggest there is no contribution to either [5,51]. These
data reveal a need for additional research to resolve the role
of the flexor carpi ulnaris at the elbow. Like the other mus¬
cles of the forearm discussed so far, the flexor carpi ulnaris
is unlikely to be an important participant in elbow function
in individuals with normal elbow function. However, this
muscle may provide small but useful elbow function in the
absence of normal elbow musculature.

EFFECTS OF WEAKNESS

Weakness of the flexor carpi ulnaris weakens the strength
of wrist flexion with ulnar deviation. In many activities
the wrist moves in a diagonal pattern from extension and
radial deviation to flexion and ulnar deviation as force is

Figure 15.9: The wrist position in some forceful activities. The
wrist is positioned in flexion with ulnar deviation during many
forceful activities such as (A) cutting a piece of meat and (B)
hammering.

transmitted from the forearm to the wrist and hand, such
as in chopping or hammering. In the pounding phase of
hammering, the hand holding the hammer moves into
wrist flexion and ulnar deviation [56]. When contact is
made with the nail, the nail pushes the hammer and wrist
back toward extension and radial deviation, requiring
the flexor carpi ulnaris to control the hammer and avoid

304

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Triquetrum Hamate

Figure 15.10: Attachment of the
flexor carpi ulnaris (FCU) into the
pisiform increases the angle of
application and therefore the
flexion moment arm of the FCU
at the wrist.

a rebound from the nail. Patients with weakness of the
flexor carpi ulnaris may report a sense of weakness during
such activities [8].

Effects of Tightness

Tightness of the flexor carpi ulnaris is found in some patients
with central nervous system disorders resulting in spasticity of
the flexor carpi ulnaris. It also is seen in patients with wrist
instability, such as those with rheumatoid arthritis. In these
cases, the wrist is pulled into and held in a position of flexion
and ulnar deviation. Since wrist extension is important for
powerful grasp, tightness of the flexor carpi ulnaris interferes
with powerful grasp and, consequently, may result in signifi¬
cant functional impairment.

SUPERFICIAL MUSCLES ON THE DORSAL
SURFACE OF THE FOREARM

The superficial muscles on the dorsal surface of the forearm
include the remaining muscles dedicated to wrist function,
the extensor carpi radialis longus and brevis and the extensor
carpi ulnaris (Fig. 15.11). The other muscles found in the
superficial dorsal group include the extensor digitorum and
extensor digit! minimi. All of these muscles have a common
proximal attachment on the lateral epicondyle of the humerus
by way of the common extensor tendon. Their individual
attachments and actions are presented below.

Extensor Carpi Radialis Longus
and Extensor Carpi Radialis Brevis

The extensor carpi radialis longus and extensor carpi radialis
brevis lie so close together at the elbow and follow such sim¬
ilar paths to the hand that they have similar actions at the
wrist (Muscle Attachment Boxes 15.5 and 15.6). However,
they are distinct muscles and have unique roles at the wrist
and elbow, presented here. The extensor carpi radialis longus
is the most proximal muscle of those attaching to the common
extensor tendon. It is covered proximally by the brachioradi-
alis muscle. The extensor carpi radialis brevis is covered ante¬
riorly by the extensor carpi radialis longus.

Extensor carpi
radialus longus

Extensor carpi
radialus brevis

Extensor digitorum

Extensor digiti
minimi

Extensor carpi
ulnaris

Figure 15.11: The superficial muscles on the dorsal surface of the
forearm from radial to ulnar are the extensor carpi radialis longus
(ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum
(ED), extensor digiti minimi (EDM), and extensor carpi ulnaris (ECU).

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305

MUSCLE ATTACHMENT BOX 15.5

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR CARPI RADIALIS
LONGUS

Proximal attachment: Distal one third of the lateral
supracondylar ridge and intramuscular septum and
from the common extensor tendon attached to the
lateral epicondyle of the humerus.

Distal attachment: Radial aspect of the dorsal surface
of the base of the index finger's metacarpal bone.

Innervation: Radial nerve, C6 and C7.

Palpation: The tendon of the extensor carpi radialis
longus is palpable on the dorsolateral aspect of the
wrist joint just proximal to the base of metacarpal
bone of the index finger.

ACTIONS

MUSCLE ACTION: EXTENSOR CARPI RADIALIS
LONGUS AND BREVIS

Action

Evidence

Wrist extension

Supporting

Wrist radial deviation

Supporting

Elbow flexion

Inadequate

Elbow pronation

Supporting

Elbow supination

Supporting

The actions of wrist extension and radial deviation by both of
these muscles are well accepted in the literature [34,55,75].
EMG data also support their roles in these movements [5,12].

Although the extensor carpi radialis longus and extensor
carpi radialis brevis are anatomically similar, they are not
identical. Reports suggest that they make unique contribu¬
tions to the wrist [8,12,39]. The extensor carpi radialis
brevis has a larger extension moment arm and a larger
physiological cross-sectional area than the extensor carpi
radialis longus [40] (Fig. 15.12). Evidence suggests that the
extensor carpi radialis brevis contributes the most to wrist
extension strength [37] and is the main wrist extensor,
although individual variability exists [5,8,24]. The moment
arm for radial deviation is greater in the extensor carpi radi¬
alis longus than in the extensor carpi radialis brevis, but
their participation in radial deviation appears more equal
[8,67], although the extensor carpi radialis longus may play
a larger role than the extensor carpi radialis brevis in radial
deviation [8,12,43,65] (Fig. 15.13).

The roles of the extensor carpi radialis longus and exten¬
sor carpi radialis brevis at the elbow are somewhat less clear.
The proximity of the extensor carpi radialis longus to the bra-
chioradialis, an elbow flexor, suggests that the extensor carpi
radialis longus has the potential to flex the elbow [8]. Studies
of their moment arms at the elbow suggest that both the
extensor carpi radialis longus and extensor carpi radialis bre¬
vis have the mechanical potential to flex the elbow
[20,41,42,45]. However there are no known in vivo studies
supporting this role [1,8,39].

Direct electrical stimulation studies of the extensor carpi
radialis longus and extensor carpi radialis brevis suggest that
each can generate a small supination moment when the fore¬
arm is pronated and a more significant pronation moment
when the forearm is supinated, particularly with the elbow
flexed [10,46]. As with the other wrist muscles that cross the
elbow, the contribution of the extensor carpi radialis longus
(and perhaps the extensor carpi radialis brevis) at the elbow
may be significant only in the absence of the primary mus¬
cles of the elbow.

MUSCLE ATTACHMENT BOX 15.6

ATTACHMENTS AND INNERVATION OF
THE EXTENSOR CARPI RADIALIS BREVIS

Proximal attachment: Lateral epicondyle of the
humerus by way of the common extensor tendon.

Distal attachment: Radial aspect of the dorsal surface
of the base of the long finger's metacarpal bone.

Innervation: Radial nerve, C7 and C8, perhaps C6.

Palpation: The tendon of the extensor carpi radialis
brevis is palpable on the dorsolateral aspect of the
wrist joint just proximal to the base of the
metacarpal bone of the long finger and ulnar to the
extensor carpi radialis longus tendon.

EFFECTS OF WEAKNESS

Weakness of both of these muscles results in a substantial
loss of strength in both wrist extension and radial deviation.
The loss of the extensor carpi radialis brevis alone results in
more impairment than the singular loss of the extensor carpi
radialis longus, since the former plays a larger role in wrist
extension. Weakness in wrist extension produces difficulty in
forceful grasp and pinch because of the necessary interaction
between wrist extension and finger flexion. This interaction
is explained later in this chapter.

Effects of Tightness

Tightness of these two muscles is not common but results in
decreased flexibility in the directions of flexion and ulnar
deviation. Such restriction may cause difficulty in perform¬
ing some personal hygiene tasks that typically require wrist
flexion with ulnar deviation [56].

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 15.12: Extension moment arms
of the extensor carpi radialis longus
and brevis. A sagittal view of the
wrist demonstrates that the extensor
carpi radialis brevis (ECRB) has a
larger extension moment arm at the
wrist than the extensor carpi radialis
longus (ECRL).

Figure 15.13: Moment arms of the extensor carpi radialis longus
and brevis for radial deviation. A frontal view of the wrist
demonstrates that the extensor carpi radialis longus (ECRL) has a
larger moment arm for radial deviation than the extensor carpi
radialis brevis (ECRB).

Clinical Relevance

FUNCTIONAL SIGNIFICANCE OF LIMITED WRIST
FLEXION ROM (CASE REPORT): The position of function
of the forearm and wrist in most activities is wrist extension
[11,56]. Thus loss of flexion ROM may seem less important.
However ; one patient's disability reveals the significance of
wrist flexion ROM in normal daily function. This patient under¬
went bilateral wrist and distal radioulnar fusions because of
painful instability secondary to rheumatoid arthritis. Both
wrists were fused in extension for the purpose of facilitating
activities of daily living. The patient functioned well in most
activities but was unable to perform one simple yet essential
task, that of cleaning herself after using the toilet. This exam¬
ple is a warning to all clinicians to recognize the diversity of
movements required to perform the normal spectrum of daily
activities. Careful analysis of a patient's daily tasks is needed to
appreciate the functional impact of many impairments.

Extensor Digitorum (Also Known
As Extensor Digitorum Communis)

The tendons of the extensor digitorum fan out to the four
fingers after crossing the dorsal surface of the wrist (Muscle
Attachment Box 15.7). The extensor tendons of all the fingers
are interconnected by fibrous bands between adjacent fingers
known as juncturae tendinae (Fig. 15.14). The extensor
digitorum tendon to the little finger is frequently deficient or
even absent [31,70,73] and often receives a slip from the ring
finger by way of the juncturae tendinae.

The juncturae tendinae affect both active and passive move¬
ments of the fingers. Actively, these interconnections allow an
increase in extension force to individual fingers during extensor
digitorum contraction by increasing the number of muscle
fibers pulling on a single tendon [8]. Passively, flexion of the
MCP joint of one or two fingers pulls the extensor tendons of
the remaining fingers distally via the juncturae tendinae
(Fig. 15.15). This distal migration puts the extensor tendons to

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

307

MUSCLE ATTACHMENT BOX 15.7

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR DIGITORUM

Proximal attachment: Lateral epicondyle of the
humerus by way of the common extensor tendon
and from the surrounding fascia and intermuscular
septum.

Distal attachment: A tendon to each finger broad¬
ens into a flat expansion, the extensor hood, at the
level of the MCP joint. Distal to the MCP joint, the
hood receives attachments from the intrinsic muscles.
At the distal end of the proximal phalanx, the hood
splits into a central tendon and two lateral bands.
The central tendon, or slip, inserts into the base of
the middle phalanx on its dorsal surface. The lateral
bands pass along the medial and lateral borders of
the dorsal surface of the middle phalanx and con¬
verge at the DIP joint. The two bands reunite and
attach together on the dorsal surface of the base of
the distal phalanx.

Innervation: Radial nerve, C7 and C8.

Palpation: The tendons of the extensor digitorum
are palpated as they cross the wrist and along the
metacarpals of each finger.

all the fingers on slack, allowing the fingers to flex [50]. The
juncturae tendinae also may provide important medial and lat¬
eral stabilizing forces to the MCP joints while the fingers are
flexed during forceful grip. However, these fibrous intercon¬
nections also can impede independent finger movement [38].

Figure 15.14: Juncturae tendinae of the extensor digitorum. The
tendons of the extensor digitorum to the fingers are connected
to one another by the juncturae tendinae.

Figure 15.15: Effect of finger flexion
on the juncturae tendinae and on
flexion excursion of the other fin¬
gers. Flexion of the MCP joints of the
index or long finger pulls on the
juncturae tendinae of the ring and
little fingers, putting the extensor
tendons to the ring and little fingers
on slack and allowing full flexion
excursion.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Clinical Relevance

INDEPENDENT FINGER MOVEMENT: With the fingers
relaxed in slight flexion, independent active extension of
each MCP joint is readily achieved. Independent extension at
the MCP joints of the index and little fingers is possible even
with flexion of the MCP joints of the long and ring fingers
(Fig. 15.16). However ; when the MCP joints of the index and
little fingers are flexed, independent extension of the long
and ring fingers is severely impaired. This impairment results
from the distal pull on the tendons of the long and ring fin¬
gers by the juncturae tendinae from the index and little fin¬
gers. A distal pull by the juncturae tendinae puts the exten¬
sor digitorum tendons to the long and ring fingers on slack
and renders them ineffective in producing active extension
[62]. At the same time there may be inhibition of any active
extension at the ring and long fingers; since the index and
little fingers, connected to the others by the juncturae tendi¬
nae, are kept flexed. Independent extension at the index and
little fingers results from the accessory extensor muscle to
each finger.

The difficulty in independent finger movement presents
a particular challenge to musicians such as pianists, who
need independent finger movement to play intricate com¬
positions. This difficulty has led some to undergo surgical
release of the juncturae tendinae [8,38]. Yet a study of 21
cadaveric limbs reveals that independent movement is
only slightly improved following transection of the junc¬
turae tendinae [73]. The study lists other structures,
including the skin in the web spaces of the fingers, which
also contribute to the functional interdependence of the
fingers.

ACTIONS

MUSCLE ACTION: EXTENSOR DIGITORUM

Action

Evidence

Finger MCP extension

Supporting

Finger PIP extension

Supporting (only with
intrinsic activity)

Finger DIP extension

Supporting (only with
intrinsic activity)

Wrist extension

Supporting

Wrist radial deviation

Refuting

Wrist ulnar deviation

Inadequate

Elbow flexion

Conflicting

Elbow extension

Conflicting

Elbow pronation

Inadequate

Elbow supination

Inadequate

The extensor digitorum is the primary extensor of the MCP
joints of the fingers. The muscle to the long finger is the
strongest of the four [7,8]. The extensor digitorum contributes
to the extension of the PIP and DIP joints of the fingers,
although it is unable to accomplish extension at the PIP and
DIP joints without the simultaneous activity of the
lumbricals or interossei [62,73]. EMG studies consis¬
tently reveal activity of the extensor digitorum during
MCP extension with the interphalangeal (IP) joints flexed, but
combined extensor digitorum and intrinsic muscle activity
when the IP joints are extended [13,41]. The coordinated
interplay of the extensor digitorum and the intrinsic muscles in
producing extension at the IP joints is discussed in Chapter 18.

The extensor digitorum also has been implicated in
abduction of the fingers and extension and abduction at the

Figure 15.16: Effect of the juncturae tendinae on active extension of the long and ring fingers. A. Independent active extension of the
index and little fingers is possible even with simultaneous flexion of the MCP joints of the long and ring fingers. B. Active extension of
the long and ring fingers is difficult when the MCP joints of index and little fingers are flexed.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

309

wrist [34,75], but there are no known studies verifying
these actions. The role of the extensor digitorum in abduc¬
tion of the fingers is complicated by the complex movement
of the MCP joints during flexion and extension. As noted in
the previous chapter, the fingers converge on the thenar
eminence during finger flexion and return to a spread posi¬
tion during extension. This movement gives the appearance
of active abduction at the MCP joint but probably is the
consequence of the articular shapes, with no need for active
abduction from any muscle.

Studies of the extensor digitorum s moment arms at the
wrist reveal a potential for wrist extension and ulnar deviation
[8,51]. Despite the extensor digitorum s mechanical capacity
to extend the wrist, the extensor carpi radialis longus and bre¬
vis and the extensor carpi ulnaris remain the primary wrist
extensors. However, the clinician must recognize that substi¬
tutions from the extensor digitorum can mask weakness in the
primary extensor muscles of the wrist.

Although the extensor digitorum crosses the elbow, there
are no known reports verifying its participation in elbow
motion. Studies of the muscles moment arms at the elbow
are conflicting, reporting both an extension moment arm [51]
and a flexion moment [1]. Isolated stimulation of the extensor
digitorum in three subjects produced a small supination
moment with the forearm pronated and a larger pronation
moment with the forearm in supination [46]. Like the other
muscles of the forearm, the extensor digitorum appears to
have the potential to affect the elbow, but its effects may be
negligible unless the normal musculature of the elbow is so
compromised that the small contributions from the extensor
digitorum are more important.

EFFECTS OF WEAKNESS

Isolated weakness of the extensor digitorum may result from
trauma such as tendon lacerations. Such injuries result in
weakness or loss of extension of the MCP joints of the fingers.
However, if the accessory extensors to the index and little fin¬
gers remain, some function can be maintained.

EFFECTS OF TIGHTNESS

Prior to considering the effects of tightness, it is necessary to
appreciate the role the distal attachment of the extensor dig¬
itorum plays in normal finger flexion ROM. It is the elegant
arrangement of this attachment that allows a finger to flex at
all of the joints simultaneously, as in making a fist (Fig. 15.17).
The central tendon of the extensor digitorum attaches to the
middle phalanx and is pulled taut during PIP flexion. Tension
on the central tendon pulls the extensor digitorum distally.
This distal migration of the extensor digitorum puts its lateral
bands on slack, thus allowing flexion mobility of the DIP
(Fig. 15.18). Conversely, flexion of the DIP joint tightens
the lateral bands, producing a distal pull on the extensor
digitorum tendon. In this case the distal migration of the
extensor digitorum puts the central tendon on slack and
allows full flexion ROM at the PIP (Fig. 15.19).

Figure 15.17: Extensive distal attachment of the extensor digito¬
rum. Just distal to the MCP joint the tendon splits into the cen¬
tral tendon that attaches to the base of the middle phalanx and
into the lateral bands that pass distally along the radial and
ulnar sides of the dorsal surface until they rejoin and insert on
the base of the distal phalanx.

Clinical Relevance

FLEXION ROM AT THE PIP AND DIP JOINTS: To assess
the flexion ROM at the PIP and DIP joints , the extensor digi¬
torum must be put on slack. Thus the PIP joint must be
flexed when assessing DIP joint flexion ROM (Fig. 15.20).

PIP flexion ROM is maximized by allowing the DIP joint to
remain relaxed (Fig. 15.21).

Tightness of the extensor digitorum limits full flexion
ROM of the fingers, but the manifestations of tightness of
the extensor digitorum are complex, since the muscle crosses
many joints. In the presence of extensor digitorum tightness,
extension or hyperextension of the MCP joints is accompa¬
nied by flexion of the IP joints. This combination of MCP
extension and IP flexion results from the pull of the extensor
digitorum and the responding pull of the antagonistic flexor
digitorum profundus. Tightness of the extensor digitorum
may result from adhesions or loss of muscle extensibility in
the forearm or hand. Tightness may also follow from the loss

310

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Central

tendon

Figure 15.18: Effect of PIP flexion on the extensor
mechanism. Flexion of the PIP joint stretches the
central tendon, which in turn pulls the lateral
bands distally, putting the lateral bands on slack
at the level of the DIP joints.

of muscular balance resulting from weakness of the intrinsic
muscles.

Extensor Digiti Minimi (Also Known
As Extensor Digiti Quinti)

The extensor digiti minimi lies just medial to the extensor
digitorum in the forearm (Muscle Attachment Box 15.8). It is
distinguished from the extensor digitorum tendon in the
finger by palpation, lying on the ulnar side of the extensor
digitorum tendon crossing the MCP joint of the little finger.

ACTIONS

MUSCLE ACTION: EXTENSOR DIGITI MINIMI

Action

Evidence

Finger MCP extension

Supporting

Finger PIP extension

Supporting (only with
intrinsic activity)

Finger DIP extension

Supporting (only with
intrinsic activity)

Wrist extension

Inadequate

Wrist ulnar deviation

Inadequate

Elbow flexion

Conflicting

The actions of the extensor digiti minimi are almost identical to
those of the extensor digitorum at the fingers. However, if the
muscle affects the wrist, it is likely to result in ulnar deviation
because the tendon is ulnar to the capitate as it crosses the wrist.

EFFECTS OF WEAKNESS

Weakness or loss of the extensor digiti minimi results in an
inability to extend the little fingers MCP joint independently
Because the extensor digitorum to the little finger is fre¬
quently deficient and may even be absent, weakness of the
extensor digiti minimi is characterized by significant weak¬
ness of MCP extension of the little finger.

EFFECTS OF TIGHTNESS

Isolated tightness of the extensor digiti minimi is unlikely.
However, the effects of tightness of this muscle mirror the
effects of tightness of the extensor digitorum.

Extensor Carpi Ulnaris

The extensor carpi ulnaris is the last of the dedicated wrist
muscles (Muscle Attachment Box 15.9). It is a pennate muscle,
similar in size to the extensor carpi radialis brevis [1,7,40].

Central

Figure 15.19: Effect of DIP flexion on the extensor
mechanism. Flexion of the DIP joint stretches the
lateral bands, which pulls the central tendon distally,
putting it on slack as it crosses the PIP joint.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

311

Figure 15.20: Standard position to measure the passive flexion ROM
of the DIP joint includes flexion of the PIP joint of the same finger.

ACTIONS

MUSCLE ACTION: EXTENSOR CARPI ULNARIS

Action

Evidence

Wrist extension

Supporting

Wrist ulnar deviation

Supporting

Elbow extension

Inadequate

Elbow pronation

Inadequate

Figure 15.21: Standard position to measure the passive flexion
ROM of the PIP joint includes a relaxed position of the DIP joint,
which generally assumes a position of slight flexion.

mm.

MUSCLE ATTACHMENT BOX 15.8

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR DIGITI MINIMI

Proximal attachment: Lateral epicondyle of the
humerus by way of the common extensor tendon
and from the surrounding fascia.

Distal attachment: The extensor hood of the little
finger by two slips. The lateral portion merges with
the tendon of the extensor digitorum. Thus the
ulnar tendon at the MCP joint of the little finger
belongs solely to the extensor digiti minimi.

Innervation: Radial nerve, C7 and C8.

Palpation: The tendon of the extensor digiti minimi is
palpable on the ulnar side of the extensor digitorum
tendon to the little finger as it crosses the MCP joint.

The role of the extensor carpi ulnaris in wrist extension and
ulnar deviation is well accepted and supported by EMG
evidence [5,12,34,75]. However, some data suggest that the
extensor carpi ulnaris is able to extend the wrist only when
the forearm is supinated [8]. Analysis of the extension
moment arm of the extensor carpi ulnaris reveals that the
moment arm decreases as the forearm moves from supination
to pronation [43]. This lends support to the notion that the
extensor carpi ulnaris is most effective in extending the wrist
when the forearm is supinated. EMG analysis reveals no sig¬
nificant difference in the level of recruitment of the extensor
carpi ulnaris during wrist extension with different forearm
positions [72]. There is no known report that quantifies the

MUSCLE ATTACHMENT BOX 15.9

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR CARPI ULNARIS

Proximal attachment: Lateral epicondyle of the
humerus by way of the common extensor tendon
and from the surrounding fascia and from the
posterior border of the ulna along with the
flexor carpi ulnaris.

Distal attachment: Medial aspect of the metacarpal
of the little finger.

Innervation: Radial nerve, C7 and C8.

Palpation: The tendon of the extensor carpi ulnaris
is palpable on the dorsal and ulnar aspect of the
wrist joint during resisted wrist extension with ulnar
deviation.

312

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

extension moment applied to the wrist by the extensor carpi
ulnaris in different forearm positions. Additional research is
needed to determine if the extensor role of the extensor carpi
ulnaris is altered by forearm position.

The extensor carpi ulnaris has the largest moment arm
for ulnar deviation of the dedicated wrist muscles [8,12,43].
The moment arm changes very little with wrist position,
making the muscle particularly effective for ulnar devia¬
tion. The extensor carpi ulnaris lies just ulnar to the head of
the ulna and fibrocartilaginous disc of the triangular fibro-
cartilage complex (TFCC). It plays an important role in
supporting the distal radioulnar joint [19,25,60,74]. In
pronation, the muscle is stretched and helps prevent dorsal
dislocation; in supination, the muscle may help anterior
glide of the ulna.

The extensor carpi ulnaris crosses the elbow and may
affect that joint. Measurement of the muscle s moment arms
at the elbow reveals substantial extension and pronation
moment arms, suggesting that the muscle is at least mechan¬
ically capable of elbow extension and forearm pronation
[1,50]. Yet EMG data demonstrate no activity of the extensor
carpi ulnaris in forearm pronation [5]. Like the other fore¬
arm muscles crossing the elbow, additional investigation is
needed to determine if the extensor carpi ulnaris does func¬
tion at the elbow.

EFFECTS OF WEAKNESS

Weakness of the extensor carpi ulnaris results in weakness in
wrist extension and ulnar deviation. As noted with the other
wrist muscles, the extensor carpi ulnaris participates in the
delicate balance exhibited in a healthy wrist. Disruption of
this balance resulting from impairments of any of these mus¬
cles is likely to produce significant dysfunction. Weakness in
wrist extension is particularly disruptive to the ability to pro¬
duce a strong grip and pinch.

EFFECTS OF TIGHTNESS

Isolated tightness, although uncommon, decreases the avail¬
able ROM of the wrist in flexion and radial deviation.

COMBINED ACTIONS OF THE FIVE
PRIMARY WRIST MUSCLES

From the discussion of each of the dedicated wrist muscles, it
is apparent that there is no single muscle that moves the wrist
in any of the cardinal planes of motion. Therefore, to produce
pure motions of flexion and extension or radial and ulnar
deviation, pairs of muscles must contract together (Fig. 15.22).
For example, the flexor carpi radialis and the flexor carpi
ulnaris are both necessary for pure wrist flexion.

During functional activities the wrist commonly moves on
a diagonal path from wrist extension with radial deviation to
wrist flexion with ulnar deviation [43,56]. It is not surprising

Figure 15.22: Pairs of dedicated wrist muscles that move the
wrist in the cardinal planes. A cross-sectional view of the wrist
joint demonstrates that pairs of dedicated wrist muscles are
needed to produce wrist motion in the cardinal planes of
flexion-extension or radial-ulnar deviation. FCR, flexor carpi
radialis; FCU, flexor carpi ulnaris; PL, palmaris longus; ECRL,
extensor carpi radialis longus; ECRB, extensor carpi radialis
brevis; ECU, extensor carpi ulnaris.

to find that the flexor carpi ulnaris has a larger physiological
cross-sectional area than the flexor carpi radialis [1]. Similarly,
the combined physiological cross-sectional area of the exten¬
sor carpi radialis longus and extensor carpi radialis brevis is
larger than that of the extensor carpi ulnaris. These wrist mus¬
cles appear specialized to support and move the wrist and
hand in this diagonal pattern.

Clinical Relevance

IMPAIRMENT OF A SINGLE DEDICATED WRIST
MUSCLE: Impairment of a single wrist muscle results in an
impairment of the unique motion provided by that muscle
and interferes with the motions in the cardinal planes in
which that muscle participates. For example, the extensor
carpi ulnaris contracts and produces the combined move¬
ment of wrist extension and ulnar deviation. Weakness of
that muscle results in weakness of that combined movement
as well as in pure wrist extension and pure wrist ulnar devi¬
ation, since the extensor carpi ulnaris participates in both of
those motions. Consequently, when a clinician identifies a
patient's difficulty or inability to perform wrist motion in a
cardinal plane, further evaluation is needed to determine the
distribution of weakness across the contributing muscles.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

313

DEEP MUSCLES ON THE VOLAR SURFACE
OF THE FOREARM

The deep muscles of the volar surface of the forearm include
the flexor digitorum profundus, the flexor pollicis longus, and
the pronator quadratus (Fig. 15.23).

Flexor Digitorum Profundus

The flexor digitorum profundus is a large muscle com¬
posed of multiple bundles of unipennate muscles [8], with
a large physiological cross-sectional area and a large poten¬
tial force (Muscle Attachment Box 15.10). Based on its
physiological cross-sectional area the flexor digitorum pro¬
fundus is up to 50% stronger than the flexor digitorum
superficialis, and both are stronger than the extensor digi¬
torum [7,8,40].

Figure 15.23: Deep muscles of the volar surface of the forearm
include the flexor digitorum profundus, flexor pollicis longus,
and pronator quadratus.

ACTIONS

MUSCLE ACTION: FLEXOR DIGITORUM PROFUNDUS

Action

Evidence

Finger DIP flexion

Supporting

Finger PIP flexion

Supporting

Finger MCP flexion

Supporting

Wrist flexion

Conflicting

The flexor digitorum profundus is the only muscle that can
flex the DIP of the fingers. The standard procedure to test
the strength and integrity of the muscle is active DIP flexion
[28,34]. However, the tendons of the fingers are mechanically
linked to one another at the muscle belly and perhaps even
in the carpal tunnel, making discrete activation of the flexor
digitorum profundus at a single finger difficult if not impossi¬
ble, except at the index finger, which appears to have more
independent control [13,65].

MUSCLE ATTACHMENT BOX 15.10

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITORUM PROFUNDUS

Proximal attachment: The proximal two thirds or
three quarters of the anterior and medial surfaces
of the ulna and the medial half of the interosseous
membrane, the medial surface of the coronoid
process, and from the posterior border of the ulna
by way of the aponeurosis of the flexor carpi ulnaris.

Distal attachment: A tendon to each finger inserts
on the palmar surface of the base of distal phalanx.

Innervation: The tendons to the index and long
fingers are supplied by the anterior interosseous
branch of the median nerve, C8, T1, perhaps C7.
The tendons to the little and ring fingers are
supplied by the ulnar nerve, C8 and T1.

Palpation: The tendons of the flexor digitorum
profundus may be palpable along the volar surface
of the middle phalanx of each finger.

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Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Clinical Relevance

MMT OF THE FLEXOR DIGITORUM PROFUNDUS: The

classical MMT procedure for the flexor digitorum profundus
is assessment of the strength of flexion of the DIP
[28,34,53]. In cases of nerve or tendon injury or in degener¬
ative neuropathies or myopathies, it may be clinically impor¬
tant to determine the strength of the flexor digitorum pro¬
fundus at each finger. Clinicians must recognize that
although the test may be focused on a single finger, the
structure of the whole muscle dictates that the subject be
allowed to flex all the fingers at once. The mechanical link¬
age among the muscles to the fingers prevents an individ¬
ual from isolating DIP flexion of a single finger (Fig. 15.24).
Attempts to prevent flexion at the other fingers are likely to
inhibit recruitment of the muscle.

Like the flexor digitorum superficialis, the flexor digito-
rum profundus has the potential to affect each joint it crosses.
Both muscles are capable of producing flexion at the PIP
and MCP joints of the fingers and at the wrist. However,
EMG studies suggest that the flexor digitorum profundus is
the primary flexor of the finger, activated during free, unre¬
sisted finger closure [5,13,37]. The flexor digitorum superfi¬
cialis appears to be held in reserve for additional strength,
particularly during forceful pinch and grasp [8,41]. Because
the flexor digitorum superficialis has more independent
activation of individual fingers, it also is used when individ¬
ual finger movements are needed [13,37]. Both the flexor
digitorum profundus and the flexor digitorum superficialis
are strongest at the middle finger [8,15,27,70].

The flexor digitorum profundus crosses the wrist and has
the potential to affect it. The tendons of the flexor digitorum

Figure 15.24: The standard MMT procedure to assess the strength
of the flexor digitorum profundus allows flexion of all of the fin¬
gers, even when a single finger is being assessed.

Figure 15.25: Substitution by the flexor digitorum profundus
during MMT of the wrist flexors is common and can lead to an
overestimate of wrist flexion strength.

profundus are the deepest in the carpal tunnel and have small
moment arms for flexion of the wrist. Analysis suggests that
the moment arms increase with wrist flexion as the tendons
bulge anteriorly in the tunnel [8]. EMG studies are conflict¬
ing regarding the activity of the flexor digitorum profundus
during wrist flexion [5,17]. Observations during MMT of
wrist strength reveal a frequent tendency for subjects to flex
the fingers in what appears to be an attempt to recruit the
flexor digitorum profundus as a wrist flexor (Fig. 15.25).
Clinicians are warned again to watch for the use of the finger
flexor muscles to enhance wrist flexion strength.

EFFECTS OF WEAKNESS

Effects of weakness of the flexor digitorum profundus are
manifested directly as a decrease in the strength of DIP flexion,
but the overall strength of finger flexion and consequently the
strength of pinch and grip also are affected.

EFFECTS OF TIGHTNESS

Tightness of the flexor digitorum profundus leads to
decreased extension excursion of the fingers. In extreme
cases, usually in the presence of muscle spasticity, the fin¬
gers may close into the palm. However, tightness is also the
consequence of balance lost across all of the muscles in the
hand, resulting from weakness of the intrinsic muscles. In

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

315

Figure 15.26: Clawhand deformity in an individual with significant
weakness of the intrinsic muscles of the hand with concomitant
tightness of the flexor digitorum profundus and the extensor
digitorum.

this situation the finger flexors and extensors become tight
together, causing the hand to collapse into a claw deformity
(Fig. 15.26). The factors causing a claw deformity are
detailed in Chapter 18.

Flexor Pollicis Longus

The flexor pollicis longus along with the flexor digitorum pro¬
fundus lies on the floor of the carpal tunnel (Muscle
Attachment Box 15.11). It is a bipennate muscle crossing the
IP, MCP, and carpometacarpal (CMC) joints of the thumb
and the wrist joint.

ACTIONS

MUSCLE ACTION: FLEXOR POLLICIS LONGUS

Action

Evidence

Thumb IP flexion

Supporting

Thumb MCP flexion

Conflicting

Thumb CMC flexion

Conflicting

Thumb CMC adduction

Supporting

Wrist flexion

Inadequate

Wrist radial deviation

Inadequate

The flexor pollicis longus is the only muscle able to flex the IP
joint of the thumb. Unlike the long flexors to the fingers, the
flexor pollicis longus is independent, and isolated IP flexion in
the thumb is easily accomplished. The flexor pollicis longus
has a smaller moment arm for flexion at the MCP and CMC
joints of the thumb than the intrinsic muscles of the thumb.
EMG assessment reveals vigorous activation of the flexor
pollicis longus with flexion of the IP but zero activation with
isolated MCP flexion in the thumb [13]. In fact, activation of
the flexor pollicis longus in the absence of other muscle activity
at the thumb causes flexion of the IP but hyperextension of
the MCP joint of the thumb [8,64]. As is seen throughout the
wrist and hand, stable movement of the thumb requires the
balanced activation of several muscles.

MUSCLE ATTACHMENT BOX 15.11

ATTACHMENTS AND INNERVATION
OF THE FLEXOR POLLICIS LONGUS

Proximal attachment: The anterior surface of the
radius and adjacent interosseous membrane from
the radial tuberosity to the attachment of the
pronator quadratus. It may also attach to the
medial aspect of the coronoid process.

Distal attachment: Palmar surface of the base of the
thumb's distal phalanx.

Innervation: Anterior interosseous branch of the
median nerve, C7, C8, and perhaps T1.

Palpation: The tendon of the flexor pollicis longus
may be palpable on the volar surface of the proxi¬
mal phalanx of the thumb.

Few studies examine the flexor pollicis longus muscles
contribution to CMC adduction. Biomechanical analyses
reveal very small moment arms generating either abduction
(palmar abduction) [63] or adduction moments [47]. EMG
analyses reveal activity of the flexor pollicis longus during
adduction, with little or no activity during abduction [14,33].
Like the flexor digitorum profundus, the flexor pollicis longus
has only a small flexion moment arm at the wrist [8]. It may
also produce a slight radial deviation moment [51]. As with
other extrinsic muscles to the digits, the contribution of the
flexor pollicis longus to these motions may only be apparent
with the loss of the primary muscles for these actions.

EFFECTS OF WEAKNESS

Weakness of the flexor pollicis longus results in weakness in
flexion at the thumb s IP joint. Isolated weakness of the flexor
pollicis longus is unusual but can result from impingement of
the anterior interosseous nerve.

Clinical Relevance

A CASE REPORT: A 45-year-old male reported a sudden
onset of difficulty in buttoning his shirt The difficulty was
noted one evening after a day filled by a home-repair job
involving prolonged use of a screwdriver in an overhead
position. The subject denied pain , noting that his only com¬
plaint was difficulty with tasks requiring fine motor manipu¬
lation such as buttoning shirts and tying a bow tie. Physical
examination revealed full passive ROM throughout the
thumb , fingers , and wrist. No active flexion was visible at the
IP joint of the thumb or at the DIP joint of the index finger.
All other strengths were within normal limits. An evaluation

( continued )

316

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

(Continued)

by a neurologist confirmed impingement of the anterior
interosseous nerve\ likely precipitated by prolonged repetitive
forearm pronation while using the screwdriver. The nerve
emerges between the two heads of the pronator teres and was
likely compressed against the interosseous membrane during
prolonged and repeated contraction of the pronator teres.

One remarkable aspect of this case was how few func¬
tional deficits resulted from apparently complete loss of the
flexor pollicis long us and the flexor digitorum profundus to
the index finger. (Electrodiagnostic tests were not per¬
formed to quantify the degree of denervation.) Pinch was
characterized by hyperextension of the thumb's IP joint
and the index finger's DIP joint. The subject reported only
minor inconvenience , particularly with buttoning clothes.
The muscles gradually recovered nearly normal strength
over a l-year period.

EFFECTS OF TIGHTNESS

Like extrinsic muscles to the fingers, the flexor pollicis longus
is rarely tight in isolation. Tightness of the flexor pollicis longus
can be seen in cases of upper motor neuron lesions leading to
spasticity of the flexor pollicis longus and other muscles of the
hand. In cases of severe spasticity, the thumb is pulled into the
palm by the combined pull of the flexor pollicis longus, adduc¬
tor pollicis, and extensor pollicis longus. This thumb-in-palm
deformity impairs or even prevents pinch and grasp. In severe
cases, the thumb s location in the palm interferes with normal
hand hygiene and can lead to skin breakdown [52]. Surgical
correction of the deformity may be required to improve func¬
tion or to facilitate skin care.

The flexor pollicis longus is frequently tight with the exten¬
sor pollicis longus in the absence of the intrinsic muscles of
the thumb. This loss of balance results in the typical ape
thumb deformity (Fig. 15.27). The deformity is discussed in
more detail in Chapter 18.

Pronator Quadratus

The pronator quadratus is the second of the primary muscles
of pronation (Muscle Attachment Box 15.12). The pronator
teres is discussed in Chapter 12 with the elbow flexor muscles.

ACTIONS

MUSCLE ACTION: PRONATOR QUADRATUS

Action

Evidence

Elbow pronation

Supporting

The reported action of the pronator quadratus is pronation of
the forearm. As noted in Chapter 12, EMG studies reveal that
the pronator quadratus is the primary pronator, participating

Figure 15.27: Ape thumb deformity. An ape thumb deformity
is seen in an individual with significant weakness of the intrinsic
muscles of the thumb with concomitant tightness of the flexor
pollicis longus and the extensor pollicis longus.

in active forearm pronation regardless of condition [5]. The
moment arm of the pronator quadratus is at least as large as
that of the pronator teres (6-8 mm) and relatively constant
through much of the pronation and supination ROM,
decreasing slightly at the extreme of pronation and decreas¬
ing more at the extreme of supination [10]. The pronator
teres appears to have the supportive role in active pronation,
contracting only against resistance and during rapid movements.

MUSCLE ATTACHMENT BOX 15.12

ATTACHMENTS AND INNERVATION
OF THE PRONATOR QUADRATUS

Proximal attachment: Distal one fourth of the anter¬
ior surface of the ulna.

Distal attachment: Distal one fourth of the anterior
surface of the radius.

Innervation: Anterior interosseous branch of the
median nerve, C7, C8, and perhaps T1.

Palpation: The pronator quadratus may be palpated
during unresisted pronation by placing a finger on
the volar surface of the distal radius or ulna. The
palpating finger must slide deep to the overlying
tendons.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

317

Studies also demonstrate that the pronator quadratus plays
an essential role in supporting the distal radioulnar joint
[35,48,60,61].

EFFECTS OF WEAKNESS

Weakness of the pronator quadratus weakens pronation
strength. However, if the pronator teres remains intact, it pro¬
vides substantial pronation force. Weakness of the pronator
quadratus may lead to difficulty in pronation with elbow
extension, since the pronator teres flexes the elbow as it
pronates. Weakness of the pronator quadratus may also com¬
promise stability of the distal radioulnar joint.

EFFECTS OF TIGHTNESS

Tightness of the pronator quadratus alone is unlikely.
However, it is tight in combination with other structures such
as the interosseous membrane and ligaments of the distal
radioulnar joint, leading to restricted supination ROM.

DEEP MUSCLES ON THE DORSAL
SURFACE OF THE FOREARM

The deep muscles of the dorsal surface of the forearm include
the supinator, the three “snuff box” muscles (the abductor
pollicis longus, the extensor pollicis brevis, and the extensor
pollicis longus), and the extensor indicis (Fig. 15.28).

Supinator

The supinator muscle is presented in Chapter 12 with the
other major supinator of the forearm, the biceps brachii.
EMG studies reveal that the supinator is an important supina¬
tor of the elbow, particularly when the elbow is extended,
effectively inhibiting the biceps brachii. Weakness of the
supinator results in significant loss of supination strength
when the elbow is extended. The case report presented in
Chapter 12 reveals how supinator weakness can be missed if
supination strength is evaluated only in the standard MMT
position, which is with the elbow flexed [28,53]. In this posi¬
tion the biceps brachii is a powerful supinator and may mask
any weakness of the supinator muscle.

Tightness of the supinator alone is improbable but in com¬
bination with the biceps brachii limits pronation ROM.
Chapter 12 discusses how assessment of pronation ROM with
the elbow flexed and extended helps distinguish the contri¬
butions of the biceps brachii and the supinator to limited
pronation ROM.

Abductor Pollicis Longus

The abductor pollicis longus forms the anterior border
of the anatomical snuffbox, composed of the abductor

Figure 15.28: Deep muscles of the dorsal surface of the forearm
include the supinator, abductor pollicis longus, extensor pollicis
brevis, extensor pollicis longus, and extensor indicis.

pollicis longus, the extensor pollicis brevis, and the extensor
pollicis longus (Muscle Attachment Box 15.13). Investi¬
gators report up to seven separate tendons of the abductor
pollicis longus as it passes through the first dorsal compart¬
ment of the wrist with the extensor pollicis brevis [18,59].
It is a powerful muscle that affects both the thumb and the
wrist [8].

318

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

MUSCLE ATTACHMENT BOX 15.13

ATTACHMENTS AND INNERVATION
OF THE ABDUCTOR POLLICIS LONGUS

Proximal attachment: Posterior surface of the ulna
and interosseous membrane distal to the attach¬
ment of the anconeus and the middle one third of
the posterior radius distal to the supinator muscle.

Distal attachment: Trapezium and radial surface of
the base of the thumb's metacarpal bone.

Innervation: Radial nerve, C7 and C8.

Palpation: The abductor pollicis longus tendon is
palpated forming the anterior border of the
anatomical snuff box of the thumb as it inserts into
the base of the thumb's metacarpal bone.

ACTIONS

MUSCLE ACTION: ABDUCTOR POLLICIS LONGUS

Action

Evidence

Thumb CMC abduction

Conflicting

Thumb CMC extension

Supporting

Wrist radial deviation

Supporting

Wrist flexion

Supporting

Wrist extension

Supporting

Forearm pronation

Inadequate

Forearm supination

Inadequate

Essential to the understanding of the actions of the abductor
pollicis longus is a clear image of the motions of the CMC joint
of the thumb. These motions are presented in Chapter 14 and
are reviewed in Fig. 15.29. Abduction (palmar abduction) of
the CMC joint occurs in a plane perpendicular to the plane of
the palm. Extension (radial abduction) occurs in a plane par¬
allel to the plane of the palm. Despite the muscles name, the
abductor pollicis longus appears to play only a supportive role
in abduction at the CMC joint. Most investigators report
some participation of the muscle in abduction of the thumb
[5,13,14,33,34], although one report states that the abductor
pollicis longus does not participate in abduction at all [8].
Several studies also report that it contributes to extension of
the CMC [8,13,14,30,33,34].

Careful analysis of the distal attachments of the abductor
pollicis longus and its moment arms helps to explain the role
of the abductor pollicis longus at the CMC joint of the thumb.
The attachment is both extensive and variable, and this vari¬
ability may be the source of the variety of interpretations of
the muscles actions [9,63,71]. The primary attachment of the
abductor pollicis longus is on the dorsal surface of the base of
the thumbs metacarpal (Fig. 15.30). Because the thumb lies
slightly volar and medially rotated with respect to the rest of

Figure 15.29: Motions available at the CMC joint of the thumb
include flexion and extension in a plane parallel to the plane of
the palm, abduction and adduction in a plane perpendicular to
the plane of the palm, and opposition that combines flexion,
abduction, and medial rotation.

Figure 15.30: Attachment of the abductor pollicis longus on the
dorsal aspect of the thumb's metacarpal. Because of the position
of the thumb with respect to the hand, attachment of the
abductor pollicis longus on the dorsal aspect of the thumb's
metacarpal produces extension of the CMC joint.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

319

Figure 15.31: Moment arms of the abductor
pollicis longus and brevis. The attachment of
the abductor pollicis longus on the trapezium
may create a slight abduction moment.
However, the muscle's moment arm is shorter
than the abduction moment arm of the abduc¬
tor pollicis brevis.

the hand (see Fig. 14.38), attachment of the abductor pollicis
longus on the dorsum of the metacarpal is consistent with the
action of extension of the CMC. An attachment of the abduc¬
tor pollicis longus tendon on the lateral aspect of the palmar
surface of the trapezium also is described [44,71]. This attach¬
ment is consistent with an action of abduction of the CMC
joint of the thumb, although the abduction moment arm is
much smaller than that of the abductor pollicis brevis [47,63]
(Fig. 15.31). In fact, the abductor pollicis longus has a larger
moment arm for extension than for abduction.

EMG studies demonstrate activity of both the abductor
pollicis longus and the intrinsic abductor during active
abduction, but these studies demonstrate more activity of the
abductor pollicis longus during extension of the CMC joint
of the thumb than during abduction [5,9,14,33,72]. Studies
of individuals following nerve blocks of the median nerve,
which innervates the abductor pollicis brevis, also help clarify
the abductor pollicis longus muscle s contribution to abduc¬
tion of the thumb [6,33]. These subjects demonstrate severe
loss of abduction strength at the CMC joint (from 75 to
100%). The variations in response to the nerve blocks are
consistent with the anatomical variability already described.
Thus it appears that while the abductor pollicis longus par¬
ticipates in abduction of the thumb, the intrinsic abductor is
much better at it. The abductor pollicis longus is more
important as an extensor of the thumb s CMC joint.

The abductor pollicis longus appears well suited for radial
deviation of the wrist. It has one of the largest moment arms
for radial deviation of any muscle crossing the wrist [6,8,9].
It also is reported to flex the wrist [6,8,34,55] and to be
active during wrist extension [9,72]. Assessment of the mus¬
cles moment arm for wrist flexion and extension suggests
that it crosses the wrist through the axis of flexion and exten¬
sion and thus has neither a flexion nor a extension moment
[51]. However, movement of the thumb in the direction of
abduction allows the tendon of the abductor pollicis longus
to glide anteriorly. In this position it is likely that the muscle

can flex the wrist [9] (Fig. 15.32). This is a reported substi¬
tution pattern in patients with wrist flexion weakness.
Similarly, adduction of the thumb may move the tendon
posteriorly so that the muscle is posterior to the wrist s axis

Figure 15.32: Function of the abductor pollicis longus at the
wrist. A. When the thumb is abducted at the CMC joint, the
abductor pollicis may contribute to wrist flexion. B. When the
thumb is adducted at the CMC joint, the abductor pollicis may
contribute to wrist extension.

320

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

of flexion and extension and is capable of wrist extension.
The large and variable attachment of the muscle at the wrist
and thumb may also explain some of the differences in
actions attributed to the abductor pollicis longus at the
wrist. Clinicians must note that these anatomical variations
suggest that patients may demonstrate varied responses to
contractions of the abductor pollicis longus. Mechanical and
cadaver models of the muscle s moment arm suggest that
the abductor pollicis longus can contribute to supination
when the forearm is supinated, and to pronation with the
forearm in the mid-range [10]. Clinicians need to remain
alert for substitutions by the abductor pollicis longus during
a variety of wrist movements.

EFFECTS OF WEAKNESS

Weakness of the abductor pollicis longus is manifested pri¬
marily in weakness in CMC extension. Extension at the CMC
joint of the thumb is an essential component of normal pinch.
Therefore, weakness of the abductor pollicis longus compro¬
mises an individuals ability to perform a powerful pinch.
Details of the mechanics and pathomechanics of pinch are
presented in Chapter 19.

EFFECTS OF TIGHTNESS

There are no known reports of isolated tightness of the
abductor pollicis longus. It is likely to be tight in the presence
of tightness of the other extrinsic muscles of the thumb. Its
tightness may contribute to decreased flexion ROM at the
thumb s CMC joint.

Extensor Pollicis Brevis

MUSCLE ATTACHMENT BOX 15.14

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR POLLICIS BREVIS

Proximal attachment: Posterior surface of the radius
and adjacent interosseous membrane distal to the
abductor pollicis longus.

Distal attachment: Dorsal surface of the base of the
thumb's proximal phalanx.

Innervation: Radial nerve, C7 and C8.

Palpation: The tendon of the extensor pollicis
brevis is palpable along with the abductor pollicis
longus as the radial side of the anatomical snuff
box of the thumb. It lies just dorsal to the tendon
of the abductor pollicis longus.

The extensor pollicis brevis and abductor pollicis longus lie
in the same tendon sheath and thus have almost identical
actions at the wrist and CMC joint of the thumb [8,49,55].
The extensor pollicis brevis lies slightly dorsal and medial
to the abductor pollicis longus. It is aligned to extend the
CMC joint of the thumb but has only a slight contribution
to abduction. It has a similar moment arm for radial deviation
of the wrist compared to the abductor pollicis longus has but
less ability to flex the wrist [34,51] (Fig. 15.33). As with the
abductor pollicis longus, movement of the thumb may alter
its effect on wrist flexion and extension.

The extensor pollicis brevis travels together with the abductor
pollicis longus in the 1st dorsal compartment at the wrist and
consequently has almost identical actions except at its distal
attachment at the MCP joint of the thumb (Muscle
Attachment Box 15.14). The extensor pollicis brevis is consid¬
erably smaller than the abductor pollicis longus [8].

ACTIONS

MUSCLE ACTION: EXTENSOR POLLICIS BREVIS

Action

Evidence

Thumb MCP extension

Supporting

Thumb CMC abduction

Supporting

Thumb CMC extension

Supporting

Wrist radial deviation

Supporting

Wrist flexion

Supporting

Wrist extension

Supporting

There is little dispute about the ability of the extensor pollicis
brevis, lying on the dorsum of the MCP joint, to extend the
MCP joint of the thumb [8,34,55,75]. The extensor pollicis
longus also can extend the MCP joint, and it is difficult to
recruit the extensor pollicis brevis in isolation.

EFFECTS OF WEAKNESS

Weakness of the extensor pollicis brevis weakens MCP and
CMC extension of the thumb. However, if the abductor pol¬
licis longus and the extensor pollicis longus remain intact, the
functional consequences are small.

EFFECTS OF TIGHTNESS

Tightness of the extensor pollicis brevis alone is unlikely. It
may contribute to limited CMC motion along with the larger
abductor pollicis longus. It is too small to affect MCP joint
motion by itself.

Clinical Relevance

DE QUERVAIN S DISEASE: De Quervain's disease is a

thickening and narrowing of the connective tissue com¬
partment containing the extensor pollicis brevis and
abductor pollicis longus. It is a disorder commonly found
in people who use repetitive thumb flexion and extension ,
for example , computer keyboard operators [47]. A classic

(< continued )

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

321

Figure 15.33: The extensor pollicis brevis lies slightly ulnar and
dorsal to the abductor pollicis longus and thus has a slightly improved
moment arm for extension of the wrist joint.

(Continued)

test for the disorder involves stretching these muscles to
reproduce the patient's complaints of pain. The test ,
known as Finkelstein's test, places the joints of the
thumb in flexion and the wrist in ulnar deviation [58,76].
The test appears to stretch the extensor pollicis brevis
more than the abductor pollicis longus , which is consis¬
tent with the more distal attachment of the extensor polli¬
cis brevis [36].

Extensor Pollicis Longus

Another pennate muscle of the forearm, the extensor pollicis
longus, takes a circuitous route through the forearm to the
thumb (Muscle Attachment Box 15.15). It loops around the
dorsal tubercle of the radius, which serves as a pulley to redi¬
rect the tendon toward the thumb. The muscle s effect on the
thumb is influenced directly by the tendons angle of pull that
results from its route around the dorsal tubercle.

MUSCLE ATTACHMENT BOX 15.15

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR POLLICIS LONGUS

Proximal attachment: Lateral aspect of the middle
one third of the ulna on its posterior surface and
the adjacent interosseous membrane.

Distal attachment: Dorsal surface of the base of the
thumb's distal phalanx.

Innervation: Radial nerve, C7 and C8.

Palpation: The extensor pollicis longus is palpated as
the ulnar border of the anatomical snuff box.

ACTIONS

MUSCLE ACTION: EXTENSOR POLLICIS LONGUS

Action Evidence

Thumb IP extension Supporting

Thumb MCP extension Supporting

Thumb CMC extension Conflicting

Thumb CMC adduction Supporting

Thumb retropulsion Supporting

Wrist radial deviation Supporting

Wrist extension Supporting

The extensor pollicis longus is the primary extensor of the IP
joint of the thumb. It is the only muscle capable of extending
the IP joint through its full range of motion. However, it is
important to recognize that other muscles (i.e., the intrinsic
muscles to the thumb) also contribute to IP extension [32,66].
The extensor pollicis longus also contributes to extension of
the MCP joint along with the extensor pollicis brevis.

The role of the extensor pollicis longus at the CMC joint is
more controversial. The muscle is commonly described as an
extensor of this joint [8,34,55,75]. However, the extensor pol¬
licis longus crosses on the ulnar side of the CMC as it winds
around the dorsal tubercle of the radius (Fig. 15.34).
Recognition that extension of the CMC occurs in the plane of
the palm and adduction occurs in a plane perpendicular to the
plane of the palm reveals that the extensor pollicis longus is
better aligned to adduct the CMC joint than to extend it [14].
As the thumb adducts, the extensor pollicis longus becomes an
even better adductor and less an extensor. It is the only mus¬
cle capable of adducting the thumb past the palm of the hand.
This action is known as retropulsion and can serve as a test
of the extensor pollicis longus (Fig. 15.35). As the thumb
abducts the muscles moment arm for extension improves [8].
The extensor pollicis longus crosses the wrist dorsally and
slightly to the radial side of the capitate and has moment arms
for both wrist extension and radial deviation [8,34,51,75].

322

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 15.34: The extensor pollicis longus crosses the thumb's
CMC dorsally, thus producing a large adduction moment at the
CMC joint.

Figure 15.35: Thumb retropulsion. The extensor pollicis longus
is the only muscle that is able to pull the thumb into adduction
past the palm, or retropulsion.

without hand pathology revealed that most subjects were able to
extend the IP joint through at least half its available excursion
with the intact intrinsic muscles [66]. This study suggests that
the extensor pollicis longus is important for full active extension
of the joint, but it is not the only extensor of the IP joint.

EFFECTS OF TIGHTNESS

Tightness of the extensor pollicis longus is seen most com¬
monly in conjunction with tightness of the other extrinsic
muscles, particularly the flexor pollicis longus. This combined
tightness is often the result of weakness in the intrinsic mus¬
cles of the thumb, altering the muscle balance necessary for
normal function in the wrist and hand.

like the abductor pollicis longus, the extensor pollicis longus
may be capable of supination when the forearm is supinated
and of pronation when the forearm is in the mid-position [10].

Clinical Relevance

USE OF THE SNUFF BOX MUSCLES IN WRIST
MOTIONS: Careful observations are required for a clini¬
cian testing wrist strength to notice a subject's use of any of
the extrinsic muscles of the thumb for additional strength
during wrist motions. However ; the capacity of these muscles
to contribute to wrist strength may greatly enhance the func¬
tional abilities of a patient with wrist weakness (Fig. 15.36).

EFFECTS OF WEAKNESS

The primary effect of weakness of the extensor pollicis longus is
weakness of extension at the IP joint of the thumb. However,
some IP extension is preserved if the intrinsic muscles of the
thumb remain intact. A study in which a nerve block produced
temporary paralysis of the extensor pollicis longus in individuals

Extensor Indicis (Also Known
As the Extensor Indicis Proprius)

The extensor indicis is a small muscle that lies deep in the
dorsum of the forearm (Muscle Attachment Box 15.16).

Figure 15.36: Thumb substitutions. The extensor pollicis longus
and other snuff box muscles can assist in wrist extension and may
substitute for weak dedicated wrist extensor muscles.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

323

MUSCLE ATTACHMENT BOX 15.16

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR INDICIS

Proximal attachment: The posterior surface of the
ulna and adjacent interosseous membrane just dis¬
tal to the extensor pollicis longus.

Distal attachment: Extensor hood of the index finger.
The tendon lies on the ulnar side of the extensor dig-
itorum tendon.

Innervation: Radial nerve, C7 and C8.

Palpation: The tendon of the extensor indicis is pal¬
pable on the ulnar side of the extensor digitorum
tendon as the two tendons cross the MCP joint of
the index finger.

ACTIONS

MUSCLE ACTION: EXTENSOR INDICIS

Action

Evidence

Index finger DIP extension

Supporting

Index finger PIP extension

Supporting

Index finger MCP extension

Supporting

Index finger MCP ulnar deviation

Inadequate

Wrist extension

Inadequate

The actions of the extensor indicis are virtually identical to
those of the extensor digitorum at the index finger.

The role of the extensor indicis at the index finger, like that
of the extensor digitorum, is extension of the MCP joint.
Extension of all of the joints of the index finger requires the
simultaneous contraction of both the extrinsic extensors and
the intrinsic muscles to the index finger. The primary functional
role of the extensor indicis is to allow independent extension
of the index finger. This is essential since the primary extrinsic
extensor of the fingers, the extensor digitorum, has intercon¬
nections (juncturae tendinae) among the fingers, making inde¬
pendent movement difficult. EMG data suggest that the
extensor indicis is more active than the extensor digitorum
during unresisted MCP extension in individuals without hand
pathology [13]. The tendon of the extensor indicis lies on the
ulnar side of the tendon of the extensor digitorum, creating a
slight moment arm for ulnar deviation of the index finger.

The extensor indicis is positioned at the center of the wrist
on the dorsum and theoretically is capable of contributing to
wrist extension. However, it is a small muscle and unable to
add much additional force.

EFFECTS OF WEAKNESS

Although weakness of the extensor indicis may produce some
weakness in extension of the MCP joint, the primary limitation

in the presence of extensor indicis weakness is difficulty in
independent movement of the index finger. The functional
impairment can be significant in individuals such as computer
operators and musicians.

EFFECTS OF TIGHTNESS

Tightness of the extensor indicis alone is unlikely but may
accompany tightness of the extensor digitorum. Together
they may contribute to hyperextension of the MCP joints of
the fingers.

SYNERGISTIC FUNCTION OF THE
FOREARM MUSCLES TO THE WRIST
AND HAND

Active Coordination of the Dedicated
Wrist Muscles and the Finger Muscles

A muscle affects every joint that it crosses. Thus the finger
flexor muscles tend to flex the wrist, the MCP joint, and the
PIP and DIP joints, while the extensor muscles of the fingers
tend to extend these joint. Attempts to make a tight fist with
the wrist maximally flexed fail and usually cause discomfort
on the volar and/or dorsal surface of the forearm (Fig. 15.37).
Conversely, it is difficult to extend the fingers completely

Passive
insufficiency
of extensors

Figure 15.37: Active and passive insufficiency. A. Full closure of
the fingers with the wrist fully flexed is prevented by active
insufficiency of the finger flexors and passive insufficiency of the
finger extensors. B. Full opening of the fingers with the wrist
fully extended is prevented by active insufficiency of the finger
extensors and passive insufficiency of the finger flexors.

324

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

when the wrist is maximally extended. There are two impor¬
tant factors contributing to the difficulty of these tasks. First
in both situations, the agonists of the movement are required
to contract while in their shortest position. This results in
active insufficiency of the finger flexor or extensor muscles.
In Chapter 4, active insufficiency is defined as the inability
of a muscle to shorten enough to pull the limb through its
complete available ROM. Every muscle has a maximum
shortening capacity that is defined by the length of its fibers.
If the finger flexors are allowed to flex each joint they cross,
they reach their maximally shortened length before pulling
the joints through their full excursions.

At the same time that the agonists are actively insufficient,
the antagonists are being stretched and may produce passive
insufficiency. Passive insufficiency is defined as the inability
to move through the entire available range because of passive
restrictions from opposing soft tissue. As the wrist and fingers
are flexed, the antagonist finger extensors are stretched and
may limit full closure of the fingers. The involved structures
are merely reversed in the example of opening the fingers
with the wrist extended. The finger extensor tendons exhibit
active insufficiency, and the finger flexors may manifest pas¬
sive insufficiency.

The dedicated wrist muscles are essential in preventing
the active and passive insufficiencies that can occur with con¬
traction of the extrinsic muscles of the fingers. Observations
of an individual opening and closing a fist reveal the well-pro¬
grammed pattern of wrist and finger synergy (Fig. 15.38). As
the fingers actively close in a tight fist, the wrist automatically
extends. Similarly the wrist flexes as the fingers actively
extend. The dedicated wrist extensor muscles contract with
the finger flexor muscles to counteract the flexion moment at
the wrist exerted by the finger flexors. Wrist extension occurs

jt . q \. during finger flexion, thereby maintaining adequate

length of the finger flexors, allowing closure of the
fingers. At the same time wrist extension puts the fin¬
ger extensor tendons on enough slack to allow the necessary
finger flexion excursion.

Clinical Relevance

TENNIS ELBOW": "Tennis elbow" is a painful con¬
dition involving the attachment of the dorsal superfi¬
cial forearm muscles to the lateral epicondyle of the
humerus. It is also known as "lateral epicondylitis/' although
the role of inflammation is unclear ; and as "lateral epicondy-
losis," based on the notion that it includes degenerative
changes in the muscle attachments [22]. The muscles include
the dedicated wrist extensors as well as the extensor digito-
rum and the extensor digiti minimi. Frequently r , patients are
confused by a diagnosis of tennis elbow when they have
never played tennis. Individuals who perform any activity of
repeated or heavily resisted finger flexion are at risk for tennis
elbow because the activity requires concomitant contraction

Figure 15.38: Synergists for finger flexion and extension. A. The
wrist extensors are the synergists to finger flexion. B. The wrist
flexors are the synergists to finger extension.

of the wrist extensors [23]. The extensor carpi radialis brevis is
a common contributor to this pain because it is the primary
wrist extensor. Another confusing phenomenon for an individ¬
ual with tennis elbow is the classic complaint of pain on the
lateral aspect of the elbow while shaking hands; an activity
that clearly involves the finger flexor muscles. The presence of
pain at the lateral epicondyle when shaking hands demon¬
strates the role of the wrist extensors during activities using
the finger flexors.

Wrist flexion by contraction of the flexor carpi radialis and
ulnaris has a similar effect on the finger extensors. The wrist
flexors balance the wrist extension force produced by the fin¬
ger extension muscles, preventing excessive wrist extension
and allowing adequate contractile length in the extensor ten¬
dons to the fingers and adequate passive length in the flexor
tendons of the fingers. The abductor pollicis longus serves a
similar purpose at the thumb. The flexor pollicis longus flexes
the CMC, MCP, and IP joints of the thumb as well as the
wrist. Yet flexion at all of these joints simultaneously would
pull the thumb into the palm. The thumb s CMC joint must

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

325

be positioned in extension for the thumb to oppose the
fingers. Thus the abductor pollicis longus contracts to block
the undesired CMC flexion, stabilizing the joint against the
pull of the flexor pollicis longus [72].

Passive Interactions between
the Dedicated Wrist Muscles
and the Finger Muscles

Active contraction of the wrist extensor muscles pulls the
wrist into extension, putting the finger extensor muscles on
slack and the finger flexor muscles in tension. Passive place¬
ment of the wrist in extension has the same effect on the
lengths of the finger muscles. Similarly, both active and pas¬
sive flexion of the wrist loosens the finger flexors and
stretches the finger extensors. This passive effect of wrist
position on the length and passive tension of the finger mus¬
cles is known as tenodesis and has very useful applications
in rehabilitation.

Clinical Relevance

TENODESIS : Patients who lack active finger motion can
still demonstrate the opening and closing of the fingers by
active or passive movement of the wrist. A patient with a
complete C6 tetraplegia lacks control of the extrinsic finger
flexor muscles yet has the ability to actively extend the
wrist. If the finger flexor muscles are allowed to develop
some passive tightness; a functional grasp is possible by
active wrist extension resulting in passive closure of the
hand (Fig. 15.39). Careful instruction to the patient to
avoid stretching the finger flexors is essential to the
maintenance of a functional grasp.

COMPARISONS OF STRENGTHS
IN MUSCLES OF THE FOREARM

There are few known studies that examine the force pro¬
duction capabilities of individual muscles or muscle groups
at the wrist. The following presents the available data
regarding force production capabilities of the forearm mus¬
culature. There is only one known study that investigates the
effects of age, gender, and hand dominance on wrist and fin¬
ger strength [54]. This study reports extension strength of
the wrist and MCP joints of the fingers and the thumb. It
notes significantly greater strength in men than in women,
gradual decline in strength with age, and significantly
greater strength in the dominant hand. These factors may
have similar effects on the other strengths of the forearm
muscles, although research is needed to measure them.
These factors can be useful when interpreting results of
strength measurements in the clinic.

Figure 15.39: Tenodesis. An individual who lacks active finger
flexion is able to grasp an object by using wrist extension, which
passively flexes the fingers.

Pronatiori versus Supination

Results from comparisons of pronation and supination
strength are conflicting. In one study of the isometric
strengths of pronation and supination in 20 individuals with¬
out pathology, the data reveal greater supination strength
than pronation strength when the forearm is in the neutral
position [69]. Because the forces were collected as the sub¬
jects gripped and attempted to turn a handle, these data are
likely to reflect force contributions from the primary prona¬
tor and supinator muscles and from wrist and finger muscles
as well. In other studies that examine the strength of only
the primary pronators and supinators, peak pronation
strength is slightly greater than peak supination strength
[57,62]. These reports also suggest that strength of prona¬
tion increases as the forearm supinates, and supination
strength increases as the forearm pronates. These data sup¬
port the view that the performance of the pronators and
supinators is dictated primarily by muscle length. As the
muscle is stretched, its force output rises. This finding is
supported by evidence that elbow flexion tends to decrease
pronation strength [57].

326

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Clinical Relevance

ASSESSING STRENGTH OF PRONATION AND
SUPINATION: Pronation and supination strength
assessments are common clinical procedures. The clinician
is cautioned to use consistent testing positions to control for
the effects of wrist and elbow joint position on pronation
and supination strength. If testing positions are altered
because a patient's status changes (e.g., elbow ROM
changes) careful documentation of the position changes will
facilitate interpretation of the clinical measures of strength.
Similarly , the clinician must be aware of any activity in the
finger and thumb muscles that may contribute to the
strength of pronation or supination.

Wrist Flexion versus Extension

Figure 15.40 provides a cross-sectional view of the wrist and
the muscles that cross it. This figure is useful in providing an
overview of the muscles and their relationships to the axes of
motion of the wrist. The figure indicates the number of

muscles capable of causing a given motion at the wrist. It also
allows a qualitative comparison of the moment arms of each
muscle for the four motions of the wrist: flexion, extension,
and radial and ulnar deviation. This figure demonstrates the
potential effect that finger and thumb muscles have at the
wrist. These effects may help explain the variations in results
of studies examining the strength of the wrist flexors and
extensors. In addition, Table 15.1 lists the moment arms of
the five dedicated wrist muscles [1,8].

Studies assessing the strength of the dedicated muscles of
the wrist—the flexor carpi radialis, flexor carpi ulnaris, pal-
maris longus, extensor carpi radialis longus, extensor carpi
radialis brevis, and extensor carpi ulnaris—consistently sug¬
gest that the wrist extensors generate larger peak extension
moments than the peak moments exerted by the flexors
[1,8,12,43,70]. These results are based on biomechanical
models [12] and anatomical studies of muscle size and
moment arms [1,8,17,43]. These studies reveal that the total
physiological cross-sectional area of the dedicated wrist
extensors is slightly greater than that of the dedicated wrist
flexors [1,6,40]. Similarly, the extensors have larger moment
arms, putting them at a mechanical advantage over the flex¬
ors [12]. However, when subjects are allowed to use finger

Flexion

Figure 15.40: A cross section of the distal wrist reveals the number of muscles with the potential to participate in wrist flexion or exten¬
sion. Flexor carpi ulnaris and palmaris longus (not seen in this view) contribute to wrist flexion. FCR, flexor carpi radialis; FDS, flexor
digitorum superficialis; FDP, flexor digitorum profundus; FPL f flexor pollicis longus; ECRL, extensor carpi radialis longus; ECRB, extensor
carpi radialis brevis; ECU, extensor carpi ulnaris; ED, extensor digitorum; El, extensor indicis; EDM, extensor digiti minimi; APL, abductor
pollicis longus; EPB, extensor pollicis brevis; EPL, extensor pollicis longus.

Chapter 15 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FOREARM

327

TABLE 15.1: Approximate Physiological Cross-Sectional Areas (PCSA) and Moment Arms for the Five Primary
Dedicated Wrist Muscles

Muscle

Moment Arm for

Flexion/Extension (cm)

Moment Arm for Radial/Ulnar
Deviation (cm)

Flexor carpi radialis

1.0

1.75

Flexor carpi ulnaris

1.6

1.85

Extensor carpi radialis brevis

2.1

1.0

Extensor carpi radialis longus

1.25

1.35

Extensor carpi ulnaris

2.5

0.6

Data from Brand PW, Hollister A: Clinical Mechanics of the Hand.

St. Louis, MO: Mosby-Year Book, 1999.

and thumb muscles as well as the dedicated wrist muscles, a
significantly larger flexion moment is generated than exten¬
sion moment [16,26]. Examination of the total physiological
cross-sectional area of all of the muscles with the potential to
flex and extend the wrist reveals that the flexors have a larger
total cross-sectional area [1,26,40]. The total force capacity of
all of the finger extensors is less than 40% of the total capac¬
ity of all of the finger flexors. The extensor pollicis longus
contributes to active wrist extension, but the abductor
pollicis longus and extensor pollicis brevis may contribute to
flexion. Consequently, if the muscles to the fingers and thumb
are allowed to participate in wrist motion, it is not surprising
that wrist flexion strength is greater than wrist extension.

Clinical Relevance

MMT OF THE WRIST: The studies of wrist strength reveal
that measures of strength at the wrist are affected signifi¬
cantly by the presence or absence of activity in the finger
muscles. Variations in the muscles participating in the test
alter the output. Satisfactory inter- and intrarater reliability
of wrist strength requires a consistent method of strength
measurement. MMT of wrist flexion with and without finger
muscle participation is likely to produce significantly
different results.

Wrist position also affects the force of wrist flexion and exten¬
sion. Studies based on muscle architecture and biomechanical
models suggest that both the wrist flexors and wrist extensors
reach their peak forces when the wrist is extended [26,43].
The wrist flexors are on stretch when the wrist is extended,
increasing the muscles’ force-generating capacity. However,
the moment arms of the flexors increase when the wrist is
flexed. In contrast, the wrist extensors are shortened when the
wrist is extended, but their moment arms increase when the
wrist is in extension. These data suggest that force output of
the wrist flexors is influenced more by their length, while the
force of the dedicated wrist extensors is influenced more by
the muscles’ moment arms.

These relationships are consistent with the functional
demands on these muscle groups. The synergistic activity

between the wrist extensors and the finger flexors reveals that
the wrist extensors’ primary responsibility is to stabilize the
wrist against the pull of the finger flexors. It is useful for these
muscles to be strongest when the finger flexors are most
active, that is, as the wrist is extended. Thus the wrist exten¬
sors appear to be architecturally designed to be strong in their
most important functional position.

Radial versus Ulnar Deviation
of the Wrist

There is only one known study comparing the strengths of
radial and ulnar deviation of the wrist, and the finger and
thumb muscles were allowed to participate [16]. The study
reports that radial deviation is stronger than ulnar deviation
because there are many more muscles with the capacity to
deviate the wrist radially. Some of these muscles (particularly
the snuffbox muscles of the thumb) have very large moment
arms. Although there are no known studies that compare the
strengths of radial and ulnar deviation when only the dedicated
wrist muscles are considered, there may be less difference
between these strengths. The capacity of these muscles to
generate a moment based on both muscle size and mechanical
advantage is more similar, although the muscles that deviate
the wrist in a radial direction may generate a slightly larger
total moment [1,7].

Finger Flexion versus Extension

Few studies are available that assess or compare the strengths
of the fingers, and there are no known studies of the strengths
across the joints of the thumb. The flexor digitorum profun¬
dus is reportedly 50% stronger than the flexor digitorum
superficialis, based primarily on muscle mass [8]. An in vivo
study of flexion strength at the DIP, PIP, and MCP joints of
the fingers reports that PIP flexion is stronger than DIP flex¬
ion [15]. The investigator reports that the DIP joints of the
fingers were fixed in extension as the strength of PIP flexion
was measured. This could allow the flexor digitorum profun¬
dus to contract against the fixation. If this occurred, the
reported strengths at the PIP joint reflect the force of both
the flexor digitorum superficialis and the flexor digitorum
profundus.

328

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Assessment of finger extension strength at the MCP joints
using both in vivo measurements [4] and anatomical studies
[8] reveals that the long finger exerts the greatest extension
force, followed by the index and ring fingers. The little finger,
which commonly lacks an extensor digitorum tendon, exerts
the least force.

Although there is a clear lack of studies characterizing the
normal range of strengths of the wrist and hand within a
healthy population, studies of functional strengths are avail¬
able. These studies assess the strengths of pinch and grasp.
The results of some of these studies are presented in
Chapter 19 following the discussion of the mechanics of nor¬
mal pinch and grasp.

SUMMARY

This chapter reviews the muscles of the forearm. Each mus¬
cle is presented, and its contributions to the function and dys¬
function of the wrist and elbow are discussed. Discussions
reveal that the functions often depend on the precise position
of the joint of interest. Many muscles lie so close to a joint axis
that the muscles effect at the joint changes as the muscle
slides back and forth across the axis. In addition, the critical
interplay between the dedicated wrist muscles and the mus¬
cles of the fingers is analyzed, and a similar interplay between
muscles of the thumb is noted.

Although many unresolved issues remain regarding the
actions of the muscles of the forearm, several guiding princi¬
ples can be derived:

• Many forearm muscles cross the elbow and may affect it,
particularly in the absence of the primary muscles of the
elbow.

• Actions of the forearm muscles at the elbow and wrist may
change with changing positions of the elbow and wrist.

• Movements of the wrist in the cardinal planes require
pairs of muscles to contract together to accomplish the
motion; the muscles composing the pairs vary according to
the desired motion.

• Normal movements of the fingers require simultaneous
contraction of wrist muscles to block the undesired effects
of the finger muscles at the wrist.

• Function of the extrinsic muscles to the fingers is inextri¬
cably intertwined with the function of the intrinsic muscu¬
lature presented in Chapter 18.

• Impairments resulting from dysfunction of the extrinsic
muscles of the fingers are also affected by the integrity of
the intrinsic muscles.

The clinician must recognize that an evaluation of the wrist
also requires an assessment of the structures of the elbow
and hand.

Throughout this chapter the function of the extrinsic
muscles of the fingers is related to structures unique to the
hand, including the intrinsic muscles. Thus the explanation
of the functional capability of the hand has just begun.

Before proceeding to these structures, a discussion of the
mechanics of the wrist must be completed. The following
chapter discusses the loads to which the wrist is subjected
during a variety of activities. In addition, the effects of vari¬
ous pathological conditions on the loads at the wrist are con¬
sidered. An understanding of loads incurred at the wrist will
provide a better understanding of the mechanics of loading
in the hand.

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CHAPTER

Analysis of the Forces at the Wrist
during Activity

CHAPTER CONTENTS

ANALYSIS OF FORCES AT THE WRIST .332

REVIEW OF THE FORCES ON THE WRIST .333

ANALYSIS OF STRESSES APPLIED TO THE WRIST JOINT DURING ACTIVITY.336

CLINICAL IMPLICATIONS OF STUDIES ANALYZING THE FORCES AND STRESSES ON THE WRIST.336

SUMMARY .337

ecause the hand is the "working end" of the upper extremity, it participates in countless activities that gener-
ate large loads in the hand and wrist. Some commonplace examples include twisting the tightened lid from
a jar or digging up flower bulbs in a garden. Other activities that involve high loads at the wrist and hand
include activities in which the upper extremity is weight bearing. As noted in the chapters on the shoulder and elbow, the
upper extremities frequently participate in ambulation through the use of crutches and other assistive devices in
the presence of impaired function in the lower extremities. The upper extremities can even replace the propulsion of
the lower extremities entirely with the use of a wheelchair. The hand and wrist are also involved in impulsive activities
such as hitting a tennis or golf ball, hammering a nail, or handling a jackhammer. Such activities expose the wrist to
very large loads. The wrist's ability to withstand such loads is a testament to its remarkable design.

Many impairments involving the structures of the forearm and wrist are directly or indirectly associated with the loads
that are generated at the wrist. Such impairments include "tennis elbow" (lateral epicondylitis), carpal tunnel syn¬
drome, and degenerative arthritis. The purpose of this chapter is to examine the loads sustained at the wrist during
activity and to discuss the implications of such loads for patients with wrist impairments and for the clinicians who
work with these individuals. Specifically the purposes of this chapter are to

■ Provide an example of a two-dimensional analysis of the forces at the wrist

■ Examine the forces on the wrist joint and surrounding structures during activities

■ Review the loading patterns and stresses on specific structures of the wrist complex

■ Discuss the clinical implications of the magnitude and locations of loads at the wrist in individuals with
and without impairments

331

332

Part II I KINESIOLOGY OF THE UPER EXTREMITY

ANALYSIS OF FORCES AT THE WRIST

In the analyses of forces generated at the shoulder and elbow
(Chapters 10 and 13), simplifying assumptions are made to
solve the equations used to calculate the forces and moments
at those joints. These assumptions reduce the number of
unknown quantities by presuming that only one muscle or
muscle group contracts at a time. At the wrist, this is a reason¬
able assumption for some weight-bearing activities in which
the wrist muscles are essential but the finger muscles can be
somewhat relaxed. However, in an activity requiring forceful
grip, it is clear that the finger flexors and wrist extensor mus¬
cles must contract simultaneously. Calculating joint forces
under these conditions is more complicated because there are
more unknown quantities than there are equations to solve,
resulting in a case of static indeterminacy (Chapter 1).

The following example is chosen to review the methods of
calculating joint and muscle forces when the simplifying

assumptions are valid. Consider an individual who uses a cane
for support following a stroke. Examining the Forces Box 16.1
contains the free-body diagram and analysis of this example.
Evaluation reveals that the patient bears approximately 50% of
body weight on the weak side during stance on that side. Thus
the cane must be supporting the remaining 50%. During the
support phase, the cane pushes up on the hand, creating an
extension moment at the wrist (Fig. 16.1). The finger muscles
are not essential at this moment, so the example uses the
assumption that only the dedicated wrist flexors are participat¬
ing in the activity. In addition, the wrist flexors are considered
together as one flexor force. These assumptions, while clearly
not completely accurate, allow a solution for the equations of
motion to determine the muscle and joint reaction forces.

This simplified analysis suggests that the total flexor force
needed to balance the extension moment at the wrist gener¬
ated by the cane is equal to approximately one eighth of body
weight. Further analysis estimates the joint reaction force on

Chapter 16 I ANALYSIS OF THE FORCES AT THE WRIST DURING ACTIVITY

333

Figure 16.1: Extension moment at the wrist generated by the
cane. The reaction force by the cane applied to the hand creates
an extension moment (M f ) at the wrist joint.

the carpus to be slightly more than 50% of body weight. An
important assumption in this example is the location of the
reaction force of the cane. The closer the force is to the joint
axis, the smaller the extension moment created. However, if
the individual bears weight more in the palm of the hand, the
extension moment and hence the muscle and joint reaction
forces increase accordingly

The values presented in this example are very rough
approximations of reality, but the example demonstrates that
the wrist can be subjected to very large loads. A fall onto an
outstretched hand can impart an even larger load to the wrist,
since the hand may bear more than 50% of body weight.
Experimental falls even from low heights (3-6 cm) generate
loads on the wrist of 50-55% of body weight [5]. Falls from
standing height must generate much larger loads. In addition,
the velocity of the body at the instant of impact means that
the kinetic energy of the body is transmitted to the wrist. No
wonder that the radius fractures or the ligaments rupture!

REVIEW OF THE FORCES ON THE WRIST

There are no known studies that examine the loads on the
wrist during crutch walking. Wheelchair propulsion and
crutch walking are not exactly analogous, but the tasks have
enough similarities that the published data from the wheel¬
chair study can help to put crutch walking into perspective.
Neither task requires significant activity of the finger flexor
muscles. Both tasks require that the wrist bear a load that

Figure 16.2: Extension moment generated on the wrist by a
wheelchair. Propelling a wheelchair creates an extension
moment (M f ) at the wrist.

creates an extension moment at the wrist (Fig. 16.2). Loads of
up to 90 N (approximately 20 lb) applied to the wheelchair
rim are reported for manual wheelchair users [2,3,7]. The load
applied by the hand onto the crutch may be significantly
greater than the load on the wheelchair rim, since the indi¬
vidual may bear as much as 50% of body weight on the crutch
[14,16]. Therefore, it is likely that crutch walking generates
larger loads at the wrist than does wheelchair propulsion.

Clinical Relevance

USE OF ASSISTIVE DEVICES FOR AMBULATION IN
INDIVIDUALS WITH RHEUMATOID ARTHRITIS: The

wrist is commonly affected in individuals with rheumatoid
arthritis , which also typically involves the feet knees , and hips ,
making ambulation difficult and painful. Assistive devices such
as canes , crutches , and walkers can be very useful in reducing
the loads on the joints of the lower extremities and improving
ambulation. However ; the analysis in Examining the Forces
Box 16.1 demonstrates that use of these devices can lead to
large loads on the wrist as well. The clinician is faced with the
dilemma of protecting the joints of the lower extremities while
perhaps overloading the joints of the wrist. Special adapta¬
tions of the assistive device may provide an alternative. These
assistive devices can be fit with special supports that allow the
patient to bear weight on the forearm rather than on the
/AqW hand and wrist (Fig. 16.3). Such modifications provide
\M Q 0 ] relief for the lower extremities while minimizing the
risk to the wrist.

334

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 16.3: Forearm trough crutches are an example of adaptive
devices that modify canes and crutches to reduce weight bearing
through the wrist joint.

There are few studies that report direct calculations of
loads applied to the wrist joint during common activities of
daily living. Chadwick and Nicol report joint reaction forces
from 1,200 to over 2,000 N (270-450 lb) when lifting 2-4
kg (4.4-8.8 lb) loads, depending upon the type of grip used
[4]. Keir and Wells suggest that more than 25% of the max¬
imum available extensor torque is required to hold the wrist
in 30° of extension, a position often assumed by typists and
data entry personnel [9]. Such muscle requirements would
presumably produce significant joint reaction forces as
well. Additionally, since such postures are typically main¬
tained over prolonged periods, it is not surprising that wrist
and elbow complaints are common in individuals in these
professions.

Clinical Relevance

WORK-RELATED WRIST AND HAND INJURIES: Work-
related musculoskeletal disorders of the wrist and hand
injuries are associated with greater lost productivity and
wages than disorders of other regions of the body [ 7 ]. Jobs
that require prolonged or high-velocity repetitive wrist posi¬
tions or high repetitive loads have high risk of wrist and
hand problems. Workers with high incidence of such disor¬
ders range from data entry personnel to dentists to farm
workers milking cows. Job analysis to identify ways to alter
or vary the wrist position and employer and employee edu¬
cation about the standards for job safety may help to
decrease the frequency of job-related injuries.

One of the reasons for the scarcity of investigations into
the joint forces at the wrist is the complexity of the problem.
As noted above, most activities involving the wrist require the
simultaneous contraction of several wrist and finger muscles.
These cocontractions invalidate the usual assumption that
only one muscle or muscle group is acting at any instant in
time. Authors use a variety of analytical approaches to inves¬
tigate the loads sustained by the wrist during cocontractions.
Optimization techniques (Chapter 1) to calculate the joint
reaction forces in the wrist as subjects pick up a moderate
load (2-4 kg) using different grips yield joint reaction forces
at the wrist that range from 1200 to 2200 N (270-500 lb) [4].
The cocontractions of the finger and wrist muscles needed to
grip the object and stabilize the wrist are apparently respon¬
sible for these remarkably large loads.

A theoretical model to estimate the loads at the
wrist during light grasp activities (approximately a
1-kg grip) yields estimates of total forces across the
wrist of approximately 160 N (36 lb), considerably less than
those in the preceding study [19]. Two methodological differ¬
ences help explain the varied results. The grip force in the
theoretical model is one fourth to one half the grip force
examined by the optimization technique. In addition, the
optimization analysis includes the flexion moment applied to
the wrist by the lifted load, while the theoretical model con¬
siders only the effects of the compressive loads of the muscles
during pinch without applying an additional external moment
to the wrist (Fig. 16.4). Both analyses demonstrate that
cocontraction of opposing muscle groups generate larger joint
reaction forces than when only one muscle group is consid¬
ered. These studies also suggest that even in such mild tasks
as gripping and lifting a 2-kg (less than 1 lb) object, the wrist
sustains large joint reaction forces.

More-vigorous activities such as some athletic events lead
to large increases in loads at the wrist. Peak internal moments
as high as 20 Nm are reported at the wrist at the instant of
impact in the tennis stroke [6]. This can be compared to 12
Nm reported at the elbow during wheelchair propulsion [21].
Competitive gymnastics includes several events that involve

Chapter 16 I ANALYSIS OF THE FORCES AT THE WRIST DURING ACTIVITY

335

Figure 16.4: Two different models to examine the forces at the wrist during grasp and pinch. A. The model includes the muscle forces
of the fingers ( F f ) and the thumb ( F r ) as well as the flexion moment ( M F ) created by the load itself. B. The model only includes the mus¬
cle forces of the fingers (FJ and the thumb (F r ) needed to pinch an object.

upper extremity weight bearing, usually in ballistic maneu¬
vers involving very high rates of loading. The loads at the wrist
during pommel horse and high bar exercises have been exam¬
ined. Peak joint reaction forces at the wrist of up to twice
body weight are reported in elite college-age male gymnasts
performing on the pommel horse [11] (Fig. 16.5). A study of
the kinetics of the high bar exercise reports peak reaction
forces on the bar up to 2.2 times body weight in elite male
gymnasts during giant swings [12]. These loads on the bar are
balanced by tensile forces on the hands and wrist that must,
in turn, be countered by large forces in the surrounding liga¬
ments and muscles to stabilize the wrist and prevent disloca¬
tion (Fig. 16.6). The loads reported in these studies help
explain the common complaints of wrist pain reported by
gymnasts [11]. Other recreational and occupational activities
such as riding dirt bikes over mountainous terrain, doing cart¬
wheels, or driving a jackhammer into concrete may generate
similarly high loads and lead to a variety of injuries and com¬
plaints at the wrist.

These studies of the joint reaction forces at the wrist sug¬
gest that the muscles and ligaments at the wrist also sustain
large loads. Direct assessment of tendon loads generated dur¬
ing activity are also reported. Analysis of the peak forces in
the extensor carpi radialis brevis during a backhand tennis
stroke reveals loads of 90 N (20 lb) in advanced tennis players
and 65 N (15 lb) in novice players [15]. Another study based
on an anatomical wrist joint simulator examines the muscle

Figure 16.5: Load on the wrist during a pommel horse exercise.
The pommel horse exercise produces loads on the wrist that can
reach twice body weight.

336

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 16.6: Load on the wrists during high bar exercises. The
wrist sustains large distraction forces during the high bar exercise
and is stabilized by the large forces generated by the surround¬
ing ligaments and muscles.

forces needed to maintain static wrist positions [24]. The
reported loads vary from 5 N (1.2 lb) in the flexor carpi radi-
alis to approximately 30 N (6.7 lb) in the flexor carpi ulnaris
and extensor carpi ulnaris, depending upon the position. The
model supports the view that wrist functions require simulta¬
neous activity of several muscles by predicting cocontraction
of the extensor carpi ulnaris, extensor carpi radialis longus,
extensor carpi radialis brevis, flexor carpi radialis, flexor carpi
ulnaris, and abductor pollicis longus in each of the positions
analyzed, 20° of flexion or extension, and 10° of ulnar or radial
deviation.

Clinical Relevance

LOADS IN TENDONS AROUND THE WRIST DURING
ACTIVITY: Studies of the loads on the wrist reveal that the
muscles surrounding the wrist must sustain substantial
loads even in relatively low-load activities and even larger
loads during more challenging tasks such as tennis. It is
easy to imagine why complaints of pain with tennis elbow
become chronic , since even without playing tennis; most
individuals perform daily functions requiring large loads in
the surrounding muscles. Identification of the loading pat¬
terns requires careful analysis of the offending activity. The
clinician may be able to recommend modifications in the
activity and provide patient education to alter the loads.
Supportive devices may also be helpful in reducing the
loads in the forearm musculature to allow healing.

ANALYSIS OF STRESSES APPLIED TO
THE WRIST JOINT DURING ACTIVITY

Although analysis of the joint reaction forces at the wrist is
uncommon, the literature contains several studies that report
calculations and direct measurements of the stresses applied

to the wrist joint surfaces and surrounding soft tissue. Stress,
or pressure, is defined in Chapter 2 as force per unit area
(force/area). Direct measurements are performed typically in
cadaver specimens by inserting a pressure-sensing device to
measure the stress between adjacent articular surfaces during
loading [8,20,22]. Larger average stresses are reported at the
radioscaphoid articulation than at the radiolunate articulation
[8,19,20,22]. Wrist position and joint alignment are among
many factors that influence these joint stresses. There is an
increase in radioscaphoid pressure when the wrist is in radial
deviation and an increase in radiolunate and ulnocarpal
stresses when the wrist is in ulnar deviation. Stresses as large
as 4.3 MPa, (1 megaPascal = 10 6 N/m 2 , Chapter 1), are
reported in single specimens [20]. These stresses can be com¬
pared to stresses between 4.0 and 6.0 MPa reported at the hip
during ambulation [10]. Other studies demonstrate distinct
alterations in loading patterns in the presence of abnormal
joint morphology. An abnormally long ulna, described as a
positive ulnar variance , is accompanied by an increase in the
stress on the triangular fibrocartilage complex [8]. Similarly,
carpal instability results in changes in stress throughout the
carpus [22].

Clinical Relevance

PRESSURE CHANGES WITH CARPAL INSTABILITIES:

Cadaver studies suggest that there is increased pressure
between the radius and scaphoid bones in the presence of
scaphoid instability [13]. The area of increased pressure is
approximately the same area that frequently shows degen¬
erative joint disease in older adults. Pressure studies such as
these suggest direct associations among abnormal joint
alignment abnormal loading patterns , and eventual joint
destruction. Additional research is needed to clarify these
associations , since a better understanding of these links will
help guide clinical decisions regarding treatment strategies
for joint malalignments.

CLINICAL IMPLICATIONS OF STUDIES
ANALYZING THE FORCES AND STRESSES
ON THE WRIST

Osteoarthritis primarily affects the large weight-bearing joints
of the body, particularly the hips and knees, but while less
common, osteoarthritis does appear at the wrist as well [7].
The most common site of joint degeneration in the wrist is
between the scaphoid and the radius [23]. This finding is con¬
sistent with the stress data reported earlier. These data sup¬
port the existence of a relationship between joint pressures
and joint degeneration. The altered patterns of loading found
in wrist joint malalignments also suggest that wrists with
abnormal alignment may be predisposed to degenerative
changes.

Chapter 16 I ANALYSIS OF THE FORCES AT THE WRIST DURING ACTIVITY

337

Some authors suggest that individuals who depend on their
upper extremities for weight bearing may be particularly
prone to degenerative changes at the wrist [18,25]. A study of
50 individuals who used a single cane for ambulation reports
no increased incidence of arthritis in the weight-bearing wrist
compared with the non-weight-bearing wrist [25]. The mean
duration of cane use among the subjects studied was 4 years.
In contrast, another study reports a high incidence of carpal
bone instability, usually involving the lunate, in individuals
with paraplegia who used wheelchairs exclusively [18]. The
mean duration of the spinal cord injury in subjects with wrist
joint instability was 30 years.

These two studies seem to have conflicting data, although
one study assesses wrist joint instability while the other exam¬
ines the prevalence of degenerative arthritis. However, the
bigger difference in these two studies is the duration of repet¬
itive weight-bearing activity. The population with observed
wrist joint pathology had a mean duration of weight-bearing
activity approximately seven times the mean duration seen in
the population with no pathology. Although additional
research is needed to clarify the relationship between pro¬
longed weight-bearing activities and wrist joint pathology,
these data suggest the possibility that excessive loading sus¬
tained by the wrist for a prolonged period of time may con¬
tribute to the development of wrist pathology.

What do these studies mean for the clinician? Avoiding
weight-bearing activities to protect the joints is not feasible in
a population that depends on upper extremity weight bearing
for mobility. Further studies are needed to determine if there
are other factors that increase or decrease the risk of eventual
degeneration. Perhaps changes in wrist joint flexibility and
strength can alter the risks associated with prolonged weight¬
bearing activities. In addition, modifications to the wheel¬
chairs and ambulation devices can alter the mechanics to
lower the loads or redistribute the stresses at the wrist. By
being aware of the possible links between loading patterns at
the wrist and future joint pathology and by understanding the
factors that influence the loads generated during an activity,
the clinician may be able to reduce a patients risk of joint
pathology and minimize the impairments resulting from such
pathology.

SUMMARY

This chapter examines the forces and stresses to which the
wrist is subjected in daily activity. A simple two-dimensional
model is used to analyze the forces in the muscles and at the
joint during a simple weight-bearing task in which the
assumption that only one muscle group is active was valid.
The simplified model yielded a joint reaction force on the
wrist of 50% of body weight. Data from more-complex mod¬
els were reviewed, since in many daily activities the wrist
requires simultaneous activity from many muscles. Data from
these more-complex models revealed that the muscles and
the wrist joint can sustain loads of more than twice body

weight, particularly during activities requiring upper extremity
weight bearing. Studies that report the stresses (force/area)
sustained at the wrist were also reported. These data suggest
that the wrist withstands stresses similar to those at the hip
during ambulation. Stresses on the wrist are altered during
normal wrist movement and are directly affected by the
mechanics and pathomechanics of the wrist. These studies
provide a useful perspective for the clinician to appreciate the
mechanical challenges to the wrist during everyday activity.

This chapter completes the discussion of the wrist.
However, the function of the hand depends to a large extent
on the muscles of the forearm and the integrity of the wrist
joint. The following three chapters examine the interaction of
forearm structures with the special structures and muscles
found within the hand that contribute to the mechanics and
the pathomechanics of hand function.

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1994; 75: 85-92.

18. Schroer W, Lacey S, Frost FS, Keith MW: Carpal instability in
the weight-bearing upper extremity. J Bone Joint Surg 1996;
78A: 1838-1843.

19. Schuind F, Cooney WP, Linscheid RL, et al.: Force and pressure
transmission through the normal wrist. A theoretical two-dimen¬
sional study in the posteroanterior plane. J Biomech 1995; 28:
587-601.

20. Short WH, Werner FW, Fortino MD, Mann KA: Analysis of the
kinematics of the scaphoid and lunate in the intact wrist joint.
Hand Clin 1997; 13: 93-108.

21. Veeger HEJ, van der Woude LHV, Rozendal RH: Load on the
upper extremity in manual wheelchair propulsion. J
Electromyogr Kinesiol 1991; 1: 270-280.

22. Viegas SF, Patterson RM: Load mechanics of the wrist. Hand
Clin 1997; 13: 109-128.

23. Watson HK, Ryu J: Evolution of arthritis of the wrist. Clin
Orthop 1986; 202: 57-67.

24. Werner FW, Palmer AK, Somerset JH, et al.: Wrist joint motion
simulator. J Orthop Res 1996; 14: 639-646.

25. Wright V, Hopkins R: Osteoarthritis in weight-bearing wrists? Br
J Rheumatol 1993; 32: 243-244.

CHAPTER

Mechanics and Pathomechanics
of the Special Connective Tissues
in the Hand

CHAPTER CONTENTS

LANDMARKS WITHIN THE HAND.340

CONNECTIVE TISSUE IN THE HAND .340

Palmar Aponeuroses.340

Retinacular, or Pulley, Systems.342

Tendon Sheaths.344

Structures That Anchor the Flexor and Extensor Apparatus of the Fingers.346

SUMMARY .349

T he preceding three chapters discuss the structure and function of the bones and joints of the wrist and hand,
the function of the muscles of the forearm, and the loads that the wrist sustains during certain functional
activities. Throughout those chapters the reader is reminded that the function and dysfunction of all of these
structures are intimately related to the integrity of the structures found within the hand. The objectives of the next
three chapters are to discuss the soft tissue structures that are intrinsic to the hand, to discuss their contribution to the
function of the hand, and to present the functional synergies that exist between the intrinsic and extrinsic structures
of the hand.

The hand contains several special connective tissue structures that are critical to the normal function of the hand. The
purposes of the current chapter are to

■ Describe the structure of the special connective tissue elements that are intrinsic to the hand
■ Describe how these connective tissue structures contribute to the function and dysfunction of the hand
■ Discuss common hand deformities that result from disruption of these connective tissue structures

The primary function of most of the special connective tissue structures presented in this chapter is to stabilize the
other soft tissue in the hand, particularly the muscles and tendons. Some structures also are essential for the nutrition,
lubrication, and smooth glide of the tendons extending into the digits. Although most of the special connective tissue
structures of the hand are subcutaneous, the skin itself plays an important role in the function and mechanics of the
hand. In particular, the skin of the web spaces between the digits participates in limiting radial and ulnar deviation of
the metacarpophalangeal (MCP) joints of the fingers as well as abduction of the thumb. Loss of the thumb's web space
as the result of scarring, for example, can cause significant functional impairment. The skin's folds and sweat glands
reduce the chance of slippage between the hand and objects in the hand. The skin also provides useful superficial land¬
marks to the clinician who is evaluating the hand. Perhaps the most useful and reliable landmarks in the hand are the
skin creases that are most prominent on the palmar surface of the hand. The following reviews the relationships
between these skin creases in the hand and the underlying structures.

339

340

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

LANDMARKS WITHIN THE HAND

Skin creases form perpendicular to the direction of pull of the
underlying muscles. Examination of the skin creases in the
hand reveals that they are directed for the most part in a
radioulnar direction. These creases are constant, with only
slight variations among individuals without hand pathology
(Fig. 17.1). Inspection of the palmar surface of the hand from
proximal to distal reveals

• One to three creases at the wrist

• A crease at the base of the thenar eminence

• A distal palmar crease

• A pair of creases at the base of each finger

• A crease at the base of the thumb

• A pair of creases at the proximal interphalangeal (PIP)
joint of each finger

• A single crease at the distal interphalangeal (DIP) joint of
each finger and at the IP joint of the thumb

The proximal wrist crease is proximal to the radiocarpal
joint. The middle crease passes directly over the radio¬
carpal joint space, and the distal crease lies just distal to
the joint line. These creases provide a reliable means of
identifying the radiocarpal joint line. The distal palmar
crease lies just proximal to the MCP joints of the middle,
long, and little fingers, while the creases at the bases of
the fingers lie just distal to the MCP joints. Palpation of
the MCP joints occurs between the distal palmar crease
and the creases at the base of the fingers. The creases in
the fingers and thumb lie directly over, or just proximal
to, the underlying joints [14,15]. Similarly, there are
creases on the dorsum of the fingers, which lie approxi¬
mately over the MCP, PIP, and DIP joints of the fingers
(Fig. 17.2).

Figure 17.2: Creases on the dorsum of the fingers lie over the
joints of the fingers.

Clinical Relevance

EDEMA IN THE HAND: The creases on the palmar side of
the hand are rarely absent and thus are useful to the clini¬
cian who is evaluating deeper structures. In contrast the
creases on the dorsal side of the fingers are readily dis¬
placed or obscured by swelling in the digits. Swelling is
more apparent on the dorsal surface of the hand because
the connective tissue structures on the palmar surface pre¬
vent distention of the skin and deeper structures. The tissues
cannot expand to accommodate increased volume.
Consequently , edema accumulates on the dorsal surface
where the skin is readily distended. The dorsal creases are
unreliable landmarks in the presence of swelling.

Figure 17.1: Creases on the palmar surface of the hand provide
reliable landmarks for palpating the underlying structures.

CONNECTIVE TISSUE IN THE HAND

The prevalence of swelling on the dorsum of the hand is a con¬
sequence of the unique connective tissue structures found
in the hand. These special connective tissue structures consist
of the palmar aponeuroses; the retinacular, or pulley, systems of
the wrist and fingers; the tendon sheaths; and the special liga¬
ments that anchor the flexor and extensor tendon apparatus in
each finger. Each of these structures serves a slightly different
purpose and is described separately in the following sections.

Palmar Aponeuroses

There are two layers of aponeuroses in the palm of the hand,
the superficial and deep. The superficial palmar aponeurosis
has three parts, the thenar, hypothenar, and midpalmar
aponeuroses (Fig. 17.3). These aponeurotic sheets project
fibrous tentacles into the overlying skin, providing essential
stabilization of the skin for grasping activities. The ability to
pull on a rope or twist off a jar lid requires that the palmar

Chapter 17 I MECHANICS AND PATHOMECHANICS OF THE SPECIAL CONNECTIVE TISSUES IN THE HAND

341

Figure 17.3: The superficial palmar aponeurosis is divided into
the thenar, hypothenar, and midpalmar aponeuroses.

skin stay fixed to the underlying tissues of the hand.
Otherwise the hand would slide inside its skin just as a foot
slides in an oversized boot. Connections between an aponeu¬
rosis and the skin are absent on the dorsum of the hand allow¬
ing the skin to glide freely It is the absence of these tethers
between the skin and the underlying structures that allows
swelling to accumulate more readily on the dorsal surface of
the hand than on the palmar surface.

Besides the vertical fibers projecting toward the skin, the
superficial midpalmar aponeurosis contains transverse and
longitudinal fibers that extend toward the fingers. Although
most of these fibers end at the web space of the fingers, some
fibers extend into the fingers and become continuous with the
digital fascia [21].

Clinical Relevance

DUPUYTREN'S CONTRACTURE: The superficial midpal-
mar aponeurosis undergoes progressive fibrotic changes in
some individuals , particularly in men older than 50 years of
age. Although the etiology is unclear ; the disorder known as
Dupuytren's contracture is manifested by palpable thick¬
ening of the palmar fascia and a progressive flexion con¬
tracture of the ulnar two fingers [6,27] (Fig. 17.4). The
distal extension of the palmar aponeurosis into the
fingers explains the progressive flexion of the fingers.

Figure 17.4: Dupuytren's contracture produces flexion of the
ulnar fingers.

The deep palmar aponeurosis lies just anterior to the
metacarpal bones and palmar interosseus muscles. There are
fascial extensions that run between the deep and superficial
palmar aponeuroses, creating compartments within the palm
of the hand [21,32] (Fig. 17.5). Individual compartments are
created to contain the tendons of the flexor digitorum super-
ficialis and profundus to a single finger. Individual com¬
partments also are formed for the lumbricals and for the
neurovascular bundles to each finger. Consequently, the
structures to each finger are stabilized within their own fascial
tunnels as they project toward the fingers.

Figure 17.5: Fibrous compartments in the palm of the hand. The
deep palmar aponeurosis is connected to the superficial palmar
aponeurosis by fascial bands that create compartments in the
hand for tendons, nerves, and blood vessels.

342

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

The palmar aponeuroses serve several purposes. They
anchor the skin to prevent slippage during grasp. They provide
protection for the underlying muscles, nerves, and blood ves¬
sels. They also create tunnels to stabilize these structures as
they travel through the hand. In contrast, the extensor ten¬
dons are compartmentalized only as they cross the wrist. They
glide relatively freely between the deep and superficial fascial
sheaths on the dorsum of the hand.

The fingers also possess fascial compartments containing
the neurovascular bundles on the radial and ulnar aspects of
each digit as well as fibrous connections to the skin. These
include extensions from the palmar aponeurosis as well as
specialized supports at the PIP and DIP joints. They serve the
same stabilizing function as does the palmar aponeurosis, pre¬
venting translation of the neurovascular bundles as well as
slippage of the skin during grasp.

Clinical Relevance

SWELLING WITHIN THE COMPARTMENTS OF THE
HAND OR FINGERS: The fibrous compartments within
the hand and fingers are essential for stabilizing the skin ,
muscles , and neurovascular supply. However ; inflammation
within a compartment can produce severe pain and may
lead to compression and damage of the neurovascular con¬
tent. The fibrous walls of these compartments create rela¬
tively rigid spaces that cannot accommodate an increase in
volume. Consequently , edema within a compartment leads
to increased pressure that may compress and impair the
sensitive neurovascular structures. The most familiar exam¬
ple of this is compression of the median nerve within the
carpal tunnel but similar examples can occur in the com¬
partments of the fingers as well.

Retinacular, or Pulley, Systems

The retinacular systems throughout the body function to hold
tendons in place. They are found in locations where joint
motion or tension on the tendons causes the tendons to move
away from the joint. At the wrist, flexion causes the flexor ten¬
dons to project anteriorly away from the wrist joint. Similarly,
the extensor tendons migrate posteriorly away from the wrist
during wrist extension. The flexor and extensor retinacular
bands bind the tendons to the wrist to limit their bulging away
from the joint. Thus the retinacular bands help maintain a
more constant moment arm for each muscle.

The movement of a tendon away from its joint is known as
bowstringing, since the tendon is stretched away from the
joint and limb segments like the string of a bow (Fig. 17.6). One
of the dangers of bowstringing is that the tendon becomes
more prominent and hence more susceptible to injury. This is
particularly true in the wrist and fingers, where a protruding
tendon is more likely to be crushed or lacerated. Flexor ten¬
dons allowed to bowstring in the hand also interfere with the
stable contact between hand and object in a firm grasp.

Figure 17.6: The function of a stabilizing retinaculum. (>4) When
a joint is moved so that a tendon is lengthened, the tendon lies
close to the joint surface. ( B ) When the joint is moved, putting
the tendon in a slackened position, the tendon bowstrings away
from the joint, increasing the muscle's moment arm.

Bowstringing changes the mechanics of the mus¬
cle, significantly increasing the muscle s moment arm.
This puts the muscle at a mechanical advantage, since
a longer moment arm increases a muscle s ability to generate
a moment. The improved mechanical advantage is demon¬
strated by the decrease in force required to flex a digit when
the flexor tendon is allowed to bowstring [13]. However, a
muscle with a larger moment arm requires more shortening
than a muscle with a shorter moment arm to produce the
same angular excursion (Chapter 4). When a muscle bow¬
strings, increasing its moment arm, it must shorten more dur¬
ing contraction to produce the same angular displacement as
before the bowstringing. As a result, the muscle is likely to
exhibit active insufficiency, the inability to contract far
enough to pull the joint through its full excursion [30].

RETINACULAR SYSTEMS AT THE WRIST

The transverse carpal ligament (TCL) at the wrist, or flexor
retinaculum, functions to stabilize the carpal arch and the ten¬
dons that cross the volar surface of the wrist within the carpal
tunnel. Wrist flexion causes these tendons to glide in a volar
direction, and the TCL prevents excessive volar glide. The
TCL sometimes is surgically transected as part of the treat¬
ment for carpal tunnel syndrome in the hopes of relieving the
pressure on the median nerve within the tunnel. Complete
transection allows the flexor tendons to migrate anteriorly, or
bowstring, in the carpal tunnel during contraction, decreasing

Chapter 17 I MECHANICS AND PATHOMECHANICS OF THE SPECIAL CONNECTIVE TISSUES IN THE HAND

343

their contraction efficiency. Surgical transection of the
TCL decreases the muscle force needed to generate a given
pinch load following transection, consistent with the
increased muscle moment arms as the tendons bowstring fol¬
lowing the release of the flexor retinaculum. However,
reported data also reveal a 16-26% increase in shortening
required by the finger flexors to pull the fingers through the
same excursion [8,9]. Despite the improved moment arm in
the finger flexors following TCL release, many patients
demonstrate decreased grip and pinch strength [7,9]. This
decreased strength may result from the active insufficiency
that develops from the bowstringing.

Clinical Relevance

TRANSVERSE CARPAL LIGAMENT RECONSTRUCTION

Patients with carpal tunnel syndrome are frequently treated
with a surgical release of the transverse carpal ligament to
decrease the pressure within the carpal tunnel thereby decom¬
pressing the median nerve. While the majority of patients
report a decrease in pain and improved function, some
patients continue to have pain and decreased grip strength.
One explanation is that release of the transverse carpal liga¬
ment allows increased bowstringing of the flexor tendons to
the thumb and fingers with a resultant loss of flexor strength.
Reconstruction of the ligament by a "transposition flap" repair
using a segment of the palmar aponeurosis appears to stabi¬
lize the flexor tendons and improve grip strength [17,18].

The extensor tendons at the wrist are stabilized by a simi¬
lar extensor retinaculum (Fig. 17.7). Yet some extensor ten¬
dons still move away from the joint surface during wrist
extension, increasing their moment arms. This mobility and
the resultant increase in moment arms help explain why the
strength of the extensor carpi radialis longus and brevis is
greatest when the wrist is extended [10,12]. Active insuffi¬
ciency is avoided in the extensor carpi radialis longus and bre¬
vis because these muscles possess longer muscle fibers pro¬
ducing greater angular excursion during contraction [3,12].

RETINACULAR SYSTEMS AT THE DIGITS

The flexor tendons of the fingers also have an elaborate reti-
nacular, or pulley, system that stabilizes the tendons along the
entire length of the fingers on the volar surface (Fig. 17.8).
This system consists of fibrous bands that attach to the under¬
lying volar plates of the MCP, PIP, and DIP joints or to the
bones of the fingers [20]. Some of these bands encircle the
flexor tendons much like the annular ligament of the superi¬
or radioulnar joint encircles the radius. The five fibrous bands
that run circumferentially across the fingers are called annu¬
lar ligaments [4,32]. They are numbered one to five from
proximal to distal. Three pairs of cruciate pulleys are found
over the volar surface of the proximal and middle phalanges.
Although their typical arrangement is depicted in Figure 17.8,

Figure 17.7: The extensor retinaculum at the wrist stabilizes the
extensor tendons at the wrist.

it is important to recognize that the pulleys exhibit some nor¬
mal variability within the population [11].

These pulleys are essential to the function of the flexor
tendons of the fingers. Studies suggest that the annular liga¬
ments, A2 and A4, are particularly essential to the function of
the flexor digitorum profundus [23,30]. The A3 ligament is
important to the function of the flexor digitorum superficialis
[7]. In addition to affecting the efficiency of the muscles’ flex¬
ion excursion, the A1 pulley of the fingers provides an impor¬
tant radioulnar stabilizing force on the flexor tendons as they
cross the MCP joints [4]. The thumb also has a system of
annular and cruciate ligaments to stabilize the tendon of the
flexor pollicis longus as it traverses the thumb [33].

Clinical Relevance

PULLEY INJURIES IN ROCK CLIMBERS: Pulley ruptures
in the fingers can occur with hyperextension injuries but also
as the result of large forces in the flexor tendons, especially
with the fingers flexed. Rock climbers generate huge flexion
forces in the fingers as they pull themselves up along vertical
rock walls by their fingers. A common hand hold called
"crimping" uses considerable flexion at the PIP joints and
requires large forces in the flexor tendons. This grip puts the
pulleys A2, A3, and A4 at particular risk of rupture [28].

344

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 17.8: The flexor tendons of the fingers are stabilized by an
extensive series of fibrous pulleys. Typically there are five circum¬
ferential, or annular, ligaments (A1-A5) and three pairs of cruci¬
ate ligaments (C1-C3).

The tendons of the extensor digitorum are stabilized by a
less elaborate retinaculum as they cross the MCP joints of the
fingers. The tendons are secured by the sagittal bands that
run from the extensor tendons to the volar plate on the volar
surface of the joint [22] (Fig. 17.9). These bands stabilize the
tendons in the radioulnar direction to prevent dislocation of
these tendons to either side of the fingers.

Extensor digitorum

Figure 17.9: The extensor digitorum tendons of the fingers are
stabilized at the MCP joints by sagittal bands.

Tendon Sheaths

The tendons of the wrist and fingers are encased in synovial
sheaths. The flexor tendons to the radial three fingers have sep¬
arate sheaths at the wrist, palm, and fingers [25,32] (Fig. 17.10).
The sheath for the tendons to the little finger is continuous with
the palmar sheath. The extensor tendons are enclosed by
sheaths only at the wrist. These sheaths reduce the friction of
the tendons as they glide over the wrist and along the finger.
Reduced friction is critical, since the tendons to the fingers slide
several centimeters during full finger motion [1-3].

The synovial sheaths also play an important role in the
nutrition of the flexor tendons in the fingers [5,26]. There
are three vascular sources to these tendons: (a) vessels
proceeding distally from the muscle belly through the
musculotendinous junction, (b) blood vessels proceeding
proximally in the tendon from the bone at the tendons
distal attachment, and (c) vessels entering the dorsal sur¬
face of the tendons via the tiny, fragile vinculae [20] (Fig.
17.11). These three sources leave some regions of the ten¬
dons relatively far from a vascular source. Nutrition, par¬
ticularly in these regions, depends on diffusion across the
synovial sheaths similar to the mechanism for nourishing
articular cartilage.

Clinical Relevance

"NO MAN'S LAND": Hand surgeons refer to the region
of the hand that contains the finger flexor tendons within
their digital sheaths as Zone II. However ; historically this
region has been known as "no man's land" because it was
believed that primary repairs of the flexor digitorum
profundus or superficialis were unsuccessful within this
region because nutrition to the tendons was so tenuous
and scarring and adhesions were so likely [31] (Fig. 17.12).
Disruption of a tendon in this region can easily rupture
the nearby vinculaecompromising an already precarious
blood supply.

Over the last several decades; careful examination and
clinical trials have demonstrated that primary tendon repairs
in this region are not only feasible but desirable over the
alternative of tendon grafts [19,29]. Yet the success of such
repairs hinges a great deal on the postoperative rehabilita¬
tion that occurs. Recognition that synovial fluid diffusing
through the tendon sheath contributes to a tendon's nutri¬
tion has led to the use of early mobilization in the treatment
of tendon repairs. Mobilization assists in bathing the tendon
in synovial fluid while reducing the development of adhe¬
sions. However ; early mobilization also carries the risk of
disrupting the repaired tendon. The clinician must avoid
excessive active or passive tension in the tendon during the
early stages of healing. Chapter 19 discusses the loads sus¬
tained by the finger flexor tendons during activity.

Chapter 17 I MECHANICS AND PATHOMECHANICS OF THE SPECIAL CONNECTIVE TISSUES IN THE HAND

345

Figure 17.10: The synovial sheaths of the tendons to the digits. A. The extensor tendons are encased in synovial sheaths only at the
wrist. B. The flexor tendons to the digits are surrounded by synovial sheaths at the wrist and digits.

Figure 17.11: The blood supply to the flexor tendons in the fingers. Flexor tendons (FDP and FDS) in the fingers receive their blood
supply from (1) the proximal musculotendinous junction, (2) the distal bony attachment, and (3) the tiny blood vessels passing through
the vinculae.

346

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

Figure 17.12: "No man's land" is the area where healing is most
difficult for the fingers' flexor tendons. It encompasses much of
the region where the digital tendons are in their synovial
sheaths.

Pathology of the synovial sheath itself also contributes to
common functional impairments of the fingers. Infection
within the sheath can result from trauma such as puncture
wounds in the hand and can lead to impaired circulation to,
and necrosis of, the flexor tendons and adhesions within the
sheath. As a synovial tissue, the tendon sheath also is suscep¬
tible to inflammatory processes such as rheumatoid arthritis.
Excessive friction within the sheaths and fibrous tunnels also
may contribute to overuse syndromes and inflammation.

Clinical Relevance

TRIGGER FINGER: The tendon sheath complex is suscep¬
tible to inflammation as the result of disease, overuse, or
trauma. The synovial membrane then can exhibit all of the
cardinal signs of inflammation including swelling. As the
sheath swells within the relatively rigid fibrous tunnel
formed by the pulley system, the swelling causes compres¬
sion of the enclosed tendon. Compression of the tendon can
compromise the vascular flow to the tendon and lead to

Figure 17.13: A "trigger finger" becomes stuck moving into either
flexion or extension as the swollen flexor tendon or sheath tries to
squeeze into the fibrous tunnel.

swelling of the tendon itself. As the finger is flexed, the
thickened synovial sheath is pulled proximally through the
pulleys of the finger. Often the thickening can be palpated
in the palm just proximal to the MCP joint. As the finger is
extended, the thickening must reenter the fibrous tunnel, but
the swollen tendon or sheath has difficulty squeezing back
into the narrow channel (Fig. 17.13). Finger extension is
blocked, and the patient reports that the finger is "stuck."
With additional extension force the thickening suddenly
snaps into the tunnel, and the finger extends [5,16,27]. Such
blocks to motion can occur in either flexion or extension as
the thickening slides through any of the individual pulleys.
The block to motion releases so suddenly that the finger
acts like a "trigger." The mechanical block to movement
results from the initial inflammatory process and resultant
thickening. Treatments to reduce the inflammation and pre¬
vent recurrence are most beneficial. These may include anti¬
inflammatory medications, corticosteroid injections, and the
use of splints and patient education to avoid overuse.

Structures That Anchor the Flexor
and Extensor Apparatus of the Fingers

The flexor tendons to the fingers are bound firmly to each
digit by the synovial sheaths and fibrous pulleys just
described. The extensor tendons are stabilized somewhat at
the MCP joints by the intersection of the interossei and lum-
bricals forming the extensor hood mechanism, which is
known by many names including extensor mechanism,
extensor expansion, and dorsal hood (Fig. 17.14). The dis¬
tal attachment of the extensor digitorum, composed of the
central tendon attached to the middle phalanx, and lateral
bands attached to the distal phalanx form the skeleton of the
extensor hood (Chapter 15). Distal attachments of the intrin¬
sic muscles of the fingers expand into a fibrous sheet that
encompasses the extensor digitorum tendons forming a fibrous
covering over the dorsum of the phalanges of the fingers. A
similar fibrous expansion from intrinsic muscles of the thumb

Chapter 17 I MECHANICS AND PATHOMECHANICS OF THE SPECIAL CONNECTIVE TISSUES IN THE HAND

347

Figure 17.14: The extensor hood mechanism consists
of the central tendon and lateral bands of the exten¬
sor digitorum and the fibrous sheet that is an exten¬
sion of the distal attachments of the lumbricals and
interossei. A. Lateral view. B. Dorsal view.

forms a fibrous expansion over the dorsum of the phalanges of
the thumb, blending with the tendons of extensor pollicis
longus and extensor pollicis brevis.

Careful inspection of each finger reveals additional soft tis¬
sue structures that contribute to the stability of the flexor ten¬
dons and extensor hood mechanisms as they travel the length
of each finger. These additional structures play an integral role
in maintaining a balance between the flexor and extensor mus¬
cles but also participate in the development of common struc¬
tural deformities in the hand. A cross-sectional view of a finger
at the level of the MCP joint reveals interconnection among
the many structures crossing the joint. The interconnection
occurs at the lateral borders of the volar surface of the MCP
joint [31] (Fig. 17.15). The sagittal bands of the extensor digi¬
torum, the flexor tendon sheath, and the collateral ligaments
all join with the volar plate and the transverse intermetacarpal
ligaments at this intersection. The opposing flexor and exten¬
sor muscle groups also are linked at the PIP joints by the
oblique and transverse retinacular ligaments that run from the
flexor sheath on the volar surface to the extensor expansion on
the dorsal side of the finger [31] (Fig. 17.16). These ligaments
suspend the lateral bands of the extensor hood mechanism
over the lateral aspects of the dorsum of the finger. Thus the
flexor and extensor muscles are actually connected to one
another throughout much of the length of the finger.

Figure 17.15: Interconnections among the tendons and ligaments
at the MCP joint of a finger. A cross section of an MCP joint of a
finger reveals the interconnections among the flexor tendon
sheath, the extensor tendon, the collateral ligaments, the trans¬
verse intermetacarpal ligaments, and the volar plate.

Figure 17.16: The oblique and transverse retinacular
ligaments at the PIP joint of a finger connect the flexor
tendon sheaths and the extensor hood.

Extensor expansion

Central tendon Lateral band

348

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

These linkages not only help stabilize the tendons circum¬
ferentially on the finger but also contribute to the overall bal¬
ance of opposing forces within the finger.

The importance of the balance provided by these ligaments
is most evident in its absence. The classic hand deformities,
swan neck and boutonniere, found in patients with rheuma¬
toid arthritis provide vivid demonstrations of the impact of
pathology involving the structures that balance the flexion and
extension forces in the fingers [24]. Both deformities are pre¬
cipitated by synovitis at the PIP joint of any finger.

Clinical Relevance

BOUTONNIERE AND SWAN NECK DEFORMITIES: The

boutonniere deformity is characterized by flexion of the PIP
joint and hyperextension of the DIP joint of the finger (Fig.
17.17). In this deformitythe swelling of the PIP joint resulting
from the inflammation of the joint's synovium is located par¬
ticularly in the dorsum of the joint This puts prolonged ten¬
sion on the extensor digitorum tendon , especially the central
tendon and the fibrous sheet connecting the lateral bands of
the extensor mechanism. Prolonged swelling causes these
structures to stretch. As they stretch , the lateral bands gradu¬
ally slide toward the volar surface of the finger. When the
bands slide volarly past the axis of flexion and extension of
the joint they begin to exert a flexion moment on the joint
(Fig. 17.18). The resulting PIP flexion puts additional stretch on
the central tendon , which can eventually rupture. The PIP
joint then protrudes through the extensor hood like a button
through a buttonhole\ giving the deformity its name. At the
same time the volar migration of the lateral bands causes an
increased extension pull from the intact extensor tendon at
the DIP joint and that joint hyperextends.

A similar mechanism in reverse is at work in a swan neck
deformity ; This deformity is characterized by hyperextension of
a finger's PIP joint with concomitant flexion of the DIP joint (Fig.
17.19). In this deformity the PIP joint swelling has a greater
effect on the sides of the PIP joint capsule. Prolonged swelling
in this region puts a prolonged stretch on the lateral joint cap¬
sule and the retinacular ligaments. The resulting laxity in the

Figure 17.17: A boutonniere deformity in an individual with
rheumatoid arthritis exhibits flexion of the PIP joint and hyperex¬
tension of the DIP joint. (Reprinted from the AHPA Teaching Slide
Collection Second Edition now known as the ARHP Assessment
and Management of the Rheumatic Diseases: The Teaching Slide
Collection for Clinicians and Educators. Copyright 1997. Used by
permission of the American College of Rheumatology.)

Ruptured central tendon

Figure 17.18: The mechanism of a boutonniere deformity. A cross
section of a PIP joint of a finger reveals how rupture of the cen¬
tral tendon of the extensor hood allows the lateral bands to slip
to the volar side of the joint, producing flexion at the PIP. The
intact distal attachment of the lateral bands produces hyperex¬
tension of the DIP joint.

retinacular ligaments allows the lateral bands of the extensor
hood to migrate dorsally, increasing their extension moment
arms and the extension moment at the PIP (Fig. 17.20). The PIP
joint is pulled into hyperextension by this increased extension
moment stretching the tendon of the flexor digitorum profun¬
dus , which responds by pulling the DIP into flexion.

Swan neck deformities are frequently associated with
another classic hand deformity , ulnar drift of the fingers at
the MCP joint. Ulnar drift is also the result of a loss of bal¬
ance between the flexor and extensor mechanisms at the
fingers, but because the external loads on the fingers are
important contributing factors, this deformity is described in
Chapter 19 within the context of the forces sustained by the
fingers and thumb during activity.

Figure 17.19: A swan neck deformity in an individual with rheuma¬
toid arthritis consists of hyperextension of the PIP joint with flexion
of the DIP joint. (Reprinted from the AHPA Teaching Slide
Collection Second Edition now known as the ARHP Assessment and
Management of the Rheumatic Diseases: The Teaching Slide
Collection for Clinicians and Educators. Copyright 1997. Used by
permission of the American College of Rheumatology.)

Chapter 17 I MECHANICS AND PATHOMECHANICS OF THE SPECIAL CONNECTIVE TISSUES IN THE HAND

349

Plane of cross section

Figure 17.20: The mechanism of a swan neck deformity. A cross
section of a PIP joint of a finger reveals how stretch of the reti-
nacular ligaments allows the lateral bands of the extensor hood
to slip dorsally, increasing the extension moment at the PIP joint
and causing hyperextension. The hyperextension stretches the
flexor digitorum profundus, producing flexion at the DIP joint.

SUMMARY

This chapter presents the special connective tissue structures
that make critical contributions to the mechanics of the hand.
These structures serve several roles that include stabilizing
skin, muscles, and neurovascular bundles; protecting under¬
lying structures; and contributing to the balance between the
flexor and extensor apparatus. Many common hand deformi¬
ties and joint impairments result from disruption of these
connective tissue structures. A loss of balance between the
flexor and extensor mechanisms contributes to the classic
hand deformities seen in patients with rheumatoid arthritis.
Another system that is essential to the maintenance of bal¬
ance within the hand is the intrinsic muscle group. These
muscles are presented in the following chapter.

References

1. Aleksandrowicz R, Pagowski S: Functional anatomy and bio¬
engineering of the third finger of the human hand. Folia
Morphol (Warsaw) 1981; 40: 181-192.

2. An KN, Ueba Y, Chao EY, et al.: Tendon excursion and moment
arm of index finger muscles. J Biomech 1983; 16: 419-425.

3. Brand PW, Beach RB, Thompson DE: Relative tension and
potential excursion of muscles in the forearm and hand. J Hand
Surg [Am] 1981; 6: 209-219.

4. Brand PW, Cranor KC, Ellis JC: Tendon and pulleys at the
metacarpophalangeal joint of a finger. J Bone Joint Surg 1975;
57A: 779-784.

5. Ferlic DC: Rheumatoid flexor tenosynovitis and rupture. Hand
Clin 1996; 12: 561^72.

6. Gelberman RH, Amiel D, Rudolph RM, Vance RM:
Dupuytrens contracture. J Bone Joint Surg 1980; 62-A: 425^32.

7. Hamman J, Ali A, Phillips C, et al.: A biomechanical study of the
flexor digitorum superficialis: effects of digital pulley excision
and loss of the flexor digitorum profundus. J Hand Surg [Am]
1997; 22A: 328-335.

8. Kang HJ, Lee SG, Phillips CS, Mass DP: Biomechanical
changes of cadaveric finger flexion: the effect of wrist position

and of the transverse carpal ligament and palmar and forearm
fasciae. J Hand Surg [Am] 1996; 21A: 963-968.

9. Kiritsis PG, Kline SC: Biomechanical changes after carpal tun¬
nel release: a cadaveric model for comparing open, endoscopic,
and step-cut lengthening techniques. J Hand Surg [Am] 1995;
20: 173-180.

10. Lieber RL, Friden J: Musculoskeletal balance of the human
wrist elucidated using intraoperative laser diffraction. J
Electromyogr Kinesiol 1998; 8: 93-100.

11. Lin GT, Amadio PC, An KN, Cooney WP: Functional anatomy
of the human digital flexor pulley system. J Hand Surg [Am]
1989; 14A: 949-956.

12. Loren GJ, Shoemaker SD, Bmkholder TJ, et al.: Human wrist
motors: biomechanical design and application to tendon trans¬
fers. J Biomech 1996; 29: 331-342.

13. Low CK, Pereira BP, Ng RTH, et al.: The effect of the extent of
Al pulley release on the force required to flex the digits. A
cadaver study on the thumb, middle and ring fingers. J Hand
Surg [Br] 1998; 23B: 46^9.

14. Magee DA: Orthopedic Physical Assessment. Philadelphia: WB
Saunders, 1998.

15. Moore KL: Clinically Oriented Anatomy. Baltimore: Williams &
Wilkins, 1980.

16. Mulpruek P, Prichasuk S, Orapin S: Trigger finger in children. J
Pediatr Orthop 1998; 18: 239-241.

17. Netscher D, Lee M, Thornby J, Polsen C: The effect of division
of the transverse carpal ligament on flexor tendon excursion. J
Hand Surg 1997; 22A: 1016-1024.

18. Netscher D, Mosharrafa A, Lee M, et al.: Transverse carpal lig¬
ament: its effect on flexor tendon excursion, morphologic
changes of the carpal canal, and on pinch and grip strengths
after open carpal tunnel release. Plast Reconstr Surg 1997; 100:
636-642.

19. Newmeyer WL 3rd, Manske PR: No man’s land revisited: the
primary flexor tendon repair controversy. J Hand Surg 2004;
29 A: 1-5.

20. Ochiai N, Matsui T, Miyaji N, et al.: Vascular anatomy of flex¬
or tendons. I. Vincular system and blood supply of the profun¬
dus tendon in the digital sheath. J Hand Surg [Am] 1979; 4:
321-330.

21. Rayan GM: Palmar fascial complex anatomy and pathology in
Dupuytrens disease. Hand Clin 1999; 15: 73-86.

22. Rayan GM, Murray D, Chung KW, Rohrer M: The extensor
retinacular system at the metacarpophalangeal joint. Anatomical
and histological study. J Hand Surg [Br] 1997; 22B: 585-590.

23. Rispler D, Greenwald D, Shumway S, et al.: Efficiency of the
flexor tendon pulley system in human cadaver hands. J Hand
Surg [Am] 1996; 21A: 444-450.

24. Rizio L, Belsky MR: Finger deformities in rheumatoid arthritis.
Hand Clin 1996; 12: 531-540.

25. Romanes GJE: Cunninghams Textbook of Anatomy. Oxford:
Oxford University Press, 1981.

26. Rosenblum NI, Robinson SJ: Advances in flexor and extensor
tendon management. In: Moran CA, ed. Hand Rehabilitation.
New York: Churchill Livingstone, 1986; 17-44.

27. Salter RB: Textbook of Disorders and Injuries of the
Musculoskeletal System. 3rd ed. Baltimore: Williams & Wilkins,
1999.

28. Schoffl VR, Einwag F, Strecker W, Schoffl I: Strength measure¬
ment and clinical outcome after pulley ruptures in climbers.
Med Sci Sports Exerc 2006; 38: 637-643.

350

Part II I KINESIOLOGY OF THE UPPER EXTREMITY

29. Su BW, Solomons M, Barrow A, et al.: Device for zone-II flexor
tendon repair. J Bone Joint Surg 2005; 87: 923-935.

30. Tomaino M, Mitsionis G, Basitidas J, et al.: The effect of partial
excision of the A2 and A4 pulleys on the biomechanics of finger
flexion. J Hand Surg [Br] 1998; 23B: 50-52.

31. Tubiana R, Thomine JM, Mackin E: Examination of the Hand
and Wrist. Philadelphia: WB Saunders, 1996.

32. Williams P, Bannister L, Berry M, et al.: Grays Anatomy The
Anatomical Basis of Medicine and Surgery, Br. ed. London:
Churchill Livingstone, 1995.

33. Zissimos AG, Szabo RM, Yinger KE, Sharkey NA:
Biomechanics of the thumb flexor pulley system. J Hand Surg
[Am] 1994; 19A: 475-479.

CHAPTER

Mechanics and Pathomechanics
of the Intrinsic Muscles
of the Hand

CHAPTER CONTENTS

PRIMARY INTRINSIC MOVERS OF THE THUMB .352

Abductor Pollicis Brevis .352

Flexor Pollicis Brevis .353

Opponens Pollicis .354

Adductor Pollicis.354

PRIMARY INTRINSIC MOVERS OF THE LITTLE FINGER.356

Abductor Digiti Minimi (Also Known As the Abductor Digiti Quinti) .357

Flexor Digiti Minimi (Also Known As Flexor Digiti Quinti) .358

Opponens Digiti Minimi (Also Known As Opponens Digiti Quinti) .358

INTEROSSEI AND LUMBRICALS.359

Dorsal Interossei.359

Palmar Interossei .361

Lumbrical Muscles.361

CLASSIC DEFORMITIES RESULTING FROM MUSCLE IMBALANCES IN THE HAND .364

Ulnar Nerve Injury .364

Median Nerve Injury.365

Radial Nerve Injury.366

Sensory Deficits Associated with Nerve Injuries to the Hand .367

SUMMARY.368

C hapter 15 discusses the muscles of the forearm including the extrinsic muscles of the hand. However, the nor¬
mal function of the extrinsic muscles is inextricably intertwined with the function of the intrinsic muscles. Few
if any normal functional movements of the hand use only the extrinsic or the intrinsic muscle groups. The
hand functions by using a delicately balanced combination of muscles from both groups. To understand the integrated
activity of these muscle groups, the clinician must first appreciate the potential of the individual muscles. The purposes
of this chapter are to

■ Describe the structure and function of the individual intrinsic muscles of the hand
■ Review the activity of the intrinsic muscles during movements of the hand

■ Discuss the contribution to dysfunction in the hand made by impairments of the intrinsic muscles
■ Explain the mechanics of the deformities of the hand resulting from weakness of the intrinsic muscles

351

352

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Although the intrinsic muscles of the hand frequently are classified by muscle attachment, this chapter arranges the
muscles into four functional groups: (a) the primary intrinsic movers of the thumb, (b) the primary intrinsic movers of
the little finger, (c) the interossei, and (d) the lumbrical muscles. This classification scheme helps the clinician recognize
the interaction of individual muscles.

PRIMARY INTRINSIC MOVERS
OF THE THUMB

The muscles that are the primary intrinsic movers of the
thumb are the abductor pollicis brevis, flexor pollicis brevis,
opponens pollicis, and adductor pollicis (Fig. 18.1). The first
three of these muscles are referred to as the thenar muscles,
forming the muscle mass overlying the metacarpal of the
thumb. These muscles are innervated by the median nerve.
The adductor pollicis is critical to the function of the thumb,
particularly in pinch, but is distinct from the thenar muscle
mass. It lies in the palm of the hand and is innervated by the
ulnar nerve. The thenar muscles are rarely tight. Therefore,
only impairments associated with weakness of the thenar mus¬
cles are discussed below. In contrast, the adductor pollicis
exhibits abnormal tightness in some hands, so tightness of the
adductor pollicis is discussed as a possible impairment.

Figure 18.1: The primary intrinsic movers of the thumb include
the abductor pollicis brevis, the flexor pollicis brevis, the oppo¬
nens pollicis, and the adductor pollicis.

Abductor Pollicis Brevis

The abductor pollicis brevis is the most prominent muscle of the
thenar muscle mass lying on the volar and radial aspect of the
thenar prominence (Muscle Attachment Box 18.1).

ACTIONS

MUSCLE ACTION: ABDUCTOR POLLICIS BREVIS

Action

Evidence

Abduction (palmar abduction) of the
thumb's CMC joint

Supporting

Medial rotation of the thumb's CMC joint

Supporting

Opposition of the thumb's CMC joint

Supporting

Abduction of the thumb's MCP joint

Inadequate

Flexion of the thumb's MCP joint

Inadequate

Extension of the thumb's IP joint

Supporting

The primary actions of the abductor pollicis brevis are those
at the CMC joint. The abductor pollicis brevis is ideally posi¬
tioned on the volar surface of the CMC joint of the thumb to
abduct (palmar abduction) that joint. It has a larger abduction
moment arm than the abductor pollicis longus and is opti¬
mally positioned to pull the thumb into abduction [7,27,34].
It is approximately only one third the size of the abductor pol¬
licis longus [5] and consequently is limited in force produc¬
tion [15]. However, abduction of the CMC joint rarely occurs
against resistance, so large forces are not necessary [5].
Electromyographic (EMG) studies support its role as one of
the primary abductors of the CMC of the thumb, with the
abductor pollicis longus at best an accessory abductor [7-9,39].

MUSCLE ATTACHMENT BOX 18.1

ATTACHMENTS AND INNERVATION
OF THE ABDUCTOR POLLICIS BREVIS

Proximal attachment: Flexor retinaculum and the
tubercles of the scaphoid and trapezium

Distal attachment: Radial side of the base of the
thumb's proximal phalanx and the extensor expan¬
sion of the EPL

Innervation: Median nerve, C8, and T1

Palpation: The abductor pollicis brevis can be pal¬
pated on the radial aspect of the thenar eminence
during active abduction of the thumb.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

353

There are no known studies that specifically examine the
abductor pollicis brevis as a medial rotator of the CMC joint.
Medial rotation, also known as pronation, is virtually insepara¬
ble from abduction or flexion at the CMC joint [10,14] and is
an automatic consequence of the active abduction resulting
from contraction of the abductor pollicis brevis [13]. Since
abduction and medial rotation are components of opposition,
the abductor pollicis brevis also contributes to opposition [32].

The attachment of the abductor pollicis brevis on the radial
side of the proximal phalanx of the thumb explains its reported
role as an abductor of the MCP joint of the thumb. In some
individuals the MCP joint functions more as a hinge joint,
allowing only flexion and extension [1,12]. Therefore, the role
of the abductor pollicis brevis at the MCP joint is probably
variable. The muscle is located so that it can produce
abduction of the MCP joint, but only in individuals
who possess the available motion.

The abductor pollicis brevis also is described as a flexor of
the MCP joint of the thumb [19]. EMG studies of the thumb
do not offer selective analysis of this role for the abductor pol¬
licis brevis and therefore can neither confirm nor refute this
action. Additional research is needed to determine any con¬
tribution to MCP flexion [8,9,11].

Although anatomy books typically describe an attachment
to the extensor hood of the thumb, there is little or no men¬
tion of IP extension in the list of actions by the abductor polli¬
cis brevis [18,30]. Classic studies repeatedly demonstrate
EMG activity of the abductor pollicis brevis during extension
of the thumb [2,8,11,36]. A study of 11 subjects whose exten¬
sor pollicis longus was temporarily paralyzed reveals continued
ability to extend the IP joint of the thumb, although through
less than the full range of motion (ROM) [36]. This study
demonstrates moderate activity in both the abductor pollicis
brevis and the flexor pollicis brevis during IP extension with¬
out activity in the extensor pollicis longus, suggesting that one
or both of these muscles contributes to the motion by way of
the thumbs extensor expansion. Disruption of the attachment
of the abductor pollicis brevis at the extensor hood also pro¬
duces an extension impairment [24]. These studies demon¬
strate the abductor pollicis brevis muscle s participation in IP
extension. Under normal circumstances this activity may be
important in stabilizing the tendon of the extensor pollicis
longus. In the absence of the extensor pollicis longus, however,
the thenar muscles, including the abductor pollicis brevis, may
provide functionally useful IP extension.

EFFECTS OF WEAKNESS OF THE ABDUCTOR
POLLICIS BREVIS

Weakness of the muscle is a common manifestation of a
median nerve palsy and often, but not always, occurs with
simultaneous weakness of the other thenar muscles.
Weakness of the abductor pollicis brevis usually is quite
apparent upon inspection. The muscle is superficial, so atro¬
phy of its muscle belly results in a flattening of the thenar
eminence (Fig. 18.2). Weakness of the abductor pollicis
brevis weakens abduction of the CMC joint of the thumb.

Figure 18.2: Atrophy of the APB is easily visible as a flattening
of the thenar eminence. Seen here is an individual with wasting
of the thenar eminence resulting from denervation.

Active abduction of the thumb is necessary to position the
thumb for grasp or pinch.

The antagonist to the abductor pollicis brevis is the exten¬
sor pollicis longus, which lies ulnarly on the dorsum of the
thumb and is positioned ideally to adduct the CMC joint of
the thumb. Consequently, weakness of the abductor pollicis
brevis upsets the delicate balance of strengths within the
thumb. The unbalanced pull of the extensor pollicis longus
interferes with the ability to position the thumb for pinch or
grip and contributes to the characteristic thumb deformity
ape thumb. Details of the common hand deformities result¬
ing from muscle imbalances are presented at the end of this
chapter. Weakness of the abductor pollicis brevis also leads to
decreased strength of pinch and grasp.

Flexor Pollicis Brevis

The flexor pollicis brevis lies on the medial aspect of the abduc¬
tor pollicis brevis and is approximately the same size as the
abductor pollicis brevis [5,15] (Muscle Attachment Box 18.2).

ACTIONS

MUSCLE ACTION: FLEXOR POLLICIS BREVIS

Action

Evidence

Flexion of the thumb's CMC joint

Supporting

Abduction and medial rotation of the
thumb's CMC joint

Inadequate

Flexion of the thumb's MCP joint

Supporting

Extension of the thumb's IP joint

Supporting

The primary actions of the flexor pollicis brevis are flexion
of the thumbs CMC and MCP joints. The medial alignment

354

Part II I KINESIOLOGY OF THE UPER EXTREMITY

MUSCLE ATTACHMENT BOX 18.2

ATTACHMENTS AND INNERVATION
OF THE FLEXOR POLLICIS BREVIS

Proximal attachment: The superficial portion comes
from the flexor retinaculum and tubercle of the tra¬
pezium. The deep portion arises from the capitate
and trapezoid bones.

Distal attachment: Radial side of the base of the
proximal phalanx of the thumb. There is frequently
a sesamoid bone within the tendon. The muscle may
also contribute to the extensor hood of the EPL.

Innervation: The superficial head is usually supplied
by the median nerve, T1 and perhaps C8. The deep
head is usually supplied by the same spinal roots of
the ulnar nerve.

Palpation: The flexor pollicis brevis is palpated on
the ulnar aspect of the thenar eminence during
flexion of the thumb's CMC joint while the MCP
joint is extended.

MUSCLE ATTACHMENT BOX 18.3

ATTACHMENTS AND INNERVATION
OF THE OPPONENS POLLICIS

Proximal attachment: Flexor retinaculum and the
tubercle of the trapezium

Distal attachment: The lateral half of the entire
length of the metacarpal of the thumb

Innervation: Median nerve, T1 and perhaps C8. It
may also receive innervation from the ulnar nerve.

Palpation: The opponens pollicis can be palpated along
the radial aspect of the palmar surface of the thumb's
metacarpal during gentle opposition of the thumb. The
palpating digit must be slipped between the radial bor¬
der of the abductor pollicis brevis and the metacarpal.
Vigorous opposition will generate contraction of the
abductor pollicis brevis, making palpation of the oppo¬
nens pollicis impossible.

of the flexor pollicis brevis positions it to flex the CMC and
MCP joints of the thumb [2,11]. Its attachment into the exten¬
sor hood also suggests a role in IP extension like that of the
abductor pollicis brevis. EMG data reveal activity of the flexor
pollicis brevis during IP extension in the absence of extensor
pollicis longus activity [36]. The action of the flexor pollicis
brevis also is linked with the action of the opponens pollicis
and the adductor pollicis [2,9]. The superficial portion of the
flexor pollicis brevis, innervated by the median nerve, is some¬
times attached directly to the opponens pollicis. This portion
is best suited to position the CMC joint of the thumb [5].
The deeper portion, innervated by the ulnar nerve, is aligned
more closely with the adductor pollicis and may function with
that muscle at the thumb s MCP joint during pinch.

EFFECTS OF WEAKNESS

Weakness of the flexor pollicis brevis weakens the actions of
flexion at the CMC and MCP joints of the thumb, which may
have profound functional ramifications, particularly in pinch.
This effect is examined more closely in Chapter 19.

Opponens Pollicis

The opponens pollicis is the second largest intrinsic muscle of
the thumb and thus offers considerable strength to the base
of the thumb [5,25] (Muscle Attachment Box. 18.3).

ACTIONS

MUSCLE ACTION: OPPONENS POLLICIS

Action

Evidence

Opposition of the thumb's CMC joint

Supporting

Opposition is the combination of abduction, flexion, and medial
rotation of the CMC joint of the thumb. Some references
report that the opponens pollicis performs these individual
actions, but it is important to recognize that contraction of the
opponens pollicis produces abduction, flexion, and medial
rotation simultaneously, that is, opposition. Thus the opponens
pollicis contributes to the actions of both the abductor pollicis
brevis and the flexor pollicis brevis and adds important
strength for both muscles [2,5,8]. In a study by Cooney et al.
in which the flexor pollicis brevis was studied as part of the
opponens pollicis, the opponens pollicis acted as a secondary
flexor of the thumb, with increased activity as the force of
pinch increased [9]. In this same study the opponens pollicis
and abductor pollicis brevis were the primary abductors of the
thumb. These data indicate that the opponens pollicis dupli¬
cates and reinforces the actions of the other thenar muscles.

EFFECTS OF WEAKNESS

Weakness of the opponens pollicis is usually accompanied by
weakness of either or both the abductor pollicis brevis and
flexor pollicis brevis. Weakness of the opponens pollicis leads
to difficulty in positioning and stabilizing the CMC joint of
the thumb during pinch and grasp.

Adductor Pollicis

The adductor pollicis is the largest of the intrinsic muscles of
the hand with a physiological cross-sectional area similar to that
of the extensor carpi radialis longus and the flexor carpi radialis
[5,15,25] (Muscle Attachment Box 18.4). This remarkable size
reveals that the muscle is specialized for force production.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

355

MUSCLE ATTACHMENT BOX 18.4

ATTACHMENTS AND INNERVATION
OF THE ADDUCTOR POLLICIS

Proximal attachment: The oblique head attaches to
the anterior surfaces of the bases of the second,
third, and perhaps fourth metacarpals; the
capitate; the palmar carpal ligaments; and the syn¬
ovial sheath of the flexor carpi radialis. The transverse
head attaches to the distal two thirds of the anterior
surface of the long finger's metacarpal bone.

Distal attachment: The base of the thumb's proximal
phalanx and the extensor hood of the EPL. There is
usually a sesamoid bone within the tendon of the
oblique head.

Innervation: Ulnar nerve, C8, and T1

Palpation: The adductor pollicis cannot be palpated.

ACTIONS

MUSCLE ACTION: ADDUCTOR POLLICIS

Action

Evidence

Adduction of the thumb's CMC joint

Supporting

Flexion of the thumb's CMC joint

Supporting

Flexion of the thumb's MCP joint

Supporting

Adduction of the thumb's MCP joint

Inadequate

Extension of the thumb's IP joint

Supporting

The primary actions of the adductor pollicis are flexion and
adduction of the CMC joint and flexion of the MCP joint of
the thumb. Adduction of the CMC joint of the thumb is
defined as movement of the thumb toward (or beyond) the
palm of the thumb in a plane perpendicular to the plane of
the palm. The adductor pollicis can adduct the thumb only
as far as the palm (Fig. 18.3). It is the extensor pollicis
longus that is aligned to adduct the thumb through its full
excursion. EMG data reveal that the adductor pollicis and
the extensor pollicis longus are the primary muscles of
adduction of the CMC joint [5,9]. Active adduction is most
functional in pinch when the thumb is positioned in abduc¬
tion but must be pulled toward the fingers to maintain pinch
(Fig. 18.4).

EMG data also support the role of the adductor pollicis as
a flexor of both the CMC and MCP joints of the thumb [9].
The transverse head of the adductor pollicis crosses both the
CMC and the MCP joints at almost a 90° angle of application
at both [5] (Fig. 18.5). Its moment arm at the CMC joint is
greater than the length of the metacarpal bone. Thus the
length of its moment arm and the size of its physiological cross-
sectional area make the adductor pollicis an extraordinarily
powerful muscle for flexion at the CMC and MCP joints.

Extensor pollicis
longus

Figure 18.3: The primary adductors of the thumb are the adduc¬
tor pollicis and the extensor pollicis longus. A. The adductor pol¬
licis adducts the thumb to the palm. B. The extensor pollicis
longus adducts the thumb past the palm (retropulsion).

Figure 18.4: The adductor pollicis muscle supplies much of the
force of pinch.

356

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 18.5: The angle of application of the larger, transverse
head of the adductor pollicis is almost 90° at both the CMC and
MCP joints of the thumb.

Like the abductor pollicis brevis, the adductor pollicis is
reported to move the MCP joint of the thumb in the plane
of abduction and adduction. The muscles contribution to
actual adduction excursion depends on the joints shape,
although it appears to provide important medial stability
regardless of the joints potential for adduction excursion.
The muscle also has an attachment in the extensor hood of
the thumb and may participate in IP extension with the
abductor and flexor pollicis brevis muscles [9,18,36,40].

EFFECTS OF TIGHTNESS AND WEAKNESS

Tightness of the adductor pollicis may occur in the presence of
weakness of the thenar muscles, with a resulting loss of mus¬
cle balance. Tightness of the adductor pollicis limits abduction
and extension flexibility of the CMC and extension ROM of
the MCP joint. These restrictions prevent movement of the
thumb away from the palm, which has profound negative
effects on the mechanics of pinch and grasp. Severe tightness
of the adductor pollicis and flexor pollicis longus contributes to
the thumb-in-palm deformity, rendering the thumb useless
and compromising the hygiene of the involved hand [29].

Figure 18.6: Froment's sign. Ability to hold a piece of paper
between thumb and palm tests the strength of the adductor
pollicis.

Weakness of the adductor pollicis produces weakness in
flexion and adduction of the CMC joint and flexion of the
MCP joint of the thumb. These impairments can produce
severe functional deficits in pinch and grasp [9,20].

Clinical Relevance

FROMENT'S SIGN: As noted[ the adductor pollicis is the
largest of the intrinsic muscles of the hand. Weakness in
thumb adduction may be the most reliable means of identi¬
fying an ulnar nerve palsy [21]. Froment's sign is a classic
method to assess adductor pollicis strength. A patient is
asked to hold a piece of paper between the thumb and palm
while the examiner tries to pull the paper away (Fig. 18.6).

An individual with normal strength will be able to hold the
paper without difficulty , but an individual with
weakness of the adductor pollicis wiii have difficulty
maintaining a hold on the paper.

PRIMARY INTRINSIC MOVERS
OF THE LITTLE FINGER

The hypothenar muscles provide the primary intrinsic control
of the little finger (Fig. 18.7). The palmar interosseus muscle
to the little finger supplies some additional movement and is
discussed with the other interossei. The hypothenar muscles
are innervated by the ulnar nerve. Their attachments and
actions are similar to those of their thenar counterparts. All of the
joints of the little finger are influenced by these three muscles.

The CMC articulation of the little finger usually is
described as a gliding joint, but Chapter 14 demonstrates that
this articulation is more mobile than any other CMC articula¬
tion except the thumb. Its surfaces resemble a saddle that
allows forward glide and opposition of the little finger [17].

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

357

Figure 18.7: The primary intrinsic movers of the little finger
include the abductor digiti minimi, flexor digiti minimi, and
opponens digiti minimi.

The hypothenar muscles have important effects on this joint.
The actions of the hypothenar muscles at the CMC joint are
described differently by various authors. Some sources report
that the muscular actions of these muscles produce the typi¬
cal motions of a saddle joint, including flexion, rotation, and
opposition (Fig. 18.8). Other sources describe the actions
according to the direction of movement of the metacarpal,
including volar glide and rotation. In this chapter, the actions
of the hypothenar muscles at the CMC joint are reported as
the actions at a saddle joint, but it is recognized that the joint
is only a very mobile gliding joint. Isolated weakness of any of
the hypothenar muscles is difficult to identify. The effects of
weakness are presented together following the presentation
of all three muscles. The effects of intrinsic muscle tightness
in the fingers are similar in all of the fingers and are presented
later in this chapter.

Abductor Digiti Minimi (Also Known
As the Abductor Digiti Quinti)

The abductor digiti minimi is larger than the abductor polli-
cis brevis and is capable of considerable force production
[5,15,25] (Muscle Attachment Box 18.5).

Figure 18.8: Motion at the CMC articulation of the little finger
often is described as flexion, rotation, and opposition.

ACTIONS

MUSCLE ACTION: ABDUCTOR DIGITI MINIMI

Action

Evidence

Abduction of the little finger's MCP joint

Supporting

Flexion of the little finger's MCP joint

Supporting

Flexion of the little finger's CMC joint

Supporting

Extension of the little finger's IP joints

Supporting

MUSCLE ATTACHMENT BOX 18.5

ATTACHMENTS AND INNERVATION
OF THE ABDUCTOR DIGITI MINIMI

Proximal attachment: Pisiform bone, pisohamate lig¬
ament, and the tendon of the flexor carpi ulnaris

Distal attachment: Ulnar aspect of the base of the
little finger's proximal phalanx and into the exten¬
sor hood

Innervation: Ulnar nerve, T1, and perhaps C8

Palpation: The abductor digiti minimi is palpated on
the ulnar aspect of the hypothenar eminence dur¬
ing abduction of the little finger's MCP joint.

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Part II I KINESIOLOGY OF THE UPER EXTREMITY

The primary actions of the abductor digiti minimi occur
at the MCP and CMC joints of the little finger. Abduction
of the MCP joint of the little finger is the well-recognized
role of the abductor digiti minimi [5,19,30,40]. The action of
the abductor digiti minimi at the CMC joint is often
ignored, even though its attachment on the pisiform pro¬
duces a large moment arm for flexion at this joint. EMG and
clinical studies indicate a clear role for the abductor digiti
minimi in stabilizing and flexing the CMC joint. Despite
noting an attachment to the extensor hood of the little
finger, few anatomy sources report any participation in
extension of the IP joints [30,31,40]. Clinical studies and
EMG data support the role of the abductor digiti minimi in
both MCP flexion and IP extension [2,5].

Flexor Digiti Minimi (Also Known
As Flexor Digiti Quinti)

the muscle s attachment to the hook of the hamate reveals
that the flexor digiti minimi acts with the abductor digiti min¬
imi to flex the CMC articulation of the little finger, and EMG
studies support this role [3,11].

Opponens Digiti Minimi (Also Known
As Opponens Digiti Quinti)

The opponens digiti minimi is the largest and strongest of the
hypothenar muscles [5,15,25] (Muscle Attachment Box 18.7).
Like the opponens pollicis, the opponens digiti minimi affects
only the CMC joint.

ACTIONS

MUSCLE ACTION: OPPONENS DIGITI MINIMI

Action

Evidence

Opposition of the little finger's CMC joint

Supporting

The flexor digiti minimi is the smallest and weakest of the
hypothenar muscles [5,15,25] (Muscle Attachment Box 18.6).

ACTIONS

MUSCLE ACTION: FLEXOR DIGITI MINIMI

Action

Evidence

Flexion of the little finger's MCP joint

Supporting

Abduction of the little finger's MCP joint

Supporting

Flexion of the little finger's CMC joint

Supporting

Extension of the little finger's IP joints

Supporting

The flexor digiti minimi is functionally important at all of the
joints of the little finger. The accepted action of the flexor dig¬
iti minimi is flexion of the MCP joint, but its attachment into
the extensor hood suggests a role in IP extension. This role is
supported by EMG and clinical observations [2,5,11]. These
same studies report participation in abduction of the MCP
joint along with the abductor digiti minimi. Observation of

MUSCLE ATTACHMENT BOX 18.6

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITI MINIMI

Proximal attachment: Flexor retinaculum and the
hook of the hamate

Distal attachment: Ulnar aspect of the base of the
little finger's proximal phalanx and into the exten¬
sor hood. Its tendon may contain a sesamoid bone.

Innervation: Ulnar nerve # T1, and perhaps C8

Palpation: The flexor digiti minimi is palpated on
the radial aspect of the hypothenar eminence.

Opposition of the CMC joint of the little finger is the
acknowledged action of the opponens digiti minimi [40] and
is supported by EMG data [2,5,9,19,30]. Opposition of the
little finger is defined as flexion of the CMC joint with
rotation so that the pulp of the finger turns toward the thumb.
Consequently, some sources also note that the opponens
digiti minimi flexes the CMC joint. Opposition of the little
finger contributes to the volar arch that is formed by cupping
the hand (Fig. 18.9).

EFFECTS OF WEAKNESS OF THE HYPOTHENAR
MUSCLES

A review of the actions of the hypothenar muscles listed
above reveals that the three muscles have very similar actions.

MUSCLE ATTACHMENT BOX 18.7

ATTACHMENTS AND INNERVATION
OF THE OPPONENS DIGITI MINIMI

Proximal attachment: Flexor retinaculum and the
hook of the hamate

Distal attachment: The ulnar half of the palmar sur¬
face of the little finger's metacarpal bone

Innervation: Ulnar nerve, T1, and perhaps C8

Palpation: The opponens digiti minimi can be pal¬
pated along the ulnar aspect of the palmar surface
of the little finger's metacarpal during gentle oppo¬
sition of the little finger. As with palpation of the
opponens pollicis, the palpating digit must be
slipped between the abductor digiti minimi and the
metacarpal. Vigorous opposition will generate con¬
traction of the abductor digiti minimi, making pal¬
pation of the opponens digiti minimi impossible.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

359

Figure 18.9: Opposition of the little finger moves the little finger
toward the thumb and creates the volar arch.

The abductor digiti minimi and flexor digiti minimi flex the
CMC joint, and the opponens digiti minimi flexes and rotates
the CMC of the little finger toward the thumb. Thus the
three muscles all contribute to opposition of the CMC joint
of the little finger. The opponens digiti minimi is the largest
and strongest of the three. Weakness of the opponens digiti
minimi has the greatest effect on opposition strength; however,
weakness of any of the three diminishes opposition strength
somewhat. Similarly, both the abductor digiti minimi and the
flexor digiti minimi are abductors of the MCP joint of the lit¬
tle finger. Because the abductor digiti minimi is larger than
the flexor digiti minimi and has a larger abduction moment
arm, weakness of the abductor digiti minimi is manifested
most clearly by weakness of abduction of the MCP joint, but
weakness of the flexor digiti minimi may reduce abduction
strength as well. Consequently, it is difficult to ascertain the
exact level of weakness of any of these muscles. Despite these
difficulties, the clinician can use the following guide to assess
hypothenar strength:

• Opposition remains the best indicator of opponens digiti
minimi strength.

• MCP abduction strength best indicates abductor digiti
minimi strength.

• MCP flexion with IP extension best reflects the strength of
the flexor digiti minimi.

Clinical Relevance

WEAKNESS OF THE HYPOTHENAR MUSCLES: Weakness
of the hypothenar muscles impairs the intricate movements
of the little finger ; especially abduction. This may result in a
serious disability in individuals whose occupations depend
upon discrete finger movements, such as computer operators
or musicians (Fig. 18.10). At least as important and used by

Figure 18.10: Functional activity requiring activity of the intrinsic
muscles of the little finger. Abduction or MCP flexion with IP
extension of the little finger is a common position required while
typing.

almost all humans is the ability to create the volar arch. The
volar arch is essential to produce a tight fist (see Fig. 14.45).
Because the hypothenar muscles provide the active control
of the volar arch, weakness of the hypothenar muscles may
impair the ability to make the volar arch, leading to a
significant loss in grip strength [3,19].

Weakness of the hypothenar muscles also creates a muscle
imbalance in the little finger. In most cases, weakness of the
hypothenar muscles is accompanied by weakness of the other
muscles of the hand innervated by the ulnar nerve.
Consequently, the extrinsic muscles overpower the intrinsic
muscles, resulting in a characteristic hand deformity known as
a claw hand. A more complete discussion of this deformity is
presented later in this chapter.

INTEROSSEI AND LUMBRICALS

Dorsal Interossei

There are four dorsal interosseous muscles in the hand
(.Muscle Attachment Box 18.8)(Fig. 18.11). They are bipen-
nate muscles of varying sizes. The first dorsal interosseous

360

Part II I KINESIOLOGY OF THE UPER EXTREMITY

MUSCLE ATTACHMENT BOX 18.8

ATTACHMENTS AND INNERVATION
OF THE DORSAL INTEROSSEI

Proximal attachment: Each interosseous muscle arises
from the adjacent sides of two metacarpal bones

Distal attachment: The radial two dorsal interossei
insert on the radial sides of the bases of the proxi¬
mal phalanges of the index and long fingers; the
ulnar two dorsal interossei insert on the ulnar side
of the bases of the proximal phalanges of the ring
and long fingers. Each of the dorsal interossei also
inserts on the extensor hood of its respective finger.

Innervation: Ulnar nerve, T1, and perhaps C8

Palpation: The first dorsal interosseus can be pal¬
pated on the dorsal surface of the web space
between thumb and index finger, particularly dur¬
ing pinch. Palpation of the remaining dorsal
interossei occurs on the dorsal surface of the inter-
metacarpal spaces.

muscle is the second largest intrinsic muscle of the hand,
slightly smaller than the adductor pollicis [15]. Its size indi¬
cates that it is capable of considerable force production.
The size and strength of the remaining dorsal interossei
decrease from the radial to the ulnar side of the hand. The
actions of the dorsal interossei are similar and are presented
as a group.

Figure 18.11: Interossei and lumbrical muscles. A. The lumbricals and palmar interossei lie in the palm of the hand, with distal attach¬
ments to the fingers. B. The dorsal interossei are on the dorsum of the hand.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

361

ACTIONS

MUSCLE ACTION: DORSAL INTEROSSEI

Action

Evidence

Abduction of the index, long, and ring
fingers' MCP joints

Supporting

Flexion of the index, long, and ring
fingers' MCP joints

Supporting

Extension of the index, long, and ring
fingers' IP joints

Supporting

Adduction of the thumb's CMC joint by
the first dorsal interosseus

Inadequate

To understand the attachments of the dorsal interossei on
only the index, ring, and long fingers, it is useful to recognize
that the abductor digiti minimi fulfills the role of a dorsal
interosseous muscle to the little finger, providing abduction
and flexion of the MCP and extension of the IP joints. The lit¬
tle finger has no dorsal interosseous muscle. In addition,
because the long finger is the reference for abduction and
adduction, it participates in abduction in both the radial and
ulnar directions. Thus the long finger has both a radial and an
ulnar dorsal interosseous muscle. The role of the dorsal
interosseous muscles in abduction and flexion of the MCP
joints and in IP extension is well accepted [2,5,19,30,40].
EMG and in vivo muscle stimulation studies confirm their
activity in these motions [16,26].

Although the first dorsal interosseous muscle is said to
adduct the CMC joint of the thumb and to stabilize the
thumb [5,19], an EMG study of isometric abduction and
adduction of the thumb reveals minimal activity of the first
dorsal interosseous in both directions [9]. EMG data consis¬
tently reveal that the muscle is active during pinch, stabilizing
either the thumb or index finger or perhaps both [5,9].

EFFECTS OF WEAKNESS

Weakness of the dorsal interossei is manifested most clearly
as weakness in abduction of the index, long, and ring fingers.
Weakness also contributes to a loss of muscle balance in the
hand and thus to the formation of the claw hand. In addition,
weakness of the dorsal interossei results in a loss of both pinch
and grasp strength [20].

Palmar Interossei

Sources describe either three [5,15,31] or four [19,30,40]
palmar interossei (Muscle Attachment Box 18.9). The source
of variation is the first, or most radial, palmar interosseous
muscle, which is described as part of the flexor pollicis brevis
or adductor pollicis by those who list only three palmar
interossei [37]. Only three palmar interossei are described in
this book. The role of the radial muscle belly is included
in the description of the actions of the flexor pollicis brevis.
The palmar interossei muscles are unipennate, and the three
possess similar physiological cross-sectional areas that are
generally smaller than those of the dorsal interossei [5,15].

MUSCLE ATTACHMENT BOX 18.9

ATTACHMENTS AND INNERVATION
OF THE PALMAR INTEROSSEI

Proximal attachment: The palmar interosseous to
the index finger arises from the ulnar surface of the
index finger's metacarpal bone. The palmar interos¬
sei to the ring and little fingers arise from the radial
surface of the ring and little fingers' metacarpal
bones, respectively.

Distal attachment: Each of the palmar interossei
attaches to the extensor hood of the same finger.
The palmar interosseus to the little finger may also
insert on the radial side of the proximal phalanx of
the little finger.

Innervation: Ulnar nerve, T1, and perhaps C8
Palpation: The palmar interossei cannot be palpated.

ACTIONS

MUSCLE ACTION: PALMAR INTEROSSEI

Action

Evidence

Adduction of the index, ring, and little
fingers' MCP joints

Inadequate

Flexion of the index, ring, and little
fingers' MCP joints

Inadequate

Extension of the index, long, and ring
fingers' IP joints

Inadequate

The absence of a palmar interosseous muscle to the long fin¬
ger is consistent with the presence of two dorsal interossei to
that finger. The actions of the palmar interossei are broadly
accepted, although there are few EMG data available assess¬
ing their function directly.

EFFECTS OF WEAKNESS

Weakness of the palmar interossei is manifested by weakness
in adduction of the fingers. Individuals with palmar interossei
weakness are unable to hold their fingers close together with
the MCP joints of the fingers extended (Fig. 18.12). Weakness
also contributes to weakness of MCP flexion with IP extension.
Thus, as with the dorsal interossei, weakness of the palmar
interossei contributes to weakness in grasp and pinch [6].
jQ. Similarly, weakness of the palmar interossei also con-
tributes to the muscular imbalances leading to the
claw hand deformity.

Lumbrical Muscles

The lumbrical muscles are the smallest muscles of the hand
and possess the longest muscle fibers of the intrinsic muscles
[5,15,25]. They also are among the most unusual muscles in

362

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 18.12: Weakness of the palmar interossei. An individual
with weakness of the palmar interossei has difficulty holding the
fingers together while the MCP joints are extended.

the body, possessing no bony attachment, instead attaching
proximally and distally to tendons that are antagonists to one
another [30,40] (Muscle Attachment Box 18.10).

ACTIONS

MUSCLE ACTION: LUMBRICALS

Action

Evidence

Flexion of the MCP joints of the fingers

Supporting

Extension of IP joints of the fingers

Supporting

Radial deviation of the MCP joints of the fingers

Inadequate

EMG studies verify the activity of the lumbrical muscles
during MCP flexion and IP extension [2,26]. Since these
actions are the same as those of the interossei, there has
been considerable debate regarding the relative contribu¬
tion of each muscle group to these motions. The moment
arms of the lumbrical muscles are greater than those of the
interossei at the MCP joints [5] (Fig. 18.13). The moment
arms of the dorsal interossei are the smallest. However, the
dorsal interossei are the largest of these muscles, and the
lumbrical muscles have less than one tenth the physiological

MUSCLE ATTACHMENT BOX 18.1

ATTACHMENTS AND INNERVATION
OF THE LUMBRICAL MUSCLES

Proximal attachment: The tendons of the flexor
digitorum profundus. The lumbricals to the index
and long fingers arise from the radial and palmar
surfaces of the tendons to the index and long
fingers, respectively. The lumbrical to the ring finger
arises from the tendons to the long and ring fingers
and the lumbrical to the little finger from the ten¬
dons to the ring and little fingers.

Distal attachment: The radial side of the extensor
expansion to each finger

Innervation: The lumbricals to the index and long fin¬
gers are innervated by the median nerve, T1, and per¬
haps C8. The lumbricals to the ring and little finger are
innervated by the ulnar nerve, T1, and perhaps C8.

Palpation: The lumbricals cannot be palpated.

cross-sectional area of the dorsal interossei [5,15]. Therefore,
the interossei are considered the primary flexors of the
MCP joints when the IP joints are extended, although bio¬
mechanical models, cadaver studies, and studies in human
subjects consistently demonstrate the lumbricals’ ability
to flex the MCP joints [23,28,35]. In a study of 80 fingers
with interosseus paralysis but intact lumbrical muscles,
Srinivasan demonstrated a reduced, but real, capacity to flex
the MCP joint while maintaining IP extension [35].

The lumbrical muscles’ effects on radial deviation are
unclear. Examination of their structure reveals consistent
moment arms for radial deviation, but their moment arms
are smaller than the moment arms of the corresponding
interossei. Since the physiological cross-sectional areas of
the interossei are much larger than those of the lumbrical
muscles, it is likely that the lumbricals are, at best, acces¬
sory muscles for radial deviation. There is no known
research that demonstrates that lumbrical contraction
produces any functional radial deviation in the absence of
the interossei.

Dorsal

interossei

Palmar

interossei

Figure 18.13: The lumbricals have the longest flexion
moment arms of the intrinsic muscles at the MCP
joints; the dorsal interossei have the shortest.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

363

TABLE 18.1: Muscles Active during Combined Movements and Postures of the MCP and IP Joints
of the Fingers

Concentric MCP
Extension

Static Position in

MCP Extension

Concentric MCP
Flexion

Static Position
in MCP Flexion

Static position

Interossei

in IP extension

Lumbricals

Concentric

Interossei

IP extension

Lumbricals

Static position

FDP

in IP flexion

FDP

FDS

Concentric IP

FDP

flexion

FDP

Data from Long C, Brown ME: Electromyographic kinesiology of the hand: muscles moving the long finger. J Bone Joint Surg 1964; 46A: 1683-1706.
FDS, flexor digitorum superficialis; ED, extensor digitorum; NR, not reported; FDP, flexor digitorum profundus.

In contrast, the role of the lumbrical muscles in IP exten¬
sion is uncontested. The importance of the lumbricals’ con¬
tribution to IP extension is best understood by analyzing the
EMG activity of the intrinsic and extrinsic muscles of the
fingers. The results of the classic study of the intrinsic and
extrinsic finger muscles by Long and Brown [26] are presented
in Table 18.1. This study examined muscle activity during
various combinations of unresisted isometric or concentric
contractions producing flexion or extension of the MCP and
IP joints of the long finger. The following conclusions can be
drawn from these data:

• The flexor digitorum profundus is active whenever the IP
joints are flexed or flexing.

• The flexor digitorum profundus is active with MCP flexion
only when the IP joints are flexed or flexing.

• The extensor digitorum is active whenever the MCP joints
are extended or extending.

• The extensor digitorum is active in IP extension only when
the MCP joints are extended or extending.

• The interossei are active in any combination of isometric
or concentric MCP flexion with IP extension.

• The lumbrical muscles are active whenever the IP joints
are extended or extending, regardless of MCP position or
motion.

• The lumbricals are active during MCP flexion when the IP
joints are extended or extending.

The data reported by Long and Brown help explain the criti¬
cal role played by the lumbrical muscles. Extension by the
extensor digitorum is resisted by the passive tension of the
tendons of the flexor digitorum profundus. As noted, the lum¬
bricals have unique attachments to tendons. As a lumbrical
contracts, it pulls on the flexor digitorum profundus. This pull
puts the portion of the flexor digitorum profundus tendon
that is distal to the lumbrical attachment on slack, reducing
the passive resistance to extension that could be applied
by the flexor tendon at the level of the IP joints (Fig. 18.14).
The primary role of the lumbrical muscles is to reduce the
resistance to extension offered by the flexor digitorum
profundus, even when extension of the MCP joints stretches

the flexor digitorum profundus. The lumbricals also assist the
extensor digitorum in extending the IP joints through the full
ROM [26,38].

The data from Long and Brown [26] apply to unresisted
motions of the fingers moving in space (open chain motions).
More recent studies of finger motion against a resistance or as
the fingers press against a load (closed chain activities) sug¬
gest a more complex interaction between the intrinsic and
extrinsic muscles of the fingers in which precise joint posi¬
tions influence the relative activity of the muscles [16].

EFFECTS OF WEAKNESS

Isolated weakness of the lumbrical muscles is unusual and dif¬
ficult to identify. The classic manual muscle test of the lum¬
brical muscles is resisted flexion of the MCP joints while the
IP joints maintain extension [19] (Fig. 18.15). However, EMG
data convincingly demonstrate that this motion uses the com¬
bined activity of the lumbricals and interossei [6,19,26].
Together with weakness of the interossei, weakness of the
lumbricals contributes to the classic deformity, the claw hand.

Extensor digitorum

Figure 18.14: Function of the lumbricals. Contraction of the
lumbricals pulls the flexor digitorum profundus tendon distally,
putting the portion of the tendon that is distal to the lumbrical
muscle on slack while increasing the tension in the distal por¬
tion of the extensor digitorum (ED). This decreases the passive
flexion moment on the IP joints and assists the ED in extending
the IP joints.

364

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 18.15: The standard manual muscle test procedure for the
lumbricals is resisted flexion of the MCP joints with the IP joints
extended. EMG data show that the interossei participate in this
activity as well.

EFFECTS OF TIGHTNESS OF THE LUMBRICAL,
INTEROSSEOUS, AND HYPOTHENAR MUSCLES

The EMG data reported in Table 18.1 reveal that both the
interossei and lumbrical muscle groups are active in com¬
bined MCP flexion and IP extension of the fingers. The
hypothenar muscles participate in the same actions at the lit¬
tle finger. This position is known as the intrinsic positive, or
intrinsic plus hand, since it results from a purely intrinsic
contraction (Fig. 18.16). Tightness of the intrinsic muscles
leads to a static posture in a similar position. Patients fre¬
quently demonstrate tightness of the intrinsic muscles after
immobilization of the hand, since immobilization must occur
with the MCP joints flexed to preserve MCP flexion ROM.

Figure 18.16: An intrinsic positive hand is positioned with the
fingers' MCP joints flexed and the IP joints extended.

CLASSIC DEFORMITIES RESULTING FROM
MUSCLE IMBALANCES IN THE HAND

To appreciate the deformities resulting from muscle imbal¬
ances, it is essential to recognize that the extrinsic muscles of
the hand are multijoint muscles, typically crossing three or four
joints. The effects of muscles traversing so many joints are sim¬
ilar to the effect of the cords supporting a window blind. The
cords lie on both sides of the blind, and are firmly attached to
the bottom of the blind. As the cords are pulled together, the
blind folds on itself (Fig. 18.17). Similarly, when only the
extrinsic flexor and extensor muscles pull together on the most
distal phalanx, the finger begins to fold on itself, collapsing into
a zigzag pattern of joint hyperextension and flexion [33].

Shorter muscles, namely the intrinsic muscles, attaching to
the more proximal bones are needed to stabilize the multi-
jointed finger. The loss of balance occurs when the intrinsic
muscles are weak or absent and is manifested in the deformi¬
ties that result from peripheral nerve injuries. The impair¬
ments, deformities, and associated functional deficits from
specific nerve injuries are presented below.

Ulnar Nerve Injury

The ulnar nerve is susceptible to injury as it wraps around the
medial epicondyle of the humerus and as it travels across the
wrist into the hand. Injuries at the hand affect the innervation

Figure 18.17: Mechanism of a claw hand deformity. When the
cords that are attached to the bottom of a window blind are
pulled, the blind folds on itself in a zigzag pattern.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

365

of the intrinsic muscles, while injuries at the elbow may affect
the muscles receiving innervation from the ulnar nerve in the
forearm as well as in the hand.

ULNAR NERVE INJURIES AT THE WRIST

The muscles that may be affected by an ulnar nerve injury at
the wrist are listed below:

• Hypothenar muscles: abductor digiti minimi, flexor digiti
minimi, opponens digiti minimi

• Dorsal interossei

• Palmar interossei

• Ulnar two lumbrical muscles

• Adductor pollicis

• Deep head of the flexor pollicis brevis

If these muscles are paralyzed, the ulnar two fingers have no
intrinsic muscle support, and the long and index fingers have
only the remaining lumbrical muscles. The extrinsic muscles
pull directly against each other. The extensor digitorum has a
larger moment arm at the MCP joint and, consequently, pulls
that joint into hyperextension. The extrinsic finger flexors
stretch as a result and pull on the phalanges, causing flexion
at the PIP and DIP joints. The resulting deformity is the claw
hand (Fig. 18.18). The deformity demonstrates the classic
zigzag pattern of joint hyperextension and flexion that follows
when the extrinsic flexor and extensor tendons function with¬
out the balance of the intrinsic muscles. Because the lumbri¬
cal muscles to the index and long fingers remain intact, the
clawing is less obvious in these fingers [35]. The claw hand
deformity also is characterized by flattening of the hypothenar
eminence.

A claw hand deformity produces significant functional
deficits. An individual with a claw hand loses grip strength as a
result of the loss of intrinsic muscle strength that contributes
directly to powerful grasp [4,20]. Loss of the hypothenar
muscles results in the inability to form the volar arch actively,
resulting in further loss of grip strength. The ability to grasp

Figure 18.18: In a claw hand deformity, the extrinsic flexor digi¬
torum profundus and the extensor digitorum pull on the distal
phalanx. Lacking the control of the intrinsic muscles attached
more proximally, the fingers collapse into the zigzag pattern
of hyperextension at the MCP joints and flexion at the PIP
and DIP joints.

Figure 18.19: Position of the hand to hold a large object. Holding
a large object requires MCP flexion of the fingers, with little or
no flexion of the IP joints.

large objects also is impaired, since it requires MCP flexion
with IP extension (Fig. 18.19). The other important loss is the
adductor pollicis, resulting in significant impairment to power¬
ful pinch but without an associated deformity.

ULNAR NERVE INJURY AT THE ELBOW

The muscles that may be affected by an ulnar nerve injury at
the elbow are listed below:

• Flexor carpi ulnaris

• Flexor digitorum profundus to the ring and little fingers

• The intrinsic muscles listed with ulnar nerve injuries at the
wrist

Paralysis of the flexor digitorum profundus to the ulnar two
fingers alters the claw deformity slightly in these two fingers.
In the absence of the flexor digitorum profundus, the pull
from the extrinsic muscles is exerted by the extensor digito¬
rum and the flexor digitorum superficialis. The MCP joints
and PIP joints are hyperextended and flexed, respectively, as
in a typical claw hand, but the DIP joints in the ring and little
fingers remain extended. Paralysis of the flexor carpi ulnaris
may result in deviation of the wrist toward extension and
radial deviation.

Median Nerve Injury

Median nerve injuries within the carpal tunnel are common,
but the nerve also is susceptible to injuries at the elbow or in

366

Part II I KINESIOLOGY OF THE UPER EXTREMITY

the proximal forearm. The associated deformities and func¬
tional deficits in each lesion are described below.

MEDIAN NERVE INJURY AT THE WRIST

The muscles affected by a median nerve injury at the wrist are
listed below:

• Thenar muscles: abductor pollicis brevis, superficial head
of the flexor pollicis brevis, opponens pollicis

• Lumbrical muscles to the index and long fingers

The primary loss with this lesion is the loss of the thenar
muscles, leaving only the adductor pollicis, the deep head of
the flexor pollicis brevis, and the first dorsal interosseus mus¬
cle as the intrinsic supply to the thumb. These muscles are
insufficient to balance the extrinsic muscles of the thumb.
The mechanics at work in the claw hand produce similar
effects in the thumb. The extensor pollicis longus with its
large adductor moment arm at the CMC joint pulls the CMC
joint of the thumb into adduction and extension.
Consequently, the flexor pollicis longus is stretched and pulls
the MCP and IP joints of the thumb into flexion. The result¬
ing deformity is known as an ape thumb, in which the
thumb is pulled onto the radial side of the hand, with the
CMC joint extended and adducted and the MCP and IP
joints flexed (Fig. 18.20). This deformity can be very debili¬
tating, since the adducted position precludes normal tip-to-
tip or pulp-to-pulp pinch [4].

Clinical Relevance

MEDIAN NERVE INJURY AT THE WRIST:

Orthopaedic surgery to transfer intact muscles to the
thumb may be useful in providing some active control of
pinch , but a simple splint to position the thumb in opposi¬
tion also is successful in improving function by placing
the thumb in a functional position and allowing the
fingers to provide the active pinch against a stable
thumb (Fig. 18.21).

MEDIAN NERVE INJURY AT THE ELBOW

The muscles affected when the median nerve is injured at the
elbow are listed below:

• Pronator teres

• Flexor carpi radialis

• Palmaris longus

• Flexor digitorum superficialis

• Flexor pollicis longus

• Flexor digitorum profundus to the index and long fingers

• Pronator quadratus

• The intrinsic muscles affected by a median nerve lesion at
the wrist

Figure 18.20: An ape thumb deformity in an individual with a
peripheral neuropathy is positioned in extension and retropul-
sion at the CMC joint and flexion at the MCP and IP joints.

These include all of the superficial flexors of the forearm
except the flexor carpi ulnaris, the primary pronators of the
forearm, the flexor digitorum profundus to the index and
long fingers, and the flexor pollicis longus. The ape thumb
deformity is altered, since the IP joint remains extended.
Similarly the DIP joints of the index and long fingers remain
extended. The wrist may be positioned in extension and
perhaps ulnar deviation.

Radial Nerve Injury

No intrinsic muscles of the hand are innervated by the radi¬
al nerve. Muscular effects are seen in more-proximal radial

Figure 18.21: Use of an abduction splint to position the
thumb in slight opposition can increase an individual's
function, even in the presence of significant weakness
of the thenar muscles.

Chapter 18 I MECHANICS AND PATHOMECHANICS OF THE INTRINSIC MUSCLES OF THE HAND

367

nerve injuries. As noted in Chapter 8, the radial nerve is
particularly susceptible to injury, as it lies against the
humeral shaft in the radial groove. The muscles of the
forearm and wrist that are affected in such a lesion are
listed below:

• Extensor carpi radialis longus (and perhaps the
brachioradialis)

• Extensor carpi radialis brevis

• Extensor digitorum

• Extensor digiti minimi

• Extensor carpi ulnaris

• Supinator

• Abductor pollicis longus

• Extensor pollicis brevis

• Extensor pollicis longus

• Extensor indicis

This list includes all of the wrist extensors and extrinsic
extensors to the fingers and thumb. This lesion results in
a drop wrist deformity. The greatest functional deficit
caused by a radial nerve injury is difficulty in positioning
the wrist for powerful grasp or pinch. The essential syner¬
gy between the wrist extensors and the finger flexors com¬
bines the contraction of the extensor carpi radialis longus
and brevis and the extensor carpi ulnaris with the flexor
digitorum profundus and superficialis. This synergy is
necessary to avoid passive insufficiency of the extensor
digitorum and active insufficiency of the finger flexors. If
the wrist is allowed to remain in flexion in the absence of
the wrist extensors, the patient is unable to develop a
powerful grasp or pinch (Fig. 18.22). A study of 10 healthy
individuals with radial nerve blocks reported more than a
75% decrease in grip strength and a 33% loss in pinch
strength [22].

Figure 18.22: An individual with a drop wrist is unable to make a
strong fist.

Figure 18.23: An individual with a radial nerve injury can
use a splint to position the wrist and fingers in a functional
position so that the fingers can develop a powerful
grasp.

Clinical Relevance

DROP WRIST DEFORMITY: The functional need for wrist
extension leads surgeons to perform a variety of tendon
transfers to restore active control. As in the ape thumb
deformity , a splint that helps position the wrist in extension
can succeed in providing stability to the wrist in a functional
position , allowing the intact finger muscles to perform their
roles in grasp and pinch (Fig. 18.23).

Sensory Deficits Associated with Nerve
Injuries to the Hand

Although this book focuses on the mechanics and pathome-
chanics of the musculoskeletal system, the functional impli¬
cations of sensory loss to the hand demand at least brief con¬
sideration. Sensory distribution to the hand is depicted in
Figure 18.24. Sensory loss secondary to a median nerve
injury presents the greatest functional challenge. Useful
pinch requires the integrated control of the intrinsic and
extrinsic muscles of the thumb, index, and long finger.
However, it also depends on the acute sensory feedback pro¬
vided by the pulps and nail beds of these digits. Anyone
whose hand has “fallen asleep” can appreciate the frustration
of lack of sensation in the finger tips.

Sensory loss from a radial or ulnar nerve injury also is a
potentially serious lesion. Although feedback from the dorsum
or ulnar surface of the hand is less important during pinch and
grasp, it is an important warning of injury to the hand. These
surfaces are easily bumped on furniture or a hot coil of a stove.
Lack of sensation prevents spontaneous recognition of such

368

Part

I KINESIOLOGY OF THE UPER EXTREMITY

Figure 18.24: Sensory distribution of the
nerves to the hand. Sensory loss in the
hand resulting from a peripheral nerve
injury can produce severe disability. Fine
motor tasks demand sensory input from
the pulps of the digits, and the dorsal
surface of the hand and fingers is sub¬
jected to trauma that, if undetected, can
lead to infection and serious impairment.

injuries. If these injuries remain undetected, infection can set
in. The clinician must teach the individual to carefully inspect
the insensitive skin regularly.

SUMMARY

This chapter presents the intrinsic muscles of the hand, which
are integral to the normal function of the hand. The normal
interplay of the intrinsic and extrinsic muscles is presented.
The primary intrinsic muscles of the thumb are the thenar
muscles, innervated by the median nerve, and the adductor
pollicis, innervated by the ulnar nerve. Weakness of the
thenar muscles leads to an ape thumb deformity, while weak¬
ness of the adductor pollicis weakens pinch and grasp. An
ulnar nerve injury also leads to reduced grip strength by
impairing production of the volar arch and the strength of the
intrinsic muscles to the fingers. The following chapter dis¬
cusses the mechanics and pathomechanics of pinch and
grasp. These functions demonstrate the coordinated activity
of the intrinsic and extrinsic muscles.

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2. Basmajian JV, DeLuca CJ: Muscles Alive. Their Function Revealed
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3. Bendz P: The functional significance of the fifth metacarpus and
hypothenar in two useful grips of the hand. Am J Phys Med
Rehabil 1993; 72: 210-213.

4. Bowden RE: Peripheral Nerve Injuries. London: HK Lewis &
Co, 1958.

5. Brand PW, Hollister A: Clinical Mechanics of the Hand.
St. Louis, MO: Mosby-Year Book, 1999.

6. Brandsma JW: Manual muscle strength testing and dynamome-
try for bilateral ulnar neuropraxia in a surgeon. J Hand Ther
1995; 8: 191-194.

7. Brandsma JW, Oudenaarde EV, Oostendorp R: The abductores
pollicis muscles: clinical considerations based on electromyo-
graphical and anatomical studies. J Hand Ther 1996; 9: 218-222.

8. Close JR, Kidd CC: The functions of the muscles of the thumb,
the index, and the long fingers. J Bone Joint Surg 1969; 51 A:
1601-1620.

9. Cooney WP III, An KN, Daube JR, Askew LJ: Electromyo¬
graphic analysis of the thumb: a study of isometric forces in
pinch and grasp. J Hand Surg [Am] 1985; 10A: 202-210.

10. Cooney WP III, Lucca MJ, Chao EYS, Inscheid RL: The kine¬
siology of the thumb trapeziometacarpal joint. J Bone Joint Surg
1981; 63A: 1371-1381.

11. Forrest WJ, Basmajian JV: Functions of human thenar and
hypothenar muscles. An electromyographic study of twenty-five
hands. J Bone Joint Surg 1965; 47A: 1585-1594.

12. Hirsch D, Page D, Miller D, et al.: A biomechanical analysis of
the metacarpophalangeal joint of the thumb. J Biomech 1974; 7:
343_348.

13. Hollister A, Giurintano DJ: Thumb movements, motions, and
moments. J Hand Ther 1995; 8: 106-114.

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369

14. Imaeda T, An KN, Cooney WP III: Functional anatomy and bio¬
mechanics of the thumb. Hand Clin 1992; 8: 9-15.

15. Jacobson MD, Raab R, Fazeli RM, et al.: Architectural design of
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16. Kamper DG, Fischer HC, Cruz EG: Impact of finger posture
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17. Kapandji IA: The Physiology of the Joints. Vol 1, The Upper
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21. Kuxhaus L, Roach SS, Valero-Cuevas FJ: Quantifying deficits in
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22. Labosky DA, Waggy CA: Apparent weakness of median and
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528-533.

23. Leijnes J, Kalker J: A two-dimensional kinematic model of the
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25. Linscheid RL, An KN, Gross RM: Quantitative analysis of the
intrinsic muscles of the hand. Clin Anat 1991; 4: 265-284.

26. Long C, Rrown ME: Electromyographic kinesiology of the
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28. Ranney DA, Wells RP, Dowling J: Lumbrical function: interac¬
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29. Rayan GM, Saccone PG: Treatment of spastic thumb-in-palm
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32. Skoff HD: The role of the abductor pollicis brevis in opposition.
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33. Smith RJ: balance and kinetics of the fingers under normal and
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36. Strong CL, Perry J: Function of the extensor pollicis longus and
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ing interphalangeal joint extension. J Am Phys Ther Assoc 1966;
46:939-945.

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and Wrist. Philadelphia: WR Saunders, 1996.

38. Wang AW, Gupta A: Early motion after flexor tendon surgery.
Hand Clin 1996; 12: 43-55.

39. Weathersby HT, Sutton LR, Krusen UL: The kinesiology of
muscles of the thumb: an electromyographic study. Arch Phys
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Anatomical Rasis of Medicine and Surgery, Rr. ed. London:
Churchill Livingstone, 1995.

CHAPTER

Mechanics and Pathomechanics
of Pinch and Grasp

CHAPTER CONTENTS

PREHENSION.371

Necessary Elements of Pinch .371

Necessary Elements of Powerful Grasp.375

FORCES ON THE FINGERS AND THUMB DURING ACTIVITIES .376

Analysis of the Forces in the Fingers.376

Review of the Forces Generated during Pinch and Grasp.378

USING FORCE ANALYSIS TO MAKE CLINICAL DECISIONS .382

How Forces Contribute to the Finger Deformity of Ulnar Drift with Volar Subluxation .382

Protecting a Surgically Repaired Tendon in the Finger.384

Relationship between the Forces in the Finger Flexor Muscles and Carpal Tunnel Syndrome.385

Forces Are Key in Ergonomic Assessments of Work-Related Musculoskeletal Disorders (WMSDs) .385

SUMMARY .386

T he previous five chapters present the structure and function of the wrist and hand. The bones and joints are
discussed to understand the motions available throughout the region. The function of the muscles of the
forearm and hand are presented with discussions of the dysfunction resulting from weakness or tightness.
One of the primary functions of the hand is to hold onto objects. Therefore a thorough understanding of the wrist
and hand requires examination of the roles of the joints and muscles during grasp and pinch.

The purposes of this final chapter on the hand are to detail the requirements of normal pinch and grasp and to discuss
the factors contributing to abnormal prehension. Specifically, the goals of this chapter are to

■ Discuss the classification schemes describing prehension

■ Examine the positions of the joints of the wrist and hand during normal pinch and grasp
■ Investigate the muscles needed for powerful pinch and grasp
■ Explore the forces to which the digits are subjected during pinch and grasp

■ Consider how the forces generated during pinch and grasp can contribute to deformities in the hand

370

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP 371

PREHENSION

Prehension pattern is an important defining characteristic of
humans. Humans with the ability to oppose the thumb to the
fingers exhibit a wide variety of grasping patterns that are typ¬
ically classified by the position of the fingers and the area of
contact between the fingers, thumb, and object grasped.
Napier offers the classic description of prehensile patterns
[48]. Prehension is classified generally as either pinch or grasp.
Pinch is a prehensile pattern that involves the thumb and the
distal aspects of the index and/or long finger (Fig. 19.1). It is
used primarily for precision and fine manipulation. In con¬
trast, grasp typically involves all of the hand, including the
digits and the palm [31] (Fig. 19.2). Although this classifica¬
tion oversimplifies the enormous variety of prehensile
patterns used in daily life, it is a useful means of inves¬
tigating the basic requirements of each pattern.

The factors distinguishing pinch from grasp are

• Area of contact within the hand

• Number of fingers involved in the activity

• Amount of finger flexion

• Position of the thumb

• Position of the wrist

The following presents the essential elements of pinch and
grasp and examines some of the variations available in each
pattern.

Necessary Elements of Pinch

Pinch is used for precise manipulation of relatively small
objects such as a needle or pen. It may be used in delicate
handling of a fragile wing of a butterfly or the powerful twist¬
ing of a key in a stubborn lock. Despite the wide variety of

Figure 19.1: Pinch uses the digits on the radial side of the hand.

applications of the pinch, certain characteristics exist. Pinch
typically uses the radial side of the hand, primarily the thumb,
index, and long fingers. The thumb moves away from the fin¬
gers but turns toward them in opposition. The thumb then
can hold an object securely against the stable post of the index
and long fingers. The thumb s position depends largely on the
mobility of its carpometacarpal (CMC) joint, while the rela¬
tive immobility of the CMC articulation of the index and long
fingers provides them the necessary stability to resist the
forces from the thumb. It is important to recall that the CMC
articulations of the index and long fingers are the least mobile
in the hand. This lack of mobility allows these fingers to
remain relatively fixed as the thumb pushes against them dur¬
ing pinch.

During pinch, the thumb and fingers assume rather
stereotypical positions that are described below. The muscles
used to achieve and maintain these positions are then pre¬
sented. Finally, the effects on the mechanics of pinch when
any of the digits is unable to achieve the appropriate position
are discussed.

REQUIREMENTS OF NORMAL PINCH

Humans use a wide variety of pinch types (Fig. 19.3).
Inspection of the basic positions used in the tip-to-tip pinch
helps identify the requirements of normal pinch. Tip-to-tip
pinch is described as the pinch that forms an “O” between the
thumb and a finger, usually the index or long finger (Fig.
19.4). It brings the very tips of the digits together and is used
to pick up a pin or tiny seed from a table. Table 19.1 lists the
position of the joints of the finger and thumb during tip-to-tip
pinch. The wrist maintains a position of extension in most
pinch activities.

372

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 19.4: Tip-to-tip pinch forms an "O" between the thumb
and index finger.

An examination of the positions used in tip-to-tip pinch
and consideration of the forces between the opposing digits
help explain the muscles needed for normal pinch (Fig. 19.5).
The DIP joint of the index finger flexes against the thumb as
the thumb applies an extension moment to the DIP joint. The
index finger requires the flexor digitorum profundus to main¬
tain this position, and the flexor digitorum profundus requires
that the wrist remain extended to have adequate contractile
length (Chapter 15). Thus the dedicated wrist extensors, par¬
ticularly the extensor carpi radialis brevis and extensor carpi

TABLE 19.1: Positions of the Joints of the Thumb
and Finger in Tip-to-Tip Pinch

Thumb

Finger

CMC joint

Opposition and extension
(radial abduction)

—

MCP joint

Flexion

IP (PIP) joint

Flexion

DIP joint

—

Flexion

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

373

Figure 19.5: Muscles required for tip-to-tip pinch include the ded¬
icated wrist extensors, extensor carpi radialis longus ( ECRL ) and
brevis ( ECRB ), the flexor digitorum profundus ( FDP), the flexor
pollicis longus {FPL), the abductor pollicis longus ( APL), the
abductor pollicis brevis {APB), the opponens pollicis (OP), the
adductor pollicis {AP), and the first dorsal interosseous muscle

ulnaris, are active during pinch [12,38,41]. Similarly, flexion of
the interphalangeal (IP) joint of the thumb demands the
activity of the flexor pollicis longus [15]. The CMC joint of the
thumb then must be stabilized in extension by the abductor
pollicis longus against the pull of the flexor pollicis longus as
described in Chapter 15 [9,14,58].

Additional intrinsic muscles are essential to normal pinch.
The abductor pollicis brevis, the primary abductor of the
thumb, is active to position the thumb in abduction (palmar
abduction) and medial rotation. The opponens pollicis also
helps to position the thumb during pinch [15,19]. Careful
observation of the thumb s position relative to the index fin¬
ger helps to explain the roles of the remaining two intrinsic
muscles. Even in tip-to-tip pinch the thumb lies slightly radial
to the index finger, exerting an ulnarly directed force on the
index finger. The adductor pollicis muscle is perfectly situated
to pull the thumb toward the index finger, particularly by
flexing the metacarpophalangeal (MCP) joint of the thumb
(Fig. 19.6). Its size and large moment arm makes the adduc¬
tor pollicis the most important flexor of the MCP joint. Hence
the adductor pollicis is an essential muscle of pinch, particu¬
larly in forceful pinch [15,29].

As the thumb exerts a force on the index finger in the ulnar
direction, the index finger is pushed ulnarly at the MCP joint.
Consequently, the remaining essential muscle in pinch is the
first dorsal interosseous muscle. This muscle stabilizes the
index finger, preventing ulnar deviation at the MCP joint [15].

Figure 19.6: The adductor pollicis {AP) applies an ulnarly directed
force on the index finger, tending to adduct the finger. This
adduction is counteracted by the abduction pull of the first dor¬
sal interosseous muscle (D/).

To summarize, the thumb muscles essential to a normal
tip-to-tip pinch are the flexor pollicis longus, abductor pollicis
longus, abductor pollicis brevis, opponens pollicis, and adduc¬
tor pollicis. At the index finger, the flexor digitorum profun¬
dus and first dorsal interosseous muscles are critical to a
normal pinch.

The other types of pinch present only minor variations of
the requirements of the tip-to-tip pinch. Pulp-to-pulp pinch
uses less DIP joint flexion. Consequently, the role of the flexor
digitorum profundus may decrease if the individual uses the
volar plate to prevent hyperextension. The flexor digitorum
superficialis may assume the role of flexor of the PIP and
MCP joints. The key, or lateral, pinch and the chuck, or three-
jaw chuck, pinches use at least three digits, the thumb, and
the index and long fingers. Therefore, these types of pinch are
stronger and are used when power is more important than
precision [25]. The joint positions and muscle requirements
in these forms of pinch are similar to those of tip-to-tip and
pulp-to-pulp pinch. Simulations of nerve injuries in cadaver
models and in healthy volunteers with temporary, sequential
nerve blocks of the median and ulnar nerves at the wrist
reveal a 60-77% loss in pinch strength with an ulnar nerve
palsy and a 60% loss with a median nerve palsy [36,57]. These

374

Part II I KINESIOLOGY OF THE UPER EXTREMITY

data demonstrate the critical contribution to pinch strength
made by the intrinsic muscles, even in the presence of intact
extrinsic musculature.

EFFECTS OF ABNORMAL JOINT POSITIONS AND
MUSCLE WEAKNESS ON PINCH MECHANICS

Inability to achieve the proper position at the thumb or fin¬
gers has a chain reaction on the other joints of the digits dur¬
ing pinch because the central goal of pinch remains to hold or
manipulate a small object. During pinch, the thumb and fin¬
gers function in a closed kinetic chain, with the distal end of
each digit fixed. The joints and muscles accommodate in a
variety of ways to keep the thumb in contact with the finger
and support the object.

Clinical Relevance

INSUFFICIENT WEB SPACE BETWEEN THE THUMB
AND INDEX FINGER: A CASE REPORT: A student found a
growing lump in the web space between the thumb and index
finger . The lump was diagnosed as a cyst and was removed
surgically. The surgery was more extensive than expected
because the cyst had an extensive blood supply Following the
surgery and 3 weeks of immobilization, the patient exhibited a
significant scar and limited abduction and extension range of
motion (ROM) of the CMC joint of the thumb. The joint end-
feel during passive ROM was consistent with soft tissue stretch
with no apparent joint capsular restriction , although the joint
ROM was limited. The patient reported a pulling discomfort in
the scar. The conclusion was that the patient's limited ROM
was due to scar formation and inadequate extensibility of the
skin in the web space between the thumb and index finger.

The patient's inability to abduct the CMC joint of the
thumb resulted in an abnormal pinch pattern. The pinch
was characterized by slight thumb CMC extension and
abduction , slight hyperextension of the MCP joint of the
thumb , and excessive flexion of the IP joint of the thumb
(Fig. 19.7). The index finger exhibited increased flexion of
the proximal interphalangeal (PIP) and distal interpha-
langeal (DIP) joints , with less flexion at the MCP joint. The
space bounded by the thumb and index finger was an oval
rather than the typical "0" of tip-to-tip pinch. The patient
was treated with stretching , gentle soft tissue massage , and
splinting. Full ROM and normal function were restored after
approximately 3 months.

There are many reasons for abnormal positioning of the
CMC joint of the thumb during pinch. These can include
weakness of the abductor pollicis longus so that the individ¬
ual is unable to stabilize the CMC joint of the thumb against
the pull of the flexor pollicis longus. Intrinsic weakness with
a resulting ape thumb deformity also affects CMC joint
position. Instability secondary to arthritic changes can also

Figure 19.7: Tip-to-tip pinch with inadequate web space between
the thumb and index finger exhibits altered positions of both
digits, with less abduction of the thumb's CMC joint and less flex¬
ion at the finger's MCP joint.

lead to inability to position the CMC joint properly. Even
the presence of long fingernails can alter the relative posi¬
tions of the fingers and thumb during pinch (Fig. 19.8). In
any of these cases the thumb lacks the ability to move into
sufficient abduction and extension to bring the tip of the
thumb to the tip of the index finger. Consequently, to
accomplish pinch, the individual alters the position of the
other joints of the thumb. In the case report, the individual
responded with hyperextension of the MCP joint of the
thumb. Other individuals may respond with flexion of the

Figure 19.8: Long fingernails alter the positions of the thumb
and index finger in pinch.

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

375

Figure 19.9: Compensation in pinch pattern resulting from
limited ROM in the thumb. Different pinch patterns can result
from inadequate abduction and extension at the thumb's CMC
joint. Both photos reveal limited abduction at the thumb's CMC
joint; however, the positions of the MCP and IP joints differ.

A. Pinch is characterized by hyperextension of the thumb's MCP
joint and excessive flexion of the IP joint. B. Pinch is character¬
ized by hyperextension of the thumb's IP joint with flexion of
the MCP joint.

MCP joint and hyperextension of the IP joint (Fig. 19.9).
The compensation depends on the available ROMs at the
remaining joints. The important element for the clinician to
recognize is that the position of the thumbs CMC joint is
critical to normal pinch mechanics. Abnormalities at this
joint are reflected in the rest of the joints of the participat¬
ing digits.

Necessary Elements of Powerful Grasp

Powerful grasp is distinguished from pinch by several factors.
Grasp uses more of the volar surface of the palm and fingers.
It generally uses all of the digits of the hand and, conse¬
quently, produces more-forceful prehension. As in pinch, the
positions of the digits are somewhat predictable but vary
slightly with the type of grasp used.

REQUIREMENTS OF NORMAL POWERFUL GRASP

Like patterns of pinch, patterns of grasp are diverse and serve
a variety of purposes (Fig. 19.10). The size of the object
grasped also affects the grasp pattern. However, certain basic
characteristics of grasp exist. The finger joints generally are
more flexed than in pinch, and the ulnar fingers exhibit more
flexion than the radial fingers. This increased finger flexion on
the ulnar side along with the volar arch formed by the move¬
ment of the CMC articulations of the little and ring fingers
draws the grasped object toward the thumb and clamps it
firmly in the palm of the hand (Fig. 19.11).

Another distinguishing characteristic of powerful grasp is
the position of the thumb. The thumb tends to flex over the
fingers and is pulled toward the palm. The CMC joint may
remain in a position of slight abduction but less than in pinch.
The adductor pollicis forcefully contracts to pull the thumb
onto the fingers and object.

The force of the grasp has a significant effect on the char¬
acteristics of the grasp. In general, an increase in the force of
grasp is accompanied by an increase in the following:

• Flexion of the fingers, particularly at the MCP joints

• Participation of the ulnar side of the hand and the use of
the volar arch

• Contact area between the object and the fingers and palm

Figure 19.10: Different patterns of grasp demonstrate varied
amounts of finger flexion and contact with the palm, leading
to varied amounts of grasp force.

376

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 19.11: Powerful grasp compresses the object into the
thenar eminence where the object is covered by the thumb.

Wrist position is more variable in powerful grasp. When mak¬
ing a tight fist, the wrist is extended using the synergy
between the dedicated wrist extensor muscles and the finger
flexors described in Chapter 15. However, in many forceful
activities, the wrist functions in neutral or even slight flexion
with ulnar deviation [44,52]. This position aligns the radial
side of the hand with the long axis of the forearm. Such posi¬
tioning is found in activities such as cutting meat or turning a
screwdriver (Fig. 19.12).

Powerful grasp requires the efforts of most of the muscles
of the wrist and hand. The extrinsic finger flexors produce IP
flexion. The interossei and lumbricals assist in flexion of the

Figure 19.12: Powerful grasp without wrist extension. In some
powerful grasps the wrist is in ulnar deviation and neutral flex¬
ion, thus aligning the hand with the long axis of the forearm.

MCP joints and increase the flexion moment at the MCP
joints. The hypothenar muscles form the volar arch for added
force, and the dedicated wrist muscles stabilize the wrist in
the appropriate position. Nerve blocks of the ulnar and median
nerves at the wrist in healthy individuals produce 38% and
32% decreases in grip strength, respectively, demonstrating
that the extrinsic muscles contribute a larger percentage of
the strength in grasp than in pinch [36].

COMPARISONS BETWEEN PINCH AND GRASP

The major difference between pinch and grasp is the part of
the hand used in each. Pinch uses the radial side of the hand,
while powerful grasp depends on the ulnar side of the hand.
Greater finger flexion ROM is required in powerful grasp
than in pinch. The integrity of the thumb s CMC joint and the
ability to maintain the thumb in abduction is essential to
pinch, but the thumb s participation in powerful grasp also is
important [10].

FORCES ON THE FINGERS AND THUMB
DURING ACTIVITIES

An understanding of the forces applied to the structures of
the hand is essential to understanding many of the deformi¬
ties that occur in the hand. An appreciation of these loads is
important to avoid loads that can undermine a patient s reha¬
bilitation. For example, the clinician must recognize activities
that can disrupt a tendon repair or contribute to joint insta¬
bility and deformity. This section reviews the analysis used to
derive the muscle and joint reaction forces in the digits. The
available data reported in the literature regarding these forces
are also presented. Finally, the application of these data to
typical clinical problems is demonstrated.

Analysis of the Forces in the Fingers

Several investigators have analyzed the forces on the joints
and in the muscles of the fingers and thumb during pinch
and grasp [2,3,7,8,11,13,20-22,27,40,50,55,57,60]. With the
exception of the DIP joint of the fingers and the IP joint of
the thumb, several muscles contract simultaneously at each
joint during pinch and grasp (Fig. 19.13). Consideration of
the moments created during pinch helps explain the need for
activity of so many muscles. Examining the Forces Box 19.1
demonstrates the moment applied to the DIP, PIP, and MCP
joints of the index finger during tip-to-tip pinch. The pinch
force creates an extension moment at each finger joint that
increases from distal to proximal. Therefore, the flexion
moments needed to stabilize the joints also must increase
from distal to proximal [13,18,30,50], which helps explain why
there are more muscles available to flex the MCP joint than
at the other joints of the fingers [40]. Electromyographic
(EMG) data reveal activity in all of these flexors during force¬
ful pinch and grasp [14,19,53].

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

377

Figure 19.13: The muscles available to flex and extend the joints of the fingers include flexor digitorum profundus ( FDP), flexor digitorum
superficialis ( FDS), extensor digitorum (ED), palmar interossei (PI), dorsal interossei (Dl), and lumbricals (L).

378

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Clinical Relevance

PINCH PATTERNS IN INDIVIDUALS WITH
WEAKNESS OF THE INTRINSIC MUSCLES: Patients with
significant weakness of the intrinsic muscles of the hand
demonstrate the claw hand and ape thumb deformities
described in Chapter 18. However ; even slight weakness of
these muscles impairs the individual's ability to generate a
forceful pinch. Because a forceful pinch produces a large
extension moment at the MCP joint of the finger,
both the intrinsic and extrinsic muscles are needed
to create sufficient flexion moment to balance this
external moment at the MCP joint. Decreasing the flexion
angle at the MCP joint reduces the moment arm of the
pinch force and the extension moment applied at the MCP
joint (Fig. 19.14). Individuals with even slight-to-moderate
weakness of the intrinsic muscles typically pinch with the
MCP joint of the finger in extension , effectively reducing the
flexion moment needed at the MCP joint.

Examining the Forces Box 19.2 presents the free-body dia¬
gram and the two-dimensional analysis of the forces at the
DIP joint of the index finger during tip-to-tip pinch. This
analysis suggests that both the force in the flexor digitorum
profundus and the joint reaction force on the distal phalanx
are about twice the applied force of the pinch. These results

Figure 19.14: Less flexion of the MCP joint reduces the extension
moment exerted on the MCP joint by the pinch force by decreas¬
ing its moment arm (of).

are consistent with results reported in the literature [2,21,60].
However, other estimates suggest that the force in the flexor
digitorum profundus may be more than four times the force
of pinch [13]. Maximum tip-to-tip pinch forces reported in
the literature range from approximately 60 to 120 N (13-27
lb) in males, less in females [17,25,39,45,61]. Based on these
data, loads in the flexor digitorum profundus and on the joint
are at least 25 lb but could be as high as 200 lb.

Because the force within a tendon is constant along its full
length, the data from the solution in Examining the Forces
Box 19.2 can be used to determine the muscle forces at the
PIP and MCP joints. Examining the Forces Box 19.3 presents
the free-body diagrams and simplified solutions for the forces
in the flexor digitorum superficialis and intrinsic muscle
group at these joints. The moment arms for the muscles at
each joint are based on data found in the literature [1,34].
This example demonstrates the need for additional flexor
muscles at each succeeding proximal joint because of the
increasing moment exerted by the pinch force.

Review of the Forces Generated during
Pinch and Grasp

More complex models than those presented in Examining
the Forces Boxes 19.2 and 19.3 allow approximations of the
loads in the muscles and ligaments and on the joints of the
fingers and thumb. Estimates of the compressive forces on
the finger joints vary but increase in magnitude from distal
to proximal, since the moment from the external force is
increasing [2,13,50]. The types of grasp and the force of the
grasp also influence the muscle and joint forces. Lateral, or
key, pinch reportedly generates larger muscle and joint
reaction forces in the index finger than other forms of pinch
[2]. Estimates of compressive loads at the DIP joint during
key pinch are as high as 12 times the pinch force [2].
Estimates of the maximum axial loads at the PIP joint dur¬
ing pinch range from 3 to almost 20 times the pinch force
[2,7,13,50]. Estimates of the maximal compressive loads at
the MCP joint are even greater, ranging from 4 to 27 times
the force of pinch.

Generally, evidence suggests that the joint reaction
forces on the fingers in grasp exceed those of pinch [2,8,13].
However, the type of grasp and the resulting joint position
significantly affects the loads on the joints. For example, a
hook grasp in which the MCP joints remain extended and
the DIP joints bear no load appears to produce smaller com¬
pressive forces on all of the joints of the fingers than any
other pinch or grasp pattern studied [2] (Fig. 19.15). These
findings are consistent with the analysis presented in
Examining the Forces Box 19.3 , in which the moment arm
of the applied load has a dramatic effect on the moments
required by the muscles and, ultimately, on the joint reac¬
tion forces. Estimates of the loads in the fingers reported by
An et al. [2] and by Purves and Berme [50] during a simula¬
tion of opening a jar lid are reported in Table 19.2.

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

379

CALCULATION OF THE FORCES AT THE DIP

sin a = -4.1/11.5

M p

JOINT DURING PINCH

moment due to the FDP

force applied by the flexor digitorum profun¬
dus (FDP)

a = 20° from x axis

x moment arm of the FDP (0.65 cm)

P pinch force (6 kg)

1.2 cm moment arm of the pinch force at the DIP
10° FDP angle of pull

2M = 0

M p - (P X 1.2 cm) = 0
(F X x) - (P X 1.2 cm) = 0
F = (6 kg X 1.2 cm)/ 0.65 cm
F = 11 kg

SF x : J x + F x = 0

J x - F X (cos 10°) = 0

J x = F X (cos 10°)

J x = 10.8 kg

£F y : J y + 6.0 kg - F x (sin 10°) = 0
J Y = F X (sin 10°) - 6.0 kg
J Y = -4.1 kg

Using the Pythagorean theorem:

J 2 = J x 2 + V

J « 11.5 kg (approximately 2 times the applied load)

The free-body diagram to calculate the forces at the DIP joint
during tip-to-tip pinch identifies the pinch force (P), the force
in the flexor digitorum profundus (F), and the joint reaction
force ( J ).

Clinical Relevance

OCCUPATIONAL HAZARDS: The piano is played by strik¬
ing the piano keys with the fingertips. Contact with the key
is sometimes very soft (pianissimo) but also can be crashing
(fortissimo). Professional pianists can develop severe hand
problems including tendinitis and joint pain. Studies using
models similar to those noted earlier report tendon forces of
about three times the force on the keys and joint reaction
forces up to seven times the key force [26]. Investigators
offer suggestions of wrist and finger positions to minimize
the forces on the soft tissue and joints. Musicians are artists ,
and the physical demands of their profession are rarely
appreciated. Recognition of the mechanical stresses of

playing some instruments may help the clinician direct treat¬
ment toward reducing joint stresses and identifying joint
protection strategies. Similar approaches are beneficial in
assessing the demands of any manual activity , including
computer operation , molding pottery , or cutting meat.

Although the data reported in the literature are varied and
are only estimates based on mathematical models, they con¬
sistently demonstrate that the fingers sustain significant loads
during daily activities such as turning a key, playing the piano,
turning a water faucet, and opening ajar [2,26,50]. The mag¬
nitude of the load on a joint is important, but how that load is
distributed across the joint surface also is important. The data

380

Part II I KINESIOLOGY OF THE UPER EXTREMITY

EXAMINING THE FORCES BOX 19.3

CALCULATIONS OF THE FORCES AT THE PIP

M p + M s - (P X d 3 ) = 0

AND MCP JOINTS DURING PINCH

(11.0 kg X 0.98 cm) + M s - (P x d 3 ) = 0

d 3

the lengths of the moment arms of the

where 11.0 kg is the force in the FDP calculated in

pinch force

Examining the Forces Box 19.2

moment applied by the flexor digitorum

profundus

d 3 = 2.5 cm

moment applied by the FDS at the PIP

10.8 kg-cm + M s - (6.0 kg X 2.5 cm) = 0

moment applied by the intrinsic muscles at

M s = 15.0 kg-cm - 10.8 kg-cm

the MCP

M s 4.2 kg-cm

force of pinch, 6 kg

FDS X 0.83 cm = 4.2 kg-cm

FDP

force applied by the flexor digitorum

profundus

FDS = 5.1 kg

FDS

force applied by the FDS

Forces at the MCP:

force applied by the intrinsic muscles

0.98 cm

moment arm of the flexor digitorum

SM = 0

profundus at the PIP [31]

M p + M s + M, - (P X d 3 ) = 0

1.01 cm

moment arm of the flexor digitorum

Using the force of the flexor digitorum superficialis

profundus at the MCP [31]

(FDS) calculated above:

0.83 cm

moment arm of the flexor digitorum

superficialis at the PIP [31]

(11.0 kg x 1.01 cm) + (5.1 kg x 1.21 cm) + M, -

moment arm of the flexor digitorum

(P X d 3 ) = 0

superficialis at the MCP [31]

d 3 = 4.1 cm

0.3 cm

moment arm of the intrinsic muscles

kg-cm + 6.2 kg-cm + M, - (6.0 kg x 4.1 cm) = 0

at the MCP [31]

M, = 7.6 kg-cm

Forces at the PIP:

1 x 0.3 cm = 7.6 kg-cm

SM = 0

1 = 25.3 kg

The free-body diagram identifies the forces at the (A) PIP joint and at the (B) MCP joint during pinch. The lengths of the distal, middle,
and proximal phalanges are 1.2 cm, 1.6 cm, and 2.4 cm, respectively.

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

381

Figure 19.15: A hook grasp produces smaller joint reaction forces
at the finger joints because smaller muscle forces are needed to
balance the decreased extension moment (M) applied by the
briefcase at the MCP joint.

thus far demonstrate that the compressive loads increase
from the distal to the proximal joints of the fingers. Analysis
of contact areas reveals that the area of contact at the three
joints of the index finger decreases from proximal to distal so
that the joint reaction forces are distributed over a smaller
area at the distal joints than at the proximal joints [38]. Since
stress is the amount of force applied to an area (force/area),
this finding suggests that the stress on the joint increases from
proximal to distal. Thus although the joint reaction forces are
greatest in the MCP joints, the stresses on the joint appear to
be greatest in the DIP joints.

TABLE 19.2: Reported Joint Reaction Forces
Generated at the PIP and MCP Joints when Twisting
a Jar Lid

Direction

An et al. [2] a

Purves and
Berme [50] b

PIP joint

Compressive

7.2-14.2

18.0 ± 13.6

Dorsal

2.4-4.9

41.6 ± 27.6

Radial

0.2-0.8

15.5 ± 16.0

MCP joint

Compressive

14.8-24.3

45.2 ± 27.1

Dorsal

6.5-9.9

15.8 ± 15.5

Radial

0.2-0.3

12.5 ± 11.4

Reported in units of applied force.

^Reported in newtons, mean ± standard deviation, from 10 male and 10 female
subjects.

Clinical Relevance

DEGENERATIVE JOINT DISEASE IN THE HAND:

Degenerative joint disease (DJD) of the fingers is most com¬
mon in the DIP joints and relatively rare in the MCP joints
[28,46,47]. Although the link between joint stress and DJD
is not clearly identified, data suggest a positive connection
between the magnitude of stress to which a joint is subject¬
ed and the incidence of DJD [4,5]. The data reported on
joint stresses in the fingers support this connection. The
joints with the highest stress are the joints that have the
greatest incidence of DJD in the hand. Additional research is
needed to verify these findings, but the clinician should use
these data to implement joint protection strategies with indi¬
viduals at risk for DJD in the hands.

Models applied to the joints of the thumb also demon¬
strate large muscle forces and increasing joint reaction forces
from distal to proximal. Results are also variable, with joint
reaction forces ranging from 2 to 24 times the applied load
[7,16,22,27,55]. One study reports that the average maximum
external load exerted by the thumb in 70 male subjects (aver¬
age age, 27 years old) is 20 lb (89 N). This suggests that the
thumb joints could sustain reaction forces of almost 500 lb
(more than 2000 N). The clinician must keep these values
in mind and help to identify individuals at risk of hand
pathology. In addition, the clinician can use the perspective
gained from such studies to assist individuals in modifying
activities to reduce the loads on the thumb and fingers.

Studies of the contact areas at the thumb s CMC joint sug¬
gest that contact during pinch occurs over a very small area,
leading to very large stresses. The areas of large stress coin¬
cide with the sites of significant degenerative change. Thus as
in the fingers, stress on the thumb may be associated with
degenerative joint diseases [4,5].

Many activities have evolved using unusual finger and
thumb positions that while commonly accepted are likely to
generate large stresses on the small joints of the hands. For
example, flautists often assume positions of extreme hyperex¬
tension of the index fingers MCP joint or hyperextension of
the thumbs IP joint [35] (Fig. 19.16). Individuals performing
manual deep tissue massage often use end range hyperexten¬
sion in finger and thumb joints as their finger flexors fatigue
or are too weak to generate adequate force for the massage
(Fig. 19.17). Such positions alter the contact area of the artic¬
ulating joint surfaces and typically decrease the total area of
contact. Consequently, the joint loads are applied over smaller
surfaces and produce increased joint stress. Although the user
may feel that such joint positions are the most efficient posi¬
tions, perhaps even the “proper” position, prolonged use of
such extreme positions may lead to overuse syndromes and
ultimately to degenerative changes within the joint surfaces.
The clinician can play an important role in prevention of
joint injuries by helping the individual to understand the

382

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 19.16: Playing the flute often requires extreme positions
of the thumb and/or index finger.

relationship among joint position, joint stress, and joint
degeneration, and then assisting the individual to adapt the
activity to utilize joint positions that maximize contact area.

Clinical Relevance

FINGER SPLINTS TO OPTIMIZE JOINT ALIGNMENT:

Sometimes an individual is incapable of maintaining good
joint alignment throughout an activity. Perhaps maintaining
good alignment requires such concentration that the indi¬
vidual is too distracted from the primary activity Perhaps
the individual lacks the muscle strength or endurance to
maintain the good alignment for the length of the activity
In such cases , the individual may be best protected by using
external supports to maintain the desired position. Simple
finger splints are often used by manual therapists them¬
selves to support their fingers while applying deep tissue
massages (Fig. 19.18). Such devices can prevent joint pain
and fatigue and ultimately may help protect the joint from
degenerative joint disease by decreasing prolonged episodes
of high joint stress.

Figure 19.17: Extreme finger positions during massage. Deep tis¬
sue massage requires strong pressure through the fingers and
often results in hyperextension in finger or thumb joints.

Figure 19.18: A finger splint can protect the fingers from hyper¬
extension during strong pressure such as while giving a massage.

USING FORCE ANALYSIS TO MAKE
CLINICAL DECISIONS

How Forces Contribute to the Finger
Deformity of Ulnar Drift with Volar
Subluxation

The data from the literature thus far focuses primarily on the
compressive forces on the joint surfaces. Studies also demon¬
strate significant forces during both pinch and grasp that pull
the fingers in a volar and ulnar direction. These forces are
particularly apparent at the MCP joints and contribute to the
MCP deformity common in individuals with rheumatoid
arthritis. There are many more flexor muscles of the MCP joint
than extensor muscles, and these muscles, particularly the
extrinsic finger flexors, sustain large forces during pinch and
grasp. The normal angle of pull of the flexor tendons is small
[34]. Therefore under normal conditions, most of the pull of
the flexor tendons is directed parallel to the adjacent phalanx,
and only a small component exerts a volar force (Fig. 19.19). If
the tendon bowstrings, however, its angle of pull increases,
and the tendon exerts a larger volar force.

In rheumatoid arthritis affecting the MCP joint, the
inflammatory process can lead to laxity in the joint capsule
and surrounding ligaments [23]. Even the A1 pulley support¬
ing the tendons at the MCP joint may weaken [53]. Once the
pulley is weakened, the pull of the tendons within the pul¬
ley contributes to the stretch of the pulley As the pulley
stretches, the tendons begin to bowstring, increasing the
volar pull on the proximal phalanx. Because the joint is unsta¬
ble as a result of the changes in the capsule and ligaments, the
proximal phalanx begins to migrate volarly. At the same time,
as the pulley loosens, the flexor tendons are able to slip side¬
ways, typically in the ulnar direction [53]. Once the tendons
have displaced in the ulnar direction, active contraction of the
flexors produces an ulnar pull across the MCP joint
(Fig. 19.20). Because the joint is unstable, the proximal pha¬
lanx migrates in an ulnar direction as it migrates volarly, and
the deformity of ulnar drift with volar subluxation begins

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

383

Figure 19.19: Under normal conditions, the pull of the flexor ten¬
dons is almost parallel to the long axis of the finger {inset).

When a tendon bowstrings, its pull exerts a pull that has a com¬
ponent parallel to the phalanx and another significant compo¬
nent aimed in a volar direction.

(Fig. 19.21). The tendons of the extensor digitorum also can
slide ulnarly and contribute additional deforming forces.

There are additional predisposing factors that contribute
to this classic deformity:

• The heads of the metacarpals are normally sloped, so that
there is naturally more ROM in ulnar deviation than radial
deviation [56,59].

• In many normal activities the fingers are pushed in an
ulnar direction by external forces (Fig. 19.22).

These predisposing factors combined with the presence of
joint instability resulting from the inflammatory process of
rheumatoid arthritis and the deforming influence of the flex¬
or tendons create a cascade of factors that can result in a
disabling deformity.

Clinical Relevance

JOINT PROTECTION PRINCIPLES: While some of the ele¬
ments contributing to an ulnar drift deformity are immutable\
others respond to intervention. First , control of the disease
process itself is important. Disease-modifying medications
and other treatments to decrease the inflammatory process
in rheumatoid arthritis are increasingly successful but

(continued)

Figure 19.20: Ulnar pull of a subluxed flexor tendon. Once the
flexor tendons are displaced, their pull increases the forces
pulling the fingers into ulnar deviation.

Figure 19.21: Ulnar deviation with volar subluxation of the MCP
joints of the fingers in an individual with rheumatoid arthritis
occurs when swelling destabilizes the joints and the tendons of
the fingers migrate and exert a deforming force. (Reprinted
from the AHPA Teaching Slide Collection Second Edition now
known as the ARHP Assessment and Management of the
Rheumatic Diseases: The Teaching Slide Collection for Clinicians
and Educators. Copyright 1997. Used by permission of the
American College of Rheumatology.)

384

Part II I KINESIOLOGY OF THE UPER EXTREMITY

Figure 19.22: Many activities of daily living exert forces that push
the fingers into ulnar deviation.

(Continued)

approaches to lessen the deforming factors also are impor¬
tant Clinicians must be active in instructing patients to mod¬
ify their activities to reduce the deforming forces of the exter¬
nal load. For examplethe throttle-shaped handles on water
faucets allow an individual to turn the water on and off
using the palm of the hand rather than the fingers , which
are pushed ulnarly by the standard tap handle (Fig. 19.23).
Similarly\ individuals can be instructed to carry items in the
palm of the hand rather than in the fingers. Finally, clinicians
should recognize the danger in exercises to strengthen the
finger flexors , such as squeezing a ball. Such exercises are
contraindicated for individuals with unstable MCP joints.

Protecting a Surgically Repaired Tendon
in the Finger

The analysis of forces in the fingers during pinch and grasp
indicates that the flexor tendons to the fingers can sustain
large forces. Awareness of the forces sustained by tendons
under various circumstances is critical as a patient resumes
motion following a tendon repair. Early motion of the
repaired tendon appears essential to enhance lubrication and
avoid adhesions and scars that prevent normal excursion of
the tendon. However, early motion of the tendon also risks
disruption of the repair. The clinician must understand the
strength of normal and healing tendons and must be able to
adjust activities to avoid excessive loads in the repaired ten¬
don. A detailed discussion of the strength of connective tis¬
sues including tendons is presented in Chapter 6.

The strength of different tendon repair techniques is
extremely variable. Reported loads at which the repair begins
to separate, or gap, range from approximately 5 to 50 N (1 to
10 lb), depending on suture technique [18,54]. Loads that pro¬
duce complete disruption also depend on repair technique
and range from less than 10 N to more than 100 N (2 to 20 lb).

Figure 19.23: Simple changes in activities of daily living help to
reduce ulnar forces on the fingers. A. Carrying objects in the
palm instead of in the fingers reduces the ulnar forces. B. Use of
push-pull controls on water faucets exerts smaller deforming
forces than twist controls.

The goal of rehabilitation is to restore full tendon and joint
function, which requires mobilizing the repaired tendon
before it has regained its preinjury strength. The therapist
responsible for rehabilitation must know how to move the
joint and apply loads to the repairing tendon that will not dis¬
rupt the repair. In general, this requires that the clinician use

Chapter 19 I MECHANICS AND PATHOMECHANICS OF PINCH AND GRASP

385

active motion only in positions in which the repaired tendon
remains on slack and recognize that most functional activities
require tendon loads that far exceed the strength of the heal¬
ing tendon.

Clinical Relevance

RECOMMENDATIONS FOR EARLY ACTIVE MOTION
OF TENDON REPAIRS: Most surgeons and therapists rec¬
ommend early active motion of a tendon repair in the hand
to facilitate tendon lubrication and excursion and limit the
effects of scarring and adhesions. Mechanical analysis and
review of clinical results suggest that active finger flexion
with the wrist extended to 20° or active finger extension
with the wrist flexed to 20° can be accomplished safely for
flexor and extensor repairs; respectively [18]. Direct measure¬
ment of tendon forces reveals loads ranging from about 1
to 5 N (0.2 to 1.1 lb) in the flexor digitorum profundus and
from 1 to 10 N (0.2 to 2.25 lb) in the flexor digitorum super-
ficialis during active finger flexion with the wrist at neutral
or in 30° of flexion [37]. The higher loads in the flexor digi¬
torum superficialis occurred with the wrist flexed to 30°.
Avoidance of extreme finger or wrist positions is necessary
to prevent overloading of the repair site. Early mobilization
of tendon repairs in the hand appears essential for a favor¬
able outcome of the surgery. However ; mobilization of a
newly repaired tendon risks disruption of the repair. By
appreciating the strength of the repair procedure as well as
the loads generated during activity , the therapist can safely
guide the patient in activities to enhance the healing
process without endangering the integrity of the repair.
Therefore , close consultation with the surgeon is essential
during the planning and implementation of rehabilitation.

Relationship between the Forces in the
Finger Flexor Muscles and Carpal Tunnel
Syndrome

Carpal tunnel syndrome (CTS) is a compression of the median
nerve within the carpal tunnel. Symptoms include pain and
paresthesia in the hand, particularly in the area of sensory dis¬
tribution of the median nerve (see Fig. 18.24). Symptoms
may also include weakness of the intrinsic muscles innervated
by the median nerve. Although there is no clear understand¬
ing of the pathomechanics causing CTS, individuals in whom
CTS is frequently observed include those whose jobs are
characterized by repetitive, high-load manual tasks [42].
Elevated pressure within the carpal tunnel is a commonly
proposed explanation for CTS. Pinch loads and finger press
activities similar to typing correlate with elevated pressures
within the carpal tunnel. These activities require activation of
the extrinsic finger flexors, and increased recruitment of these
muscles corresponds to increased symptoms in individuals

with CTS [32,51]. One theory to explain this relationship sug¬
gests that the tension on the flexor tendons produced by mus¬
cle contraction straightens the tendons in the carpal tunnel,
causing increased compression of the median nerve [33].
Clinicians may help relieve symptoms by helping patients
find ways to decrease the force of contraction in the finger
flexor muscles. Prolonged positioning in wrist flexion also
appears to pose greater risks of CTS than wrist extension [24].
Wrist flexion puts the finger flexors in a shortened position,
which may require increased contraction force, leading to
increased carpal tunnel pressures.

Clinical Relevance

CONSERVATIVE MANAGEMENT OF CTS: Conservative
management of CTS typically includes the use of splinting
devices to support the wrist to allow muscle relaxation , and
patient education to help the patient avoid activities that
aggravate the symptoms. Data suggest that resting splints for
the wrist should be positioned in slight flexion , but the patient
should be taught to perform manual tasks with the wrist in
slight extension. The clinician must remain aware of the pos¬
sible connections between muscle force and pathology. This
awareness will enable the clinician to analyze a patient's
manual activities when there are complaints of CTS. Even a
qualitative analysis of the mechanical requirements of a task
may provide insight enabling the clinician to minimize the
forces in the finger flexor muscles and thus reduce the com¬
pression on the median nerve.

Forces Are Key in Ergonomic Assessments
of Work-Related Musculoskeletal
Disorders (WMSDs)

Work-related musculoskeletal disorders are injuries or disor¬
ders of the muscles, nerves, joints, and joint tissues that are
related to the exposure to risk at work [6]. Many physically
demanding jobs can put the worker at high risk for work-
related musculoskeletal disorders (WMSDs). In order to
decrease the incidence of WMSDs, biomechanists and
ergonomists attempt to measure the forces required to per¬
form a task and the number of times an individual can sustain
that force safely. Then they create evaluations to identify indi¬
viduals who can perform the task. For example, individuals
who maintain the power lines in the electrical utility industry
often must cut aluminum cable, typically 2 cm in diameter.
Investigators have demonstrated that using long-handled
manual cable cutters to cut a 2-cm cable requires a force on
the handles of approximately 500 N (112 lb) [43]. These
investigators suggest that less than 50% of the male popula¬
tion and less than 1% of the female population is strong
enough to perform this task. Such demands help explain the

386

Part II I KINESIOLOGY OF THE UPER EXTREMITY

high incidence of upper extremity WMSDs, including wrist
sprains and carpal tunnel syndrome among these workers.
Investigations such as these can help establish standards and
guidelines for safe and efficient working conditions [49].

Clinical Relevance

FUNCTIONAL CAPACITY EVALUATIONS: Occupational
and physical therapists frequently perform functional capacity
evaluations (FCEs) to determine if an individual has the
physical capability to begin or return to a certain job. FCEs
attempt to replicate or mimic the specific demands of the
job in order to determine if the individual can perform the
task safely Being aware of the forces required of the job
allows the therapist to specifically assess the individual's
ability to perform the task the requisite number of times for
successful employment In the case of rehabilitation , it also
allows the therapist to set clear targets for performance and
construct a rehabilitation program to achieve those targets.

SUMMARY

This chapter examines the joint and muscular requirements
of pinch and grasp. Normal pinch uses the radial side of the
hand and requires activity of intrinsic and extrinsic muscles of
the thumb and index finger. Powerful grasp uses the ulnar
side of the hand as well as the thumb and also requires intrin¬
sic and extrinsic muscle activity.

A simple analysis of the forces sustained by the muscles and
joints during pinch is described. Data from more-complex bio¬
mechanical models found in the literature are presented.
These data, although varied, demonstrate that during grasp
and pinch, the structures of the hand bear loads several times
the prehensile load. Generally, loads are greater in grasp than
in pinch. Areas of high stress in the fingers and thumb corre¬
spond to areas subject to osteoarthritis, suggesting a relation¬
ship between the loads sustained in the hand and degenerative
changes within the hand. Clinical applications demonstrate
how an awareness of the forces present in the structures of the
hand during function can affect the integrity of the hand as
well as influence the treatment approach.

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125-129.

PART

Kinesiology of the Head
and Spine

Vertebral body

Inferior articular
process of
superior vertebra

Superior articular
process of
inferior vertebra

Spinous process

UNIT 4: MUSCULOSKELETAL FUNCTIONS WITHIN THE HEAD

Chapter 20: Mechanics and Pathomechanics of the Muscles of the Face and Eyes

Chapter 21: Mechanics and Pathomechanics of Vocalization

Chapter 22: Mechanics and Pathomechanics of Swallowing

Chapter 23: Structure and Function of the Articular Structures of the TMJ

Chapter 24: Mechanics and Pathomechanics of the Muscles of the TMJ

Chapter 25: Analysis of the Forces on the TMJ during Activity

Chapter 26:
Chapter 27:
Chapter 28:
Chapter 29:
Chapter 30:
Chapter 31:
Chapter 32:
Chapter 33:
Chapter 34:
Chapter 35:
Chapter 36:
Chapter 37:

UNIT 5: SPINE UNIT

Structure and Function of the Bones and Joints of the Cervical Spine
Mechanics and Pathomechanics of the Cervical Musculature
Analysis of the Forces on the Cervical Spine during Activity
Structure and Function of the Bones and Joints of the Thoracic Spine
Mechanics and Pathomechanics of the Muscles of the Thoracic Spine
Loads Sustained by the Thoracic Spine

Structure and Function of the Bones and Joints of the Lumbar Spine
Mechanics and Pathomechanics of Muscles Acting on the Lumbar Spine
Analysis of the Forces on the Lumbar Spine during Activity
Structure and Function of the Bones and Joints of the Pelvis
Mechanics and Pathomechanics of Muscle Activity in the Pelvis
Analysis of the Forces on the Pelvis during Activity

389

UNIT 4

MUSCULOSKELETAL FUNCTIONS
WITHIN THE HEAD

T he preceding three units examine the structure, function, and dysfunction of the upper extremity, which is
part of the appendicular skeleton. Since the function of the remaining appendicular skeleton, the lower
extremities, is so intimately related to the spine, it is necessary first to investigate the spine, which is part of
the axioskeleton. The axioskeleton includes the head and spine, and this text begins its examination of the axioskele-
ton at the head and proceeds in a caudal direction. The current unit examines the function and dysfunction of the mus¬
culoskeletal components of the head. These structures work in concert with each other in diverse functions including
facial expression, vocalization, chewing, and swallowing. This unit is divided rather artificially by function, and the
structures most associated with each function are described within the context of that function. However, the reader
must recognize that many anatomical components participate in multiple functions. For example, the lips participate in
facial expressions, chewing, and speech, and the tongue is equally important in swallowing and speech.

The first three chapters of this unit deviate slightly from the organization used in other parts of this textbook because
they focus on the overall functions of facial expression, vocalization, and swallowing. The structure of bones and joints
plays a smaller role in the understanding of these functions, so the chapters present a less detailed review of the rele¬
vant anatomical structures. Although plastic surgeons require a detailed knowledge of the structures within the face,
and otolaryngologists and speech and language specialists need a more detailed understanding of the larynx and phar¬
ynx, conservative management of functional deficits is typically based on more-global assessments of impairments in
these activities, and few individuals are able to isolate single muscles throughout the face, mouth, and throat.
Therefore, each of the next three chapters presents a discussion of the role of the muscles participating in the specified
function. The purposes of the first three chapters are to

■ Examine the muscles that move the face and eyes (Chapter 20)

■ Describe the intrinsic muscles of the larynx and discuss the mechanics of voice production (Chapter 21)

■ Review the muscles of the mouth and pharynx and discuss the sequence of movements that constitute the swallow
(Chapter 22)

Chapters 23 through 25 in this unit focus on the temporomandibular joint, in which a more detailed understanding of
the skeletal, articular, and muscular components is necessary to understand the function and dysfunction of the joint.
Consequently, these chapters return to the organization used in most of this text. The purposes of the last three chap¬
ters of this unit are to

■ Present the bony and articular structures of the temporomandibular joint and describe the motions that occur
(Chapter 23)

■ Review the muscles of mastication and their contribution to chewing (Chapter 24)

■ Review the forces sustained by the temporomandibular joints under various conditions (Chapter 25)

390

CHAPTER

Mechanics and Pathomechanics
of the Muscles of the Face and Eyes

CHAPTER CONTENTS

DISTRIBUTION OF THE FACIAL NERVE .391

MUSCLES INNERVATED BY THE FACIAL NERVE.392

Muscles of the Scalp and Ears .393

Facial Muscles Surrounding the Eyes.395

Muscles of the Nose.397

Muscles of the Mouth .399

MUSCLES THAT MOVE THE EYES .406

SUMMARY.410

T he muscles of the face are small and superficial, attaching at least in part to the skin of the face. The resulting
skin movement is an essential part of human communication, allowing a face to express love, rage, sadness,
fear, and a multitude of other human emotions [14,20,23].

Human expression is enhanced by movements of the eyes, such as when an individual rolls the eyes in disgust.
Appropriate and coordinated eye movement also is critical to clear and accurate vision. This chapter presents the mus¬
cles that produce facial and ocular movements and discusses the dysfunctions resulting from pathology affecting these
muscles. The specific purposes of this chapter are to

■ Present the muscles of facial expression

■ Discuss the movement dysfunctions that result from weakness in these muscles
■ Describe the muscles that move the eyes

■ Discuss the coordination of the eye muscles that produces smooth eye movements essential for proper vision

DISTRIBUTION OF THE FACIAL NERVE

The muscles of facial expression are innervated by the motor
branch of the seventh cranial nerve, known as the facial
nerve (Fig. 20.1). As it emerges from the stylomastoid fora¬
men of the temporal bone, the facial nerve gives off a branch,
the posterior auricular nerve, to the occipitalis and the pos¬
terior auricularis muscle. The terminal portion of the facial

nerve, lying within the parotid gland, divides into several
branches that go on to supply the rest of the muscles of facial
expression:

• The temporal branch supplies the anterior and superior
auricular muscles and the frontalis, orbicularis oculi, and
corrugator muscles.

• The zygomatic branch supplies the lateral portions of the
orbicularis oculi.

391

392

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 20.1: The facial nerve gives off the posterior auricular
nerve, and then its terminal portion divides into several branches:
temporal, zygomatic, buccal, mandibular, and cervical.

of motor control or may be the manifestation of muscle
weakness. The clinician requires additional evidence before
determining that a muscle is weak. Such corroborating
evidence includes the function of surrounding muscles, the rest¬
ing posture of the face, and the condition of the facial skin.

Clinical Relevance

FACIAL CREASES: As noted in Chapter 77, most normal
skin creases are formed by the pull of underlying muscles
that lie perpendicular to the creases. Most facial creases are
the consequence of activity of the facial muscles that lie just
underneath the skin. Because facial creases are the superfi¬
cial manifestations of muscle activity under the skin , the
absence of facial creases in an adult may indicate weakness
in underlying facial muscles. The clinician must be cautious
to avoid interpreting the smooth , unlined skin of an elder
patient as the consequence of a lifetime of good skin care
when it may actually indicate muscular weakness. Careful
observation of the wrinkles of both sides of the face allows
the clinician to recognize asymmetrical wrinkle patterns that
may indicate asymmetrical muscle performance and possi¬
ble pathology. Since individual palpation of single muscles
is impossibleinspection of these facial wrinkles is an impor¬
tant component of an assessment of the facial muscles.

• The buccal branch innervates the muscles of the nose and
the zygomaticus, levator labii superioris, levator anguli
oris, orbicularis oris, and buccinator.

• The mandibular branch supplies the muscles of the lower
lip and the mentalis.

• The cervical branch supplies the platysma.

An understanding of the organization of the facial
nerve helps the clinician recognize and evaluate the
clinical manifestations of facial nerve palsies.

MUSCLES INNERVATED
BY THE FACIAL NERVE

Most of the muscles innervated by the facial nerve are
muscles of facial expression, unique because they cross
no joints and attach to aponeuroses and, directly or indirectly,
to the skin of the face, producing movement of the facial
skin [39,50,51]. There are approximately 21 pairs of muscles
in the face. However, asymmetry in movements produced
by individual muscles within a pair is common among
healthy individuals [13,30,36,44]. Consequently, clinicians
must be cautious when determining the clinical significance
of asymmetrical facial excursion. For example, many indi¬
viduals can raise one eyebrow but not the other [13]. The
inability to raise an eyebrow may reflect a common lack

The muscles of facial expression surround the orifices of the
face, regulating their apertures, and pull on the skin, thereby
modifying facial expressions. The functions of the muscles of
facial expression are less well studied than that of the muscles
in the limbs and spine. The classic understanding of these
muscle actions is reported in standard anatomy texts, which
are cited in the discussions that follow [39,51]. However, there
is a growing body of literature describing the activity of facial
muscles by using electromyography (EMG) to examine the
participation of these muscles in facial movements, and these
studies also are cited in the following discussions [22,55].

Many of the muscles of the face attach to each other and,
therefore, participate together in facial movements. Other
muscles, although anatomically separated, appear to function
together routinely in certain expressions [40]. The remarkably
coordinated contractions of the zygomaticus major, a muscle of
the mouth, and the orbicularis oculi, the muscle surrounding
the eye, during a smile suggests that these two muscles may
even share a common innervation. Other muscles also appear
to function in synergies to produce facial expressions that
involve most of the face.

Few people can voluntarily contract all of the muscles of
the face individually [4]. Unlike most of the muscles of the
upper and lower extremities, the muscles of facial expression
cannot be assessed individually through palpation or manual
muscle testing. Not only do they rarely contract in isolation,
they are too small and close together to be palpated. Neely

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

393

and Pomerantz report the use of a force transducer to assess
the strength of facial movements but individual muscles cannot
be isolated [34]. The load transducer measurements indicate
that movements about the eye and lips can withstand less
than one pound of force applied through the transducer.

Since individual muscles cannot be palpated or tested
separately, the clinician must assess the muscle s perform¬
ance during function, that is, by examining the individuals
facial function. Therefore, this text groups the muscles
together according to the region of the face affected by their
contractions. The discussion includes the actions performed
by the muscles and the emotional expressions typically associ¬
ated with the muscle activity. Weakness of these muscles
affects facial expressions and facial wrinkles and also has an
impact on functional activities such as chewing and speech.
The clinical manifestations of weakness are discussed with
each muscle.

Muscles of the Scalp and Ears

The muscles of the scalp and ears include the frontalis, occip¬
italis, and the auricularis anterior, posterior and superior
(Fig. 20.2). Only the frontalis has a visible and reliable contri¬
bution to emotional expression, yet all four muscles may be
activated during looks of surprise [3].

Auricularis:

Figure 20.2: The muscles of the scalp and ears include the occipi¬
tofrontalis and the auriculares superior, posterior, and anterior.

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE OCCIPITOFRONTALIS

Bony/fascial attachment:

Occipitalis: Lateral two thirds of superior nuchal
line on the occiput, the mastoid process of the
temporal bone, and the epicranial (galea)
aponeurosis

Frontalis: Epicranial (galea) aponeurosis

Soft tissue attachment: Skin of the occipital and
frontal regions

Innervation:

Occipitalis: Posterior auricular branch of facial
nerve

Frontalis: Temporal branches of facial nerve
(7th cranial nerve)

FRONTALIS AND OCCIPITALIS

The frontalis and occipitalis actually are the anterior and pos¬
terior muscle bellies of a single muscle, the occipitofrontalis,
although they are frequently listed separately and can func¬
tion independently of one another [3,26] (Muscle Attachment
Box 20.1). They are separated by the galea aponeurotica,
which is a large fibrous sheet covering the cranium. The
action of the frontalis portion of the muscle is more observ¬
able and is the portion typically evaluated clinically

Actions

MUSCLE ACTION: FRONTALIS

Action

Evidence

Lift eyebrows

Supporting

MUSCLE ACTION: OCCIPITALIS

Action

Evidence

Pull the scalp posteriorly

Supporting

The reported action of the frontalis is to lift the eyebrows.
By lifting the eyebrows, the frontalis contributes to a look
of surprise [3,36,50]. It also pulls the galea aponeurotica
forward, creating the horizontal wrinkles in the forehead.
The occipitalis pulls the gala aponeurotica posteriorly,
thereby stabilizing it against the pull of the frontalis. The
occipitalis also is active in smiling and yawning, although its
functional significance is unclear [3].

394

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Weakness

Weakness of the occipitofrontalis is manifested in weakness of
the frontalis portion, which limits or prevents the ability to
raise the eyebrows. Consequently, the eyebrows are somewhat
drooped, stretching the skin of the forehead and reducing or

Figure 20.3: The frontalis and part of the orbicularis oculi receive
contributions from both hemispheres of the motor cortex, unlike
the rest of the facial muscles and most muscles of the body, which
receive contributions only from the contralateral hemisphere.

Figure 20.4: A facial nerve palsy produces weakness of the
frontalis because the nerve, albeit with input from both hemi¬
spheres, does not carry the stimulus to the muscle.

eliminating the forehead wrinkles. When weakness of the
frontalis is suspected, careful inspection of the forehead for
the presence or absence of wrinkles helps the clinician deter¬
mine the muscle s integrity.

Weakness of the frontalis is an important clinical finding that
helps clinicians distinguish between upper and lower motor
neuron lesions [5]. Most muscles are innervated by nerves that
are supplied by the contralateral motor cortex of the brain [31].
The frontalis and part of the orbicularis oculi, however, receive
input from the motor cortex of both the contralateral and ipsi-
lateral hemispheres via the temporal branch of the facial nerve
through synapses in the facial motor nucleus (FMN) [5,51,52]
(Fig. 20.3). As a result, a central nervous system disorder such
as a cerebral vascular accident (CVA) that affects the motor
cortex of one hemisphere may produce weakness of all of the
muscles of facial expression except the frontalis, which is only
mildly affected since it still receives input from the ipsilateral
hemisphere. In contrast, a lower motor neuron lesion to the
facial nerve produces weakness in all of the facial muscles
including the frontalis, since the facial nerve is the final common
pathway to the muscles of facial expression (Fig. 20.4). Facial
weakness with sparing of the frontalis suggests an upper motor
neuron lesion, while facial weakness including the frontalis sug¬
gests a lower motor neuron lesion.

AURICULARES ANTERIOR, SUPERIOR,

AND POSTERIOR

The auriculares muscles are much less developed in humans
than in animals who rotate their ears to localize the sounds of
prey or predators (Muscle Attachment Box 20.2).

Action

MUSCLE ACTION: AURICULARES

Action

Evidence

Wiggle the ears

Inadequate

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

395

MUSCLE ATTACHMENT BOX 20.2

ATTACHMENTS AND INNERVATION
OF THE AURICULARES

Bony/fascial attachment:

Anterior: Temporal fascia and epicranial
aponeurosis

Superior: Epicranial aponeurosis and temporal
fascia

Posterior: Surface of the mastoid process of the
temporal bone

Soft tissue attachment:

Anterior: Cartilage of the ear

Superior: Cartilage of the ear

Posterior: Cartilage of the ear

Innervation: Posterior auricular and temporal
branches of facial nerve (7th cranial nerve)

The theoretical action of the auriculares muscles is to wiggle the
ears. In a study of 442 university students, approximately 20%
exhibited the ability to move either ear, and slightly less than
20% could move both ears simultaneously [13]. Evaluation of
the auriculares muscles is not clinically relevant.

Facial Muscles Surrounding the Eyes

The facial muscles affecting the eyes are the orbicularis
oculi, levator palpebrae superioris, and corrugator (Fig. 20.5).
Contraction of these three muscles manifests a variety of
emotions such as anger, confusion, and worry. In addition, the
orbicularis oculi plays a critical role in maintaining the health
of the eye.

ORBICULARIS OCULI

The orbicularis oculi is a complex muscle that is arranged
circumferentially around the eye and is attached to the medial
and lateral borders of the orbit (Muscle Attachment Box 20.3).
Its fibers vary in size and length and are primarily type II
fibers with rapid contraction velocities [18,27].

Action

MUSCLE ACTION: ORBICULARIS OCULI

Action

Evidence

Close the eye

Supporting

Pull eyebrows medially

Supporting

The orbicularis oculi is one of the most important muscles of
facial expression [17]. By closing the eye in spontaneous
blinks, the orbicularis oculi lubricates the eye, spreading the

Figure 20.5: The muscles of the face affecting the eye include the
orbicularis oculi, the levator palpebrae superioris, and the
corrugator.

MUSCLE ATTACHMENT BOX 20.3

ATTACHMENTS AND INNERVATION
OF THE ORBICULARIS OCULI

Bony attachment:

Orbital part: Nasal part of frontal bone, frontal
process of maxilla, medial palpebral ligament

Palpebral part: Medial palpebral ligament and
adjacent bone above and below

Lacrimal part: Crest of the lacrimal bone and fascia

Soft tissue attachment:

Orbital part: Palpebral ligament after arching
around the upper and lower eyelid

Palpebral part: Palpebral raphe formed by the
interlacing of the fibers at the lateral angle of
the eye

Lacrimal part: Medial portion of the upper and
lower eyelids with the lateral palpebral raphe

Innervation: Temporal and zygomatic branches of
facial nerve (7th cranial nerve)

396

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 20.6: Weakness of the orbicularis oculi prevents eye clo¬
sure and can cause the patient to look surprised because the eye
is opened wide.

MUSCLE ATTACHMENT BOX 20.4

ATTACHMENTS AND INNERVATION
OF THE LEVATOR PALPEBRAE SUPERIORS

Bony attachment: Roof of the orbit just in front of
the optic canal

Soft tissue attachment: Skin of the upper lid and tri¬
angular aponeurosis, which attaches to the mid¬
point of the medial and lateral orbital margins

Innervation:

Somatic portion: Superior division of the oculo¬
motor nerve (3rd cranial nerve)

Visceral portion: Sympathetic nervous system

tears excreted by the lacrimal gland. Spontaneous blinks
occur at a rate of approximately 12 or 13 blinks per minute
(up to 750 blinks per hour) [18,25]. Reflex blinks are critical
to protecting the eye from foreign objects. The muscle s high
density of type II muscle fibers is consistent with the need to
perform rapid, fleeting contractions. In contrast, the orbicu¬
laris oculi, like other muscles of facial expression, is unable to
tolerate sustained contractions of several seconds duration
without fatigue [6,18].

The medial and superior muscle fibers of the orbicularis
oculi assist in drawing the eyebrows medially, and the muscle
is active during the expression of emotions such as anger and
contentment [20,50,51]. The wrinkles formed by the contrac¬
tion of the orbicularis oculi lie perpendicular to the muscle s
fibers and radiate from the corners of the eye in the charac¬
teristic “crows feet” pattern [51].

Weakness

Weakness of the orbicularis oculi results in the inability to
close the eye (Fig. 20.6). A patient with weakness of the orbic¬
ularis oculi often exhibits a perpetual look of surprise because
the affected eye is maintained in a wide-open position.

Clinical Relevance

WEAKNESS OF THE ORBICULARIS OCULI: Weakness of
the orbicularis oculi is the most serious consequence of facial
weakness because it impairs the lubricating mechanism of the
eye. if the eye is unable to close at regular and frequent inter¬
vals to spread tears over the surface of the eye, the cornea
dries , which can lead to ulceration and impaired vision [17].

In addition , foreign objects may enter the eye without the pro¬
tection of the reflex blink. Consequently, the patient with facial
weakness must obtain immediate consultation with an oph¬
thalmology specialist who can prescribe the appropriate inter¬
vention to maintain the necessary lubrication and protection
of the eye. The patient may wear a protective eye patch to
prevent drying of or trauma to the eye.

LEVATOR PALPEBRAE SUPERIORS

The levator palpebrae superioris is technically an extrinsic
muscle of the eye and, unlike the muscles of facial expres¬
sion, is innervated by the third cranial nerve, the oculomotor
nerve (Muscle Attachment Box 20.4). It is discussed here
because the levator palpebrae superioris is the antagonist to
the orbicularis oculi.

Action

MUSCLE ACTION: LEVATOR PALPEBRAE SUPERIORIS

Action

Evidence

Opens the eye

Supporting

It is because the levator palpebrae is not innervated by the
facial nerve that a patient with a facial nerve palsy affecting
the orbicularis oculi maintains a wide-eyed expression. In the
patient with facial weakness, the levator palpebrae pulls with¬
out the normal balance of its antagonist, the orbicularis oculi,
and the eye remains wide open. In a healthy awake individual,
the levator palpebrae superioris maintains a low level of activity
to keep the eye open, but activity decreases as the orbicularis
oculi closes the eye. Increased activity occurs when the eye
opens wide in a look of surprise or excitement [51].

Weakness

Weakness of the levator palpebrae superioris leads to droop¬
ing of the upper eyelid, known as ptosis. Ptosis interferes
with vision, since the eyelid droops over the eye, obscuring
the view. Surgical intervention can be useful in mechanically
lifting the eyelid to improve vision.

CORRUGATOR

The corrugator lies deep to the frontalis (Muscle Attachment
Box 20.5). Unlike the orbicularis oculi, it is composed of
approximately equal proportions of type I and type II muscle
fibers and, consequently, is more fatigue resistant [18].

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397

MUSCLE ATTACHMENT BOX 20.5

ATTACHMENTS AND INNERVATION
OF THE CORRUGATOR SUPERCILII

Bony attachment: Medial bone of the supraciliary arch

Soft tissue attachment: Skin of the medial half of
the eyebrow, above the middle of the supraorbital
margin, blending with the orbicularis oculi

Innervation: Temporal branch of facial nerve
(7th cranial nerve)

Action

MUSCLE ACTION: 6 CORRUGATOR

Action

Evidence

Pull eyebrows medially and down

Supporting

The corrugator contracts with the orbicularis oculi to pull the
eyebrows down (Fig. 20.7). It is active when an individual
squints to protect the eyes from bright lights. Its activity also
is a characteristic part of a frown and is associated with emo¬
tions such as anger and confusion [15,20,50,51]. Contraction
of the corrugator produces vertical creases at the superior
aspect of the nose.

Figure 20.8: Muscles of the nose include the procerus, the trans¬
verse and alar portions of the nasalis, the dilator naris, and the
depressor septi.

Muscles of the Nose

Weakness

There is no known functional deficit associated with weakness
of the corrugator muscle, but weakness leads to flattening of
the skin at the medial aspect of the eyebrow.

Figure 20.7: Contraction of the corrugator with the medial
portion of the orbicularis oculi draws the eyebrows together.

There are four primary facial muscles of the nose: the procerus,
the nasalis with its transverse and alar portions, the dilator naris,
and the depressor septi [9,10,12] (Fig. 20.8). The procerus
appears to function primarily in facial expressions [9,10]. The
other muscles of this group also move or stabilize the nose and
are active during respiration [9,10,12]. The functional impor¬
tance of these muscles is not well studied and, consequently, the
functional significance of weakness in these muscles is
unknown, although weakness does contribute to facial asym¬
metry. Only the actions of these muscles are discussed below.

PROCERUS

The procerus lies close to the orbicularis oculi and the corru¬
gator (Muscle Attachment Box 20.6).

MUSCLE ATTACHMENT BOX 20.6

ATTACHMENTS AND INNERVATION
OF THE PROCERUS

Bony attachment: Fascia covering the lower parts of the
nasal bone and upper part of the lateral nasal cartilage

Soft tissue attachment: Skin over the lower part of
the forehead and between the eyebrows

Innervation: Superior buccal branches of facial nerve
(7th cranial nerve)

398

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 20.9: Contraction of the procerus produces wrinkles across
the bridge of the nose. Contraction often occurs with contraction
of the levator labii superioris and the levator anguli oris in a look
of distaste.

Action

MUSCLE ACTION: PROCERUS

Action

Evidence

Pull the nose cranially

Supporting

Pull eyebrows down

Supporting

Contraction of the procerus contributes to the characteristic
look of distaste, as an individual wrinkles the nose at an
unpleasant smell, flavor, or idea [2,51] (Fig. 20.9). The mus¬
cle participates with the orbicularis oculi and corrugator in a
frown [50,51].

NASALIS

The nasalis consists of two components, the transverse and
alar segments [9,10,12,39] (Muscle Attachment Box 20.7).

Actions

MUSCLE ACTION: NASALIS, TRANSVERSE SEGMENT

Action

Evidence

Compress and stabilize
lateral wall of nose

Supporting

EMG data support the role of the transverse portion of the
nasalis muscle in compressing or flattening the nose [12].
Such movement is associated with a look of haughtiness. The

MUSCLE ATTACHMENT BOX 20.7

ATTACHMENTS AND INNERVATION
OF THE NASALIS

Bony attachment:

Transverse part: Upper end of the canine
eminence and lateral to the nasal notch of
the maxilla

Alar part: Maxilla above the lateral incisor tooth

Soft tissue attachment:

Transverse part: Aponeurosis of the nasal
cartilages

Alar part: Cartilaginous ala of the nose and skin
of the lateral part of the lower margin of the ala
of the nose

Innervation: Superior buccal branches of facial nerve
(7th cranial nerve)

movement also is important functionally in closing off the
nasal airway during speech when making vocal sounds such as
“b” and “p.”

Studies report activity in the transverse portion of the
nasalis during inspiration [9,10]. These studies suggest that
this activity stiffens the outer walls of the nose to prevent
collapse as the pressure within the nose decreases during
inspiration. Additional studies are needed to verify or refute
this explanation.

MUSCLE ACTION: NASALIS, ALAR SEGMENT

Action

Evidence

Dilate nostrils

Supporting

Draw nostrils down
and posteriorly

Inadequate

Flaring the nostrils elicits EMG activity in the alar portion of
the nasalis [12]. Although the ability to flare the nostrils seems
unimportant to most humans, studies demonstrate activity in
this muscle during inspiration, particularly during increased
respiration following exercise [9,10,12,49]. The activity of the
alar portion of the nasalis appears to stabilize the nostrils dur¬
ing inspiration while the pressure within the nose is low, tend¬
ing to collapse the nostrils.

DILATOR NARIS

The dilator naris is described by some as a part of the nasalis
[51] but is described separately in this text because recent
studies analyze and describe it separately [9,10,12] (Muscle
Attachment Box 20.8).

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

399

MUSCLE ATTACHMENT BOX 20.8

ATTACHMENTS AND INNERVATION
OF THE DILATOR NARIS

Soft tissue attachment: The cartilaginous ala of
the nose

Innervation: Superior buccal branches of facial nerve
(7th cranial nerve)

Actions

MUSCLE ACTION: DILATOR NARIS

Action

Evidence

Dilate nostrils

Supporting

The dilator naris appears to function with the alar portion of

the nasalis to maintain the shape of the nose during inspira-

tion [9,10,12].

DEPRESSOR SEPTI

The depressor septi is a

small muscle lying at the base of the

nose (Muscle Attachment Box 20.9).

Action

MUSCLE ACTION: DEPRESSOR SEPTI

Action

Evidence

Pull nose down

Supporting

Elevate upper lip

Inadequate

EMG activity is reported in the depressor septi when subjects
attempt to flatten the nose or to “look down the nose” in a
snobbish manner [9,12]. The muscle also is active during
inspiration with the other muscles of the nose, presumably to
stabilize the nose.

MUSCLE ATTACHMENT BOX 20.9

ATTACHMENTS AND INNERVATION
OF THE DEPRESSOR SEPTI

Bony attachment: Incisive fossa of the maxilla

Soft tissue attachments: Mobile part of the nasal
septum and posterior part of the ala of the nose

Innervation: Superior buccal branches of facial nerve
(7th cranial nerve)

Muscles of the Mouth

The muscles of the mouth serve several purposes:

• Control the aperture of the mouth

• Stabilize the oral chamber and alter its volume

• Change the position of the mouth and surrounding skin to
produce varied verbal sounds and convey a wide spectrum
of emotions from elation to abject sorrow

The muscles that attach to the lips and act as constrictors
of the mouth consist of the orbicularis oris and the mentalis
(Fig. 20.10). The dilators of the mouth are the zygomaticus,
risorius, levator labii superioris, levator labii superioris alaeque
nasi, levator anguli oris, depressor labii inferioris, depressor anguli
oris, and platysma (Fig. 20.11). Control of the oral aperture main¬
tains food and liquid within the oral cavity. The size and shape of
the mouth also are critical in speech, contributing to the variety of
vowel and consonant sounds in oral speech [2,29]. The volume
regulators are the buccinator muscles.

Although each muscle applies a unique pull on the lips or
cheeks, studies consistently demonstrate that muscles of the
mouth participate together during eating and speech [2,4,11,
29,53]. It is virtually impossible to activate these muscles

Figure 20.10: Constrictor muscles of the mouth are the
orbicularis oris and the mentalis muscles. The buccinator
controls the volume of the mouth.

400

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 20.11: Dilator muscles of the mouth are the zygomaticus,
risorius, levator labii superioris and levator labii superioris
alaeque nasi # levator anguli oris, depressor labii inferioris,
depressor anguli oris, and platysma.

individually through voluntary contraction and almost as diffi¬
cult to isolate them with electrical stimulation [4]. Consequently
evaluation requires the assessment of the coordinated move¬
ments of the mouth in activities such as smiling, eating, and
speaking. Weakness is most apparent in the asymmetrical and
sometimes grotesque facial movements that result from a loss
of balance among these muscles. With weakness of the muscles
of the mouth on one side of the face, the unaffected muscles
pull the mouth toward the intact side, since there is no coun¬
teracting force from the opposite side. It is important for the
clinician to recognize that this imbalance produces a mouth
that looks smooth and “normal” on the weakened side but con¬
tracted and contorted on the unaffected side. Care is needed to
correctly distinguish the weak from the unaffected side.

Clinical Relevance

BELLS PALSY: Acute idiopathic facial nerve palsy is known
as Bells palsy and is characterized by weakness of the
muscles innervated by the facial nerve (7th cranial nerve)

Figure 20.12: A facial nerve palsy on the left produces weakness
of the muscles innervated by the facial nerve on the left. This
individual displays classic signs of facial weakness, including
absence of forehead wrinkles on the left. The left eye is abnor¬
mally wide open, and the mouth is pulled to the strong side.

(Fig. 20.12). It typically is unilateral and usually temporary,
although the time course of recovery varies from days to
years [8,37]. Exercise and biofeedback have been shown to
enhance recovery in patients with facial nerve palsies [7,8].
Clinicians must be able to evaluate the integrity of the mus¬
cles of facial expression to establish goals, implement treat¬
ment, and monitor progress. It is essential that clinicians be
able to identify weakness even when unable to apply a spe¬
cific muscle assessment to each individual muscle.

ORBICULARIS ORIS

The orbicularis oris is one of the most important muscles of
facial expression because it is the primary constrictor muscle
of the mouth (Muscle Attachment Box 20.10). Although it
usually is described as a single muscle [39], its superior and
inferior portions found in the upper and lower lips, respec¬
tively, can function independently [2,43,51,54].

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE ORBICULARIS ORIS

Soft tissue attachments: To the fibrous intersection
of many muscles, known as the modulus, located
lateral to the corners of the mouth and into the soft
tissue of the lips. It is a sphincter muscle formed by
various muscles converging on the mouth.

Innervation: Lower buccal and mandibular branches
of facial nerve (7th cranial nerve)

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

401

Actions

MUSCLE ACTION: ORBICULARIS ORIS

Action

Evidence

Close lips

Supporting

The orbicularis oris is the sphincter for the mouth and is active
whenever mouth closure is needed. It is active in chewing, to
retain the food within the mouth [45,46,53]. It is used to help
slide food from a utensil such as a fork or spoon, and it is
essential during sucking through a straw or blowing on a clar¬
inet [33,35,39,51]. It participates in speech to make sounds
such as ££ p” and “b” and assists in the expression of love or
friendship, since it is the muscle used to kiss [41,53].

The orbicularis oris has a relatively large cross-sectional
area and, consequently, is capable of forceful contractions.
Studies report compression forces between the two lips up to
2-4 N (approximately 0.5-1.0 lb) [16,45].

Weakness

Weakness of the orbicularis oris diminishes the ability to close
the mouth firmly, producing oral incontinence. A patient
with weakness of the orbicularis oris muscle reports a tendency
to drool or an inability to hold liquid in the mouth. Attempts
to whistle are futile, with the air leaking out through the weak¬
ened side of the mouth. The patient may also exhibit altered
speech, with particular difficulty in pronouncing words that
include the sounds of letters such as ££ p,” “b,” and ££ w.”

A patient with weakness of the orbicularis oris exhibits
flattening of the lips on the affected side. When the muscle
contracts, the lips are pulled toward the unaffected side,
producing a distorted posture of the mouth, particularly
pronounced on the sound side (Fig. 20.13).

Figure 20.13: Contraction of the orbicularis oris with unilateral
weakness pulls the mouth to the strong side and causes the
weak side to appear smooth and without wrinkles.

MENTALIS

Although the mentalis has no direct connection to the lips, it
is the only other muscle that can assist the orbicularis oris in
closing the mouth (Muscle Attachment Box 20.11).

Actions

MUSCLE ACTION: MENTALIS

Action

Evidence

Raise and protrude lower lip

Supporting

Raise and wrinkle skin of chin

Supporting

The mentalis helps the orbicularis oris in sucking actions by
pulling the lower lip up and forward, and the muscle is active
in such actions as sucking on or blowing through a straw
[1,2,43,47,51]. Protrusion of the lower lip also is characteris¬
tic of a pouting expression (Fig. 20.14).

Figure 20.14: Contraction of the mentalis pulls the lower lip
anteriorly and superiorly, the characteristic position in a pout.

402

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE ZYGOMATICUS

Bony attachment:

Major: Zygomatic portion of zygomatic arch
Minor: Anterior and lateral zygomatic bone
Soft tissue attachment:

Major: Skin and orbicularis oris at the angle of
the mouth

Minor: Skin and muscle of the upper lip

Innervation: Buccal branches of facial nerve
(7th cranial nerve)

Weakness

Weakness of the mentalis limits the ability to protrude the
lower lip. The weakness contributes to the asymmetrical pos¬
ture of the mouth during sucking actions, with the lower lip
on the affected side appearing flat while the lip on the unaf¬
fected side appears distorted as it protrudes alone.

ZYGOMATICUS

The zygomaticus is one of the muscles that dilate the orifice
of the mouth, although its primary functional significance is to
express emotion (Muscle Attachment Box 20.12).

Actions

MUSCLE ACTION: ZYGOMATICUS

Action

Evidence

Pull edges of mouth
superiorly and laterally

Supporting

The zygomaticus is the smile muscle, contributing to the
characteristic broad full smile that brings the corners of the
mouth toward the eyes [2,32,42] (Fig. 20.15). It is important,
however, to recognize that several muscles are active in this
sort of smile. The zygomaticus does not contract alone [24].

Weakness

Weakness of the zygomaticus alters the form of an attempted
smile. As the patient smiles, the unaffected muscle pulls the
mouth vigorously toward the sound side, producing a rather
grotesque image [24] (Fig. 20.16).

Clinical Relevance

PSYCHOLOGICAL CHALLENGES FOR A PATIENT WITH
FACIAL PALSY: Weakness of the facial muscles, particularly
around the mouth, produces significant social challenges to

Figure 20.15: The primary muscle of a broad smile is the zygo¬
maticus, but most of the other dilators of the mouth also partici¬
pate, pulling the lips away from the teeth.

the patient. Weakness of the orbicularis oris may make eat¬
ing difficult and embarrassing , as the patient is unable to
avoid leakage of the food or liquid from the mouth. In addi¬
tion , facial expressions that are the natural manifestations
of emotions such as joy or sorrow are no longer the familiar
smiles or frowns but rather grotesque caricatures of such
expressions. As a result , many patients are reluctant to leave
the privacy of their own homes [48].

Figure 20.16: Contraction of the dilator muscles with unilateral
weakness pulls the mouth to the strong side, leaving the weak side
smooth and without wrinkles. (Photo courtesy of Martin Kelley
MSPT, University of Pennsylvania Health Systems, Philadelphia, PA.)

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

403

MUSCLE ATTACHMENT BOX 20.13

ATTACHMENTS AND INNERVATION
OF THE RISORIUS

Bony attachment: Zygomatic bone

Soft tissue attachment: Fascia of the parotid gland,
fascia over the masseter muscle, fascia of the platys-
ma, fascia over the mastoid process, and the skin at
the angle of the mouth

Innervation: Buccal branches of facial nerve
(7th cranial nerve)

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE LEVATOR LABI I SUPERIORS
AND LEVATOR LABII SUPERIORS
ALAEQUE NASI

Bony attachment: Maxilla and zygomatic bone
superior to the infraorbital foramen

Soft tissue attachment: Orbicularis oris of the
upper lip and the cartilaginous ala of the nose

Innervation: Buccal branches of facial nerve
(7th cranial nerve)

RISORIUS

The risorius is another dilator of the mouth and functions
with the zygomaticus (Muscle Attachment Box 20.13).

Weakness

Weakness of the risorius, like the zygomaticus, results in a dis¬
torted smile with the mouth pulled toward the unaffected side.

Actions

MUSCLE ACTION: RISORIUS

Action

Evidence

Pull edges of mouth laterally

Supporting

Although the risorius typically contracts with the zygomaticus,
when its activity is primary, the risorius produces a grimace
that can convey feelings of disgust, dislike, frustration, or other
emotions (Fig. 20.17).

Figure 20.17: When the risorius is the primary muscle active
at the mouth, the lips are pulled laterally in a grimace.

LEVATOR LABII SUPERIORS AND LEVATOR LABII
SUPERIORS ALAEQUE NASI

The two levator labii superioris muscles lie between the nose
and the mouth, contributing to the characteristic furrow
between the side of the nose and the corners of the mouth
(.Muscle Attachment Box 20.14 ).

Actions

MUSCLE ACTION: LEVATOR LABII SUPERIORIS
AND LEVATOR LABII SUPERIORIS ALAEQUE NASI

Action

Evidence

Lift the upper lip and
turn it outward

Supporting

The action of the two levator labii superioris muscles pro¬
duces the common look of disgust or revulsion and typically
coincides with contraction of the procerus [10]. These mus¬
cles also contribute to retraction of the lips during a large
smile [2,42]. The levator labii superioris alaeque nasi also con¬
tributes to the dilation of the nostrils with the alar portion of
the nasalis and the dilator naris [51].

Weakness

Weakness of the two levator labii superioris muscles con¬
tributes to a flattening of the lips in a smile. The patient also
may report a tendency to bite the upper lip, particularly while
eating. Weakness of these muscles tends to flatten the furrow
between nose and mouth. Since this furrow deepens with age
normally, weakness of the levator labii superioris muscles
tends to make an older individual appear younger.

LEVATOR ANGULI ORIS (ALSO KNOWN AS CANINUS)

The levator anguli oris also contributes to the furrow between
the nose and upper lip (Muscle Attachment Box 20.15).

404

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 20.15

ATTACHMENTS AND INNERVATION OF
THE LEVATOR ANGULI ORIS (CANINUS)

Bony attachment: Canine fossa of the maxilla imme¬
diately below infraorbital foramen

Soft tissue attachment: Fibers intermingle with the
skin and orbicularis oris at the lateral angle of mouth

Innervation: Buccal branches of facial nerve
(7th cranial nerve)

MUSCLE ATTACHMENT BOX 2

Actions

MUSCLE ACTION: LEVATOR ANGULI ORIS

Action

Evidence

Lift lateral aspect of upper lip

Supporting

ATTACHMENTS AND INNERVATION
OF THE DEPRESSOR LABII INFERIORS

Bony attachment: Oblique line of the outer surface
of the mandible between the symphysis and mental
foramen deep to the depressor anguli oris

Soft tissue attachment: Skin and mucosa of the
lower lip, mingling with the orbicularis oris

Innervation: Mandibular branches of facial nerve
(7th cranial nerve

DEPRESSOR LABII INFERIORS

Depressor labii inferioris is a dilator of the mouth, affecting
the lower lip (Muscle Attachment Box 20.16).

Actions

By lifting the lateral aspect of the lip, the levator anguli oris
exposes the canine tooth, which gives the muscle its other
name, caninus. Although many individuals are unable to iso¬
late this muscle, its action is associated with a sneering expres¬
sion (Fig. 20.18). Like the other dilator muscles, the levator
anguli oris participates in a broad smile [42].

Weakness

Weakness of the levator anguli oris contributes to a distorted
smile.

MUSCLE ACTION: DEPRESSOR LABII INFERIORIS

Action

Evidence

Pull lower lip down and
turn it outward

Supporting

Contraction of the depressor labii inferioris exposes the
lower teeth. The action of the depressor labii inferioris is
generally associated with the emotions of sadness or anger
manifested by a frown. However, the muscle also appears to
be active in large smiles in which the lips are pulled back
from both rows of teeth [38,42].

Weakness

Like all of the muscles that attach to the lips described so far,
weakness of the depressor labii inferioris contributes to dis¬
tortions of the mouth when the patient frowns or smiles, and
the mouth is pulled toward the stronger side.

DEPRESSOR ANGULI ORIS

The last of the primary depressors of the lips, the depressor
anguli oris, is active with the depressor labii inferioris (Muscle
Attachment Box 20.17).

Figure 20.18: When the levator anguli oris is active primarily, the
lip is pulled up and laterally in a sneer.

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE DEPRESSOR ANGULI ORIS

Bony attachment: Mental tubercle and oblique line
of mandible

Soft tissue attachment: Orbicularis oris and skin at
angle of mouth

Innervation: Mandibular branches of facial nerve
(7th cranial nerve)

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

405

Figure 20.19: The depressor anguli oris is primarily responsible
for the classic frown, although the other depressors of the lips
are active as well.

Figure 20.20: Contraction of the depressors of the lip with
unilateral weakness pulls the mouth to the strong side, leaving
the weak side smooth and without wrinkles. (Photo courtesy of
Martin Kelley, MSPT, University of Pennsylvania Health Systems,
Philadelphia, PA.)

PLATYSMA

The platysma is a broad, thin sheet of muscle extending from
the mouth to the upper thoracic region (Muscle Attachment
Box 20.18). It is superficial, lying just below the skin in the
cervical region.

Actions

MUSCLE ACTION: PLATYSMA

Action

Evidence

Pull corners of mouth
and lower lip down

Supporting

Assist in inspiration

Inadequate

Support skin in cervical region

Inadequate

Actions

MUSCLE ACTION: DEPRESSOR ANGULI ORIS

Action

Evidence

Pull angles of mouth
down and laterally

Supporting

The action of the depressor anguli oris is associated with the
emotion of sadness, since contraction contributes to the clas¬
sic frown (Fig. 20.19).

Weakness

Weakness of the depressor anguli oris contributes, with the
other muscles of the mouth, to the distortions of the mouth as
it is pulled toward the unaffected side. Loss of the depressor
anguli oris is particularly apparent when a patient, depressed
or saddened by the effects of the facial weakness, begins to cry.
The mouth is pulled down and laterally by the unaffected
depressor anguli oris, causing the whole mouth to deviate
toward the strong side (Fig. 20.20).

MUSCLE ATTACHMENT BOX 20.18

ATTACHMENTS AND INNERVATION
OF THE PLATYSMA

Bony attachment: Skin and superficial fascia of the
upper pectoral and deltoid regions. Fibers cross the
clavicle and pass obliquely upward and medially
along the sides of the neck.

Soft tissue attachment: Anterior fibers of either side
interlace with each other below the chin, at the sym¬
physis menti. Intermediate fibers attach at the lateral
half of the lower lip and lower border of the body of
the mandible. Posterior fibers connect with depressor
labii inferioris and depressor anguli oris and pass the
angle of the jaw to insert into the skin and subcuta¬
neous tissue of the lower part of the face.

Innervation: Cervical branch of the facial nerve
(7th cranial nerve)

406

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 20.21: Contraction of the platysma contributes to a look
of horror.

The actions of the platysma are not well studied. The attach¬
ments of the platysma are consistent with the actions listed
above [2,51]. Contraction of the platysma often contributes
to a look of horror (Fig. 20.21). Observation of an individual
in respiratory distress typically reveals contraction of the
platysma during inspiration, but the significance of such a
contraction is unknown.

Actions

MUSCLE ACTION: BUCCINATOR

Action

Evidence

Compress the cheek

Supporting

The buccinator muscle is an essential muscle in chewing. By
compressing the cheeks, the buccinator keeps the bolus of
food from getting caught in the buccal space, the space
between the mandible and the cheek. The buccinator also
controls the volume of the oral cavity and thereby controls the
pressure within the cavity. This role is particularly important
to musicians who play brass or woodwind instruments but is
used by anyone who has blown out the candles on a birthday
cake. The buccinator stiffens the cheeks so that the air can be
expelled under pressure while contraction of the orbicularis
oris muscles directs the air stream toward the target [35].

Weakness

Weakness of the buccinator produces several serious difficul¬
ties in chewing. Weakness of the muscle allows the food to
become sequestered in the buccal space, so the patient can¬
not grind the food effectively between the teeth. Prolonged
sequestering also can lead to skin breakdown and tooth decay.
In addition, with little control of the cheek, a patient is prone
to biting the inner wall of the cheek while chewing. Weakness
of the buccinator also produces difficulty in blowing air out
forcefully through pursed lips, so a patient has difficulty play¬
ing a brass or wind instrument.

Weakness

MUSCLES THAT MOVE THE EYES

The significance of platysma weakness is unknown.

BUCCINATOR

The buccinator is the muscle of the cheek, with only an indi¬
rect attachment to the lips by way of the orbicularis oris
(Muscle Attachment Box 20.19).

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE BUCCINATOR

Bony attachment: Outer surface of alveolar process
of maxilla and mandible opposite the sockets of the
molar teeth and the anterior border of the pterygo¬
mandibular raphe posteriorly

Soft tissue attachment: The orbicularis oris and the
lips and submucosa of the mouth

Innervation: Lower buccal branches of facial nerve
(7th cranial nerve)

There are seven extrinsic muscles of the eye, including the
levator palpebrae superioris, which is discussed earlier in this
chapter. The remaining six muscles are responsible for moving
the eye within the orbit and include the superior, inferior, medial,
and lateral rectus muscles and the superior and inferior oblique
muscles (Fig. 20.22). Evaluation and treatment of these muscles
are the primary responsibility of ophthalmologists and neurolo¬
gists. Rehabilitation specialists participate in the conservative
management of patients with impairments of these muscles and
require an understanding of the basic mechanisms that produce
normal eye movements described in this text.

To understand the movements produced by these muscles,
it is necessary to appreciate the axes of motion that form the
reference frame for eye movement (Fig. 20.23). Movements
of the eye are described with respect to the axes through the
eye itself. Elevation and depression occur about the medial
lateral axis; medial and lateral rotation, also known as
adduction and abduction, occur about a vertical axis; and
intorsion and extorsion occur about the anterior-posterior
axis. Intorsion is defined as the motion that rotates the superior
surface of the eye medially toward the nose. Extorsion is
motion of the same point laterally toward the ear.

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

407

Figure 20.22: The extrinsic muscles that move the eye are the
medial and lateral rectus, the superior and inferior rectus, and
the superior and inferior oblique muscles.

Figure 20.23: Motion about the vertical axis is medial and lateral
rotation (adduction and abduction, respectively). Motion about a
medial lateral axis is elevation and depression, and motion about
an anterior posterior axis is intorsion and extorsion. Intorsion is
the movement that moves the superior aspect of the eye medially,
and extorsion moves the superior surface of the eye laterally.

Figure 20.24: Axes of the eye compared with the alignment of
the orbit. The axes of the eye are aligned in the cardinal planes
of the body; however, the orbits of the eyes project anteriorly
and laterally.

The orbit of the eye projects anteriorly and laterally within
the skull, but the anterior-posterior axis of each eye lies in the
sagittal plane during normal forward vision (Fig. 20.24). The
differences between the axes of the eye and the axes
of the orbit contribute to the complexity of the motions
produced by the extrinsic muscles of the eye. Additionally, the
extrinsic muscles cannot be observed or assessed by palpation;
EMG analysis also is rarely possible. Consequently, these
muscles are not well studied. The following provides a basic
description of the current understanding of the muscles that
move the eye. Effects of weakness are discussed together
following the descriptions of all the muscles.

MEDIAL AND LATERAL RECTUS MUSCLES

Both the medial and lateral rectus muscles lie close to the
transverse plane when vision is focused on the horizon, so their
activity produces movement about a vertical axis through the
eye [51] (Muscle Attachment Box 20.20).

Actions

MUSCLE ACTION: MEDIAL RECTUS

Action

Evidence

Rotate eye medially (adduct)

Supporting

408

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 20.2

ATTACHMENTS AND INNERVATION
OF THE MEDIAL AND LATERAL RECTUS
MUSCLES

Bony attachment: The optic canal by a common
annular ligament

Soft tissue attachment: The medial and lateral
scleral surfaces of the eye respectively, posterior to
the cornea

Innervation: Medial rectus by the oculomotor nerve
(3 rd cranial nerve). Lateral rectus by the abducens
nerve (6th cranial nerve).

MUSCLE ATTACHMENT BOX 2

ATTACHMENTS AND INNERVATION
OF THE SUPERIOR AND INFERIOR
RECTUS MUSCLES

Bony attachment: Optic canal, by a common
annular ligament

Soft tissue attachment: Superior and inferior scleral
surfaces of the eye, respectively, posterior to the
cornea

Innervation: Oculomotor nerve (3rd cranial nerve)

MUSCLE ACTION: LATERAL RECTUS

Action

Evidence

Rotate eye laterally (abduct)

Supporting

The medial and lateral rectus muscles work together to turn
the gaze to the right or left [28,51]. As the head faces ante¬
riorly, gaze to the left requires contraction of the left lateral
rectus and the right medial rectus (Fig. 20.25).

Figure 20.25: Movement of both eyes to the left while the head
faces forward requires the medial rectus on the right and the lat¬
eral rectus on the left.

SUPERIOR AND INFERIOR RECTUS MUSCLES

The actions of the superior and inferior rectus muscles are
more complex than those of the medial and lateral recti
because the superior and inferior recti are more or less
aligned along the walls of the orbit and, therefore, pull
obliquely with respect to the axes of the eye (Muscle
Attachment Box 20.21).

Actions

MUSCLE ACTION: SUPERIOR RECTUS

Action

Evidence

Eye elevation

Supporting

Eye medial rotation

Supporting

Eye intorsion

Supporting

The superior rectus clearly contributes to elevation of the eye,
but its contribution to the other motions is less obvious.
Careful observation of the attachment of the superior rectus
reveals that it lies medial to the anterior-posterior and verti¬
cal axes, which explains the muscles contributions to intor¬
sion and medial rotation respectively [28,51] (Fig. 20.26).

Actions

MUSCLE ACTION: INFERIOR RECTUS

Action Evidence

Eye depression Supporting

Eye medial rotation Supporting

Eye extorsion Supporting

The attachment of the inferior rectus muscle on the inferior
surface of the eye explains its role as a depressor of the eye. It
passes medial to the vertical axis to participate in medial rota¬
tion and attaches lateral to the anterior-posterior axis to con¬
tribute to extorsion [28,51] (Fig. 20.25).

Chapter 20 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE FACE AND EYES

409

Figure 20.26: The superior rectus is aligned with the orbit of
the eye, but its position medial to the vertical and the anterior-
posterior axes explains its contributions to medial rotation and
intorsion.

SUPERIOR OBLIQUE

The superior oblique muscle travels a circuitous route to the
eye, wrapping around a pulley-like structure and traveling
posteriorly and laterally to attach posterior to the medial-
lateral and vertical axes and lateral to the anterior-posterior
axis [28,51,52] (Muscle Attachment Box 20.22) (Fig. 20.22).

MUSCLE ATTACHMENT BOX 20.22

ATTACHMENTS AND INNERVATION
OF THE SUPERIOR OBLIQUE MUSCLE

Bony attachment: Sphenoid bone superior and
medial to the optic canal

Soft tissue attachment: Sclera of the eye, posterior
to the eye's equator and on the superior lateral sur¬
face, between the attachments of the superior rec¬
tus and the lateral rectus muscles. As the muscle
progresses anteriorly through the orbit toward its
attachment on the eye, it passes through a fibrous
loop, or pulley, to redirect its fibers posteriorly and
laterally.

Innervation: Trochlear nerve (4th cranial nerve)

MUSCLE ATTACHMENT BOX 20.23

ATTACHMENTS AND INNERVATION
OF THE INFERIOR OBLIQUE MUSCLE

Bony attachment: the maxilla on the floor of the orbit

Soft tissue attachment: the sclera of the eye, on its
inferior, posterior, and lateral surfaces, between the
rectus inferior and lateralis muscles

Innervation: Oculomotor nerve (3rd cranial nerve)

Actions

MUSCLE ACTION: SUPERIOR OBLIQUE

Action

Evidence

Eye depression

Inadequate

Eye lateral rotation

Inadequate

Eye intorsion

Inadequate

INFERIOR OBLIQUE

The inferior oblique muscle travels posteriorly and laterally to
its attachment posterior and lateral to the axes of the eye
[28,51,52] (Muscle Attachment Box 20.23).

Actions

MUSCLE ACTION: INFERIOR OBLIQUE

Action

Evidence

Eye elevation

Inadequate

Eye lateral rotation

Inadequate

Eye extorsion

Inadequate

WEAKNESS OF THE MUSCLES
THAT MOVE THE EYE

Movements of the eyes appear to be the result of a complex
and rhythmic coordination of the muscles of the eye. The eye
is moving continuously in individuals with normal motor con¬
trol of the eyes, and it is likely that all of the muscles of the
eyes contract together, producing a steady gaze even when
the body or the target moves in space. An imbalance among
the extrinsic muscles of the eye produces strabismus, the
inability to direct the gaze of both eyes toward an object [52].
Strabismus in adults may produce double vision, or diplopia,
although young children are often able to accommodate by
ignoring the input from the misaligned eye. Weakness of
either medial or lateral rectus may impair the ability to scan
from side to side, creating difficulties in such activities as
reading. For example, a lesion of the abducens (sixth cranial
nerve) produces weakness of the lateral rectus muscle. The
antagonistic medial rectus pulls the eye into medial rotation,

410

Part III I KINESIOLOGY OF THE HEAD AND SPINE

producing a “crossed eye.” Peripheral vision also is challenged
if the lateral rectus is impaired, although compensations by
head movements may be available.

Clinical Relevance

RESTORING MUSCLE BALANCE SURGICALLY: Imbalance
in muscle strength of the extrinsic muscles of the eye may pro¬
duce significant vision disturbances , including double vision, or
diplopia. Muscle balance can sometimes be restored by increas¬
ing the strength of the weaker muscle through exercise. But
sometimes the intervention aims to decrease the effect of the
stronger muscle. This can be accomplished surgically by
changing the moment that the stronger muscle can generate.
Haslwanter and colleagues suggest a procedure to relocate
the attachment of the stronger muscle closer to the axis of
rotation, thereby reducing the moment generated during
muscle contraction [ 19,21]. The reader may be surprised
to recognize that the basic principles of biomechanics learned
in Chapter 1 are relevant even in ophthalmic surgery!

Weakness of the superior oblique deserves special note,
since it alone is innervated by the trochlear nerve (fourth
cranial nerve). Although both the inferior rectus and superior
oblique muscles depress the eye, only the superior oblique
can depress the eye when the eye is medially rotated. An
individual with weakness of the superior oblique muscle has
difficulty looking down and in, a requirement of many activi¬
ties of daily living such as descending stairs or examining the
keyboard of a computer [52].

Clinical Relevance

TROCHLEAR NERVE INJURY: A patient may be seen for
complaints of frequent tripping when descending stairs. Such
complaints commonly result from weakness in the lower
extremities. However, visual disturbances specifically associated
with weakness of the superior oblique muscle of the eye also
may produce complaints of difficulty descending stairs.
Trochlear nerve lesions may need to be considered in the
absence of direct associations between impairments in the
lower extremities and the functional complaints.

SUMMARY

This chapter presents the function of the muscles of facial
expression and the muscles that move the eye. The muscles
of facial expression are organized around the orifices of the
head, ears, eyes, nose, and mouth. The muscles surrounding
the eyes and mouth play a vital role in opening and closing
their respective orifices. The muscles of utmost importance
are the orbicularis oculi, which closes the eye, protecting

it from foreign matter and helping to lubricate it, and the
orbicularis oris, which closes the mouth, essential for normal
chewing and speech. The muscles surrounding the nose help
control the size of the nasal opening and passageways during
respiration and speech.

Weakness in the muscles of facial expression poses a
significant threat to the eye and produces impairments in
chewing and speech. In addition, weakness of the muscles of
facial expression alters the normal facial responses and often
results in asymmetrical and grotesque facial postures. In
many cases the facial skin is pulled toward the strong muscles,
producing smooth unwrinkled skin on the weakened side and
excessively wrinkled and puckered skin on the strong side.

The extrinsic muscles of the eye work in concert to pro¬
duce smooth, well-coordinated eye movements, allowing an
individual to maintain a steady gaze even as the individual or
target moves. Weakness in any of these muscles impairs the
coordinated movements of both eyes and may lead to double
vision or reduced vision in a specific field.

The muscles of the face and eyes work together in complex
combinations to produce finely controlled facial expressions
and discrete eye movements. Impairments of single muscles
are uncommon, and isolated examination of individual mus¬
cles is unrealistic. Therefore, the clinician needs to appreciate
the types of disturbances in movement patterns that can
occur with weakness of these muscles.

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CHAPTER

Mechanics and Pathomechanics
of Vocalization

CHAPTER CONTENTS

LARYNX.413

Laryngeal Cartilages.413

INTRINSIC MUSCLES OF THE LARYNX.416

Muscles That Close the Vocal Cords.416

Muscles That Open the Vocal Cords .418

Muscles That Alter the Tension in the Vocal Cords .418

MECHANISM OF VOICE PRODUCTION .419

Phonation .420

Resonance and Pronunciation .420

Common Abnormalities in Voice Production .421

SUMMARY .421

O ral communication is central to the function of most human beings. It is a function that involves several

regions of the body including the mouth, the larynx, and even the abdomen and several systems including
the musculoskeletal, respiratory, and nervous systems. The diagnosis and treatment of speech problems is
outside the purview of most clinicians who specialize in neuromusculoskeletal disorders. However, voice production
involves the voluntary and involuntary activity of many structures that are part of the musculoskeletal system. A basic
understanding of the structures and mechanisms used to produce voice allows rehabilitation specialists to collaborate
constructively with the specialists responsible for treating patients with speech and language impairments. In addition,
recent changes in the health care delivery system in the United States have caused patients to receive a large propor¬
tion of their health care in the home, where a neuromusculoskeletal expert may be the first, or only, practitioner to see
the patient. Therefore, clinicians must be able to recognize signs of speech and swallowing dysfunction. Many muscles
that participate in speech also contribute to facial expressions and function in swallowing. Even a basic understanding
of the mechanics of speech provides the clinician with additional tools to screen for impairments affecting any of these
functions. If impairments are suspected, clinicians are reminded to seek appropriate referrals to qualified health profes¬
sionals who treat patients with speech disorders.

The purpose of this chapter is to introduce the clinician to the structures of the musculoskeletal system that participate
in the production of speech sounds and to provide an overview of the mechanics of voice production. The specific
goals of this chapter are to

■ Describe the structures of, and movements in, the larynx that result in voice
■ Present the intrinsic muscles of the larynx and explain their function
■ Explain the musculoskeletal contributions to the production of sounds and words

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Chapter 21 I MECHANICS AND PATHOMECHANICS OF VOCALIZATION

413

LARYNX

The larynx, or voice box, consists of a cartilaginous framework
composed of three single cartilages and three pairs of cartilages
(Fig. 21 . 1 ). It lies approximately at the level of the third through
sixth cervical vertebrae in adult males and slightly more superi¬
orly in females and children [22,25] (Fig. 21 . 2 ). After puberty the
larynx is larger in males than in females, which contributes to the
differences in pitch between male and female voices. The larynx
performs important functions in swallowing, respiration, and
phonation, and the movements that occur in these functions are
similar, varying primarily in the amount of movement that is
used. The cartilages and their movements are described below.

Laryngeal Cartilages

The three single cartilages of the larynx are the cricoid carti¬
lage, the thyroid cartilage, and the epiglottis. The paired

Figure 21.1: The larynx comprises three single cartilages—the
thyroid, cricoid, and epiglottis—and three pairs of cartilages—
the arytenoid, the corniculate, and the cuneiform cartilages.

A. Posterior view. B. Lateral view.

cartilages are the arytenoid, the corniculate, and the cuneiform
cartilages.

CRICOID CARTILAGE

The cricoid cartilage is the base of the larynx, articulating with
the trachea inferiorly and attached superiorly to the thyroid
cartilage and arytenoid cartilages by synovial joints. The
cricoid cartilage forms a complete ring of cartilage with a
broad posterior surface, the lamina, and a thin anterior arch so
that the ring is shaped like a signet ring (Fig. 21 . 3 ). Laterally,
at the junctions of the lamina and arch, two articular facets
face posterolaterally for articulation with the thyroid cartilage.
The inferior surface of the cricoid cartilage lies in the trans¬
verse plane and attaches to the first tracheal cartilage, while its
superior surface slopes posteriorly and superiorly.

THYROID CARTILAGE

The thyroid cartilage is a larger cartilaginous structure than
the cricoid cartilage and lies superior to the cricoid. It is com¬
posed of two large cartilage wings, or alae, that join together
anteriorly, forming the familiar prominence in the throat, the
Adams apple, or laryngeal prominence. Viewed from above
the thyroid cartilage is V-shaped, with the opening facing pos¬
teriorly (Fig. 21 . 4 ). The angle of the thyroid cartilage is nar¬
rower in males, making the Adams apple more prominent
and the vocal cords longer, thereby contributing to the deeper
pitch of male voices. Extending superiorly and inferiorly from
the posterior aspect of each wing are projections that articu¬
late superiorly with the hyoid bone, a U-shaped bone at the
angle of the mandible, and inferiorly with the cricoid cartilage
(Fig. 21 . 5 ). The inferior projections form a mortise around
the cricoid cartilage similar to the mortise for the talus
formed by the distal tibia and fibula.

The attachment between the hyoid bone and the thyroid
cartilage causes the thyroid cartilage to move with the hyoid
bone; when the hyoid bone elevates, the thyroid cartilage ele¬
vates, and when the hyoid bone is depressed, the thyroid car¬
tilage is depressed. In contrast, movement between the thy¬
roid cartilage and the cricoid cartilage can be independent or
in unison. Elevation of the hyoid bone elevates the cricoid
cartilage with the thyroid cartilage. Consequently, elevation of
the hyoid bone produces elevation of the larynx. The mortis
joint between the cricoid and thyroid cartilages allows the
cricoid cartilage to tilt upward or downward with respect to
the thyroid cartilage, much as the talus tilts up or down in
dorsiflexion or plantarflexion at the ankle joint (Fig. 21 . 6 ).

EPIGLOTTIS

The epiglottis is a leaf-shaped fibrocartilaginous structure
that projects by its stem from the posterior surface of the
laryngeal prominence of the thyroid cartilage. The broad, flat
portion extends superiorly toward the posterior aspect of the
tongue and hyoid bone but remains free of any additional
attachment. As the larynx is elevated in swallowing, the
epiglottis folds posteriorly, forming a protective lid over the

414

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 21.2: The larynx is located between about the third and sixth cervical vertebrae.

Posterior

Figure 21.3: The cricoid cartilage is a ring of cartilage with a thin
anterior arch and a large posterior lamina, so that the cricoid
cartilage looks like a signet ring.

Arytenoid Posterior

Figure 21.4: The thyroid cartilage from the superior view forms
an angle that opens posteriorly. The anterior aspect of the angle
is the laryngeal prominence, or Adam's apple.

Chapter 21 I MECHANICS AND PATHOMECHANICS OF VOCALIZATION

415

Figure 21.5: The thyroid cartilage articulates with the hyoid bone
superiorly and with the cricoid cartilage interiorly by way of its
superior and inferior horns, respectively.

larynx so that the swallowed bolus slides over the epiglottis
into the esophagus (Fig. 21 . 7 ).

ARYTENOID CARTILAGES

The paired arytenoid cartilages rest posteriorly on the superi¬
or surfaces of the cricoid cartilage. They are roughly pyrami¬
dal, with their bases resting on the cricoid cartilage and the
pyramids projecting superiorly toward the posterior aspect of
the thyroid cartilage (Fig. 21 . 8 ). When viewed from above,
the base of each arytenoid cartilage exhibits a slight
boomerang shape with an anterior vocal process and a poste¬
rior and lateral muscular process.

Figure 21.6: Motion of the cricoid cartilage with respect to the
thyroid cartilage. The cricoid cartilage rotates about a medial lat¬
eral axis within the mortis formed by the inferior horns of the
thyroid cartilage.

Figure 21.7: Motion of the epiglottis. As the larynx elevates, the
epiglottis passively folds over the larynx.

The arytenoid cartilages articulate with the cricoid carti¬
lage by a gliding type of synovial joint supported by a joint
capsule and posterior cricoarytenoid ligament. The joints
allow rotation of the arytenoid cartilages on the cricoid carti¬
lage in the transverse plane and gliding toward and away from
each other. The slope of the cricoid cartilage produces a
simultaneous superior translation as the arytenoid cartilages
move toward each other, or adduct. Similarly, the arytenoid
cartilages glide inferiorly as they move away from each other,
or abduct (Fig. 21 . 9 ).

Figure 21.8: A. A lateral view reveals that the arytenoid
cartilages are pyramidal, with the apex extending superiorly.

B. A superior view reveals that the base of the arytenoid carti¬
lages is shaped like a boomerang, with a vocal process anteriorly
and a muscular process posteriorly and laterally.

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Part III I KINESIOLOGY OF THE HEAD AND SPINE

Posterior

Figure 21.9: Movement of the arytenoid cartilages. A. The ary¬
tenoid cartilages rotate medially and laterally on the cricoid car¬
tilage. B. The arytenoid cartilages translate and produce adduc¬
tion and abduction of the vocal folds.

CORNICULATE AND CUNEIFORM CARTILAGES

The corniculate and cuneiform cartilages are tiny, paired car¬
tilages, the former resting on the superior aspect of the ary¬
tenoid cartilages and the latter imbedded in the soft tissue
folds that encase the epiglottis and extend to the arytenoid
cartilages, the aryepiglottic folds. They provide structural sup¬
port to the mucous membrane that lines the larynx and thus
help to protect the airway [22].

Vocal Folds

The function of the vocal folds is to produce sound by
vibrating in much the same way the vibrating string of a
violin or guitar produces sound (Fig. 21.10). The vocal
folds are the thickened, medial borders of two broad liga¬
ments that connect the cricoid, thyroid, and arytenoid car¬
tilages, known as the cricothyroid ligaments. The medial
border of each cricothyroid ligament is thickened elastic
tissue, the vocal cord, which is important during vibra¬
tion of the folds. The paired vocal folds form a sort of cur¬
tain that opens and closes across the larynx and is imbed¬
ded in a mucosal lining. The opening between the two
vocal folds is known as the rima glottis, through which
air passes during respiration and vocalization and which
closes during swallowing to protect the airway. Control of
this aperture between the vocal folds is provided by the
intrinsic muscles of the larynx and is the basis for all of the
functions of the larynx.

Arytenoid

Figure 21.10: A superior view reveals that the vocal folds are
formed by the medial borders of the cricothyroid ligaments and
are covered by mucous membrane. The opening between the
vocal folds is known as the rima glottis.

INTRINSIC MUSCLES OF THE LARYNX

The extrinsic muscles of the larynx are those that raise and
lower it and include the supra- and infrahyoid muscles,
respectively. These muscles are discussed in greater detail in
Chapter 22. The intrinsic muscles of the larynx lie within
the laryngeal cartilage framework and cannot be palpated.
These muscles produce discrete motions of the cartilages of
the larynx and contribute to the unique functions of the lar¬
ynx by regulating the size of the rima glottis and the tension
of the vocal cords. The intrinsic muscles of the larynx are the
cricothyroid muscle, the lateral and posterior cricoarytenoid
muscles, the transverse and oblique interarytenoid muscles,
and the thyroarytenoid muscle. All but the transverse inter¬
arytenoid muscle are paired. Much of the knowledge of the
function of the intrinsic muscles of the larynx is based on
anatomical studies. More recently, the use of endoscopic
imaging and electromyography has advanced the understand¬
ing of the role these muscles play during swallowing and
vocalization [18,24]. However, the technical difficulty of these
measurements and the challenges in controlling for the nor¬
mal variability of the human voice explain why the number of
studies available and the total number of subjects studied
remain a fraction of the investigations available for the rest of
the musculoskeletal system [13].

Muscles That Close the Vocal Cords

The muscles that close, or adduct, the vocal cords are the
transverse and oblique interarytenoid muscles and the lateral
cricoarytenoid muscle (Muscle Attachment Boxes 21.1-21.3)
(Fig. 21.11). Adduction of the vocal cords is important in
altering pitch during phonation (described later in this chap¬
ter). Adduction also assists in protecting the airway during a
swallow by contributing to the closure of the larynx.

Chapter 21 I MECHANICS AND PATHOMECHANICS OF VOCALIZATION

417

MUSCLE ATTACHMENT BOX 21.1

ATTACHMENTS AND INNERVATION OF
THE OBLIQUE INTERARYTENOID MUSCLE

Attachments: Posterior surface of the muscular
process of one arytenoid cartilage to the apex of
the opposite arytenoid cartilage

Innervation: Recurrent laryngeal nerve, a branch of
the vagus nerve (10th cranial nerve)

ACTIONS OF THE TRANSVERSE AND OBLIQUE
INTERARYTENOID MUSCLES

MUSCLE ATTACHMENT BOX 21.3

ATTACHMENTS AND INNERVATION OF
THE LATERAL CRICOARYTENOID MUSCLE

Attachments: Superior border of the cricoid arch
anteriorly to the anterior surface of the muscular
process of the arytenoid cartilage on the same
side

Innervation: Recurrent laryngeal nerve, a branch of
the vagus nerve (10th cranial nerve)

MUSCLE ACTION: OBLIQUE AND TRANSVERSE
INTERARYTENOID MUSCLES

Action

Evidence

Adduct the vocal cords

Supporting

Both the transverse and oblique interarytenoid muscles pull
the arytenoid cartilages together, producing adduction of the
vocal cords [4]. The interarytenoid muscles are active during
both phonation and swallowing [15]. An elevation in voice
pitch correlates with increased activity of the interarytenoid
muscles. These muscles also serve as sphincters to protect the
airway during a swallow [17].

ACTION OF THE LATERAL CRICOARYTENOID MUSCLES

MUSCLE ACTION: LATERAL CRICOARYTENOID
MUSCLES

Action

Evidence

Adduct the vocal cords

Supporting

The lateral cricothyroid muscles pull the muscular process of
the arytenoid cartilage anteriorly, causing the arytenoid carti¬
lages to rotate, moving the vocal processes medially [2,21,22].
Thus the reported action of the lateral cricoarytenoid muscles
is adduction of the vocal cords. Studies show that the lateral
cricoarytenoid muscles participate in speech, contributing to
increases in pitch and helping to regulate the resonance of the
voice [4,11,18].

MUSCLE ATTACHMENT BOX 21.2

ATTACHMENTS AND INNERVATION OF THE
TRANSVERSE INTERARYTENOID MUSCLE

Attachments: Posterior surface of the muscular
processes of both arytenoid cartilages; this is the
only unpaired intrinsic muscle of the larynx

Innervation: Recurrent laryngeal nerve, a branch of
the vagus nerve (10th cranial nerve)

Arytenoid

Figure 21.11: The muscles that adduct the vocal folds include
(A) the transverse interarytenoid, the oblique interarytenoid
(posterior view), and (B) the lateral cricoarytenoid muscles
(superior view).

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Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 21.4

ATTACHMENTS AND INNERVATION OF THE
POSTERIOR CRICOARYTENOID MUSCLE

Attachments: Posterior surface of the cricoid lamina
to the muscular process of the arytenoid cartilage
on the same side

Innervation: Recurrent laryngeal branch of the
vagus nerve (10th cranial nerve)

Muscles That Open the Vocal Cords

Only the posterior cricoarytenoid muscles widen the rima
glottis (Muscle Attachment Box 21.4).

ACTION OF THE POSTERIOR CRICOARYTENOID
MUSCLES

MUSCLE ACTION: POSTERIOR CRICOARYTENOID
MUSCLES

Action

Evidence

Abduct the vocal cords

Supporting

Like the lateral cricoarytenoids, the posterior cricoarytenoids
function by rotating the arytenoid cartilages (Fig. 21.12). The
posterior cricoarytenoid muscles pull the muscular processes
posteriorly, thus moving the vocal processes laterally and pro¬
ducing abduction of the vocal cords. Studies demonstrate that
the posterior cricoarytenoid muscles actively contract during
forced inspiration, apparently to open the airway as much as
possible [4-6,18,22]. It also appears that the posterior
cricoarytenoid muscles maintain a low level of activity during

Figure 21.12: The posterior cricoarytenoid muscles are the only
muscles that abduct the vocal folds.

MUSCLE ATTACHMENT BOX 21.5

ATTACHMENTS AND INNERVATION
OF THE CRICOTHYROID MUSCLE

Attachments: Anterior and lateral aspects of the
outer surface of the cricoid cartilage passing poste¬
riorly, superiorly and laterally to the anterior border
of the thyroid cartilage's inferior horn and to the
posterior surface of the lower border of the thyroid
lamina

Innervation: External laryngeal branch of the superior
laryngeal nerve from the vagus nerve (10th cranial
nerve)

exhalation, suggesting that a patent airway is maintained
through the delicate balance between the adduction and
abduction pulls of the surrounding muscles.

Muscles That Alter the Tension
in the Vocal Cords

Alteration in the tension of the vocal cords is an important
mechanism to alter the pitch of the voice [3]. The muscles that
alter the tension of the vocal cords are the cricoarytenoid and
thyroarytenoid muscles [2,3,8,23,24] (Muscle Attachment
Boxes 21.5 and 21.6).

ACTION OF THE CRICOTHYROID MUSCLE
MUSCLE ACTION: CRICOTHYROID MUSCLE

Action

Evidence

Tense the vocal cords

Supporting

Adduct the vocal cords

Conflicting

MUSCLE ATTACHMENT BOX 21.6

ATTACHMENTS AND INNERVATION
OF THE THYROARYTENOID MUSCLE

Attachments: The posterior surface of the angle of
the thyroid cartilage to the anterolateral surface of
the arytenoid cartilage along the vocal process; the
most medial fibers, which attach to the tip of the
vocal process and to the vocal ligament, are known
as the vocalis muscle

Innervation: Recurrent laryngeal nerve, a branch of
the vagus nerve (10th cranial nerve)

Chapter 21 I MECHANICS AND PATHOMECHANICS OF VOCALIZATION

419

The cricothyroid muscle produces motion between the
cricoid and thyroid cartilages by lifting the anterior arch of
the cricoid cartilage, causing the posterior aspect of the
cartilage to tilt inferiorly on the thyroid cartilage [25] (Fig.
21.13). This posterior tilt moves the arytenoid cartilages that
rest on the cricoid cartilage posteriorly and inferiorly, putting
the vocal cords on stretch and elevating pitch [3,9,19,23,26].
There is no clear agreement regarding the cricothyroid mus¬
cles role in opening or closing of the vocal cords. Some
investigators report activity during inspiration, suggesting
that the muscle participates in abduction of the vocal cords
[24]. Others suggest that the muscle adducts the cords
because tension in the vocal cords would tend to pull the ary¬
tenoids cartilages toward each other (adduction) [22].
Rhythmic activity is also reported in respiration although the
significance of that activity is unclear [24]. Poletta et al.
report cricothyroid activity during both opening and closing
of the vocal cords in respiratory functions such as coughing
and sniffing [18]. These authors and others suggest the func¬
tion of the cricothyroid muscle may be more to stiffen the
vocal cords than to adduct or abduct them [4].

ACTION OF THE THYROARYTENOID MUSCLE
MUSCLE ACTION: THYROARYTENOID MUSCLE

Action

Evidence

Tense the vocal cords (vocalis)

Supporting

Adduct the vocal cords (muscularis)

Supporting

The thyroarytenoid muscle is a complex and poorly studied
muscle that lies within the vocal fold (Fig. 21.14). It consists
of a lateral and medial segment [4,20]. The medial segment,
also known as the vocalis muscle, attaches directly to the vocal
cord, and it is this portion that directly affects the tension

Figure 21.13: Contraction of the cricothyroid muscle lifts the arch
of the cricoid cartilage, tipping the lamina posteriorly and
stretching the vocal folds.

Figure 21.14: The thyroarytenoid muscle. A. A medial view
reveals that the thyroarytenoid muscle is imbedded within the
vocal fold. B. A posterior view of the vocal fold reveals that the
medial-most portion of the thyroarytenoid muscle is the vocalis
muscle, which tenses the vocal fold.

within the vocal cord. The reported action of the vocalis por¬
tion of the thyroarytenoid muscle is to increase the tension of
the vocal cord. As with the cricothyroid muscle, increased
activity in the thyroarytenoid muscle parallels an increase in
pitch [9,11,15,23]. The vocalis also may contribute slightly to
abduction of the vocal cords [4].

The lateral portion of the thyroarytenoid muscle is known
as the muscularis. Studies suggest that this portion contributes
significantly to adduction of the vocal cords [4]. This analysis
helps explain the finding that the thyroarytenoid muscle also
contracts during a swallow, apparently assisting in the closure
of the larynx to protect the airway [15,17].

MECHANISM OF VOICE PRODUCTION

Human voice production, or phonation, occurs as a result of
vibration of the vocal folds by air that is forced past them from
the respiratory system. The formation of words from voice
sounds is the summated outcome of contributions from the
mouth, the larynx, and the pharynx. The intensity and volume
of the sound are influenced by the velocity and pressure of
the air passing across the folds, which are affected by the

420

Part III I KINESIOLOGY OF THE HEAD AND SPINE

activity of the abdominal muscles. Thus oral communication
requires the coordinated activity of several components of the
musculoskeletal system.

Phonatiori

Phonation is the act of producing sound by vibrating the vocal
folds, which is similar to, albeit more complex than, the pro¬
duction of tones by vibrating the string of a guitar [14,22,23].
The basic mechanisms that alter the human sound can be
understood by comparing them with the methods used to
alter the pitch of any string instrument. The primary mecha¬
nisms to alter the tone of a guitar note are alterations in the
tension, length, and thickness of a string. Increasing the ten¬
sion of a string raises the pitch, while increasing the length
and thickness of the string decreases pitch. Thus the longer,
thicker vocal folds in males produce a voice with a lower
pitch. Hoarseness that accompanies a bout with laryngitis
demonstrates the effects of thickness of the vocal cords, since
it is the swelling of the cords that produces the lower pitch
characteristic of a hoarse voice. The instantaneous pitch
changes that occur during normal speech are produced
by changes in vocal cord tension and in the size of the
rima glottis as a result of intrinsic muscle activity.

Increased tension in the vocal cords raises the pitch of a
voice just as in a guitar string. Thus contraction of the vocalis
and cricothyroid muscles helps raise the pitch [3,8]. Vibration
of the surrounding folds also contributes to subtle variations
in pitch. Alterations in the width of the rima glottis also affect
pitch; adduction of the vocal cords narrows the rima glottis
and raises the pitch. The pressure of the expiring air that
passes over the vocal folds also affects pitch by producing
changes in the vibration pattern of the vocal folds. This helps
explains why singers must learn to use the abdominal muscles
to control the air pressure while singing [16,23].

All of the vowel sounds and many consonant sounds
require vocal cord vibration. However, not all sounds in
speech involve the use of the vocal cords. Voiceless conso¬
nants are consonants whose sounds do not require vocal fold
vibration. These sounds are produced typically with the vocal
cords abducted by the posterior cricoarytenoid muscle [10].
The sound of the letter “1” or “b” requires vibration, and con¬
sequently, these letters are described as voiced consonants.
Voiceless consonants include “p,” “t,” and “s.”

Resonance and Pronunciation

Phonation is only one component of normal speech. Anyone
who has watched a movie with the sound muted knows that
lip motion is frequently sufficient to communicate. Similarly,
the verbal speech of an individual who has been deaf from
early life often lacks the varied resonance and precise pro¬
nunciation that characterize most verbal communication.
Resonance of a voice is produced by the movement of the air
within the laryngeal, pharyngeal, nasal, and oral spaces. The
muscles that alter the rima glottis help modulate the reso¬
nance of the voice by modifying the size of the laryngeal

chambers [11]. Singers know that altering the size and shape
of the mouth and pharynx also has a large effect on the result¬
ing sound. Anyone who has ever had a cold with a stuffy nose
understands that the nose makes an important contribution to
the voice. The lack of resonance within the nasal cavity pro¬
duces the characteristic “cold in the nose” voice in which the
voice exhibits little or no nasal resonance.

Contraction of the muscles of the mouth and nose
(described in Chapter 20) and muscles of the tongue and soft
palate (described in Chapter 22) produces subtle changes in
resonance by altering the contributions of the nose and the
volume of the oral cavity. Similarly, the suprahyoid and
infrahyoid muscles produce changes in the size and shape of
the laryngeal and pharyngeal spaces, producing further
changes in resonance [12,22].

Another important influence on resonance is the pressure
of the airflow past the vocal cords. The quality of the airflow
depends on the status of the respiratory system and on the
abdominal musculature. Contraction of the abdominal mus¬
cles, particularly the transversus abdominis and the internal
and external oblique abdominal muscles, increases abdominal
pressure, which increases the force of expiration. Increased
airflow pressure raises the volume of the voice and allows the
speaker to project the voice, a skill essential to singers and
public speakers.

Clinical Relevance

THE ROLE OF BREATH CONTROL IN SPEECH-CASE
REPORTS: The first case involved a 35-year-old woman who
was being followed by a multidisciplinary team for rehabilita¬
tion following a closed head injury. The patient exhibited sev¬
eral functional problems including speech problems and
decreased balance with walking. Her speech problems included
vocalization impairment with difficulty in projecting her voice,
producing a "breathy" voice quality. The therapist whose pri¬
mary treatment focus was on the locomotor impairments col¬
laborated with the speech therapist to implement an exercise
program for the abdominal muscles to improve voice volume
and projection. A common understanding of the mechanics
of voice projection allowed the therapists to develop a coordi¬
nated treatment regimen directed toward a common goal of
improved volume and projection of the voice.

The second case involved a 5-year-old child with spastic
tetraplegia who was being treated by a multidisciplinary
team. One important goal of treatment was improved com¬
munication skills , including oral communication. At the time
of evaluation the patient could verbalize only three syllables
between breaths. The child also exhibited impaired trunk
control and the therapist was modifying the child's wheel¬
chair to improve trunk stability , which would also facilitate
improved breath control. Improved breath control could
improve oral communication , and the therapist collaborated
with the speech and language specialist to measure the
improvements in breath control and oral communication.

Chapter 21 I MECHANICS AND PATHOMECHANICS OF VOCALIZATION

421

Like resonance, articulation and enunciation use struc¬
tures beyond the larynx itself. Muscles of facial expression,
especially the muscles of the lips and nose, are important in
producing the varied sounds of the language [7]. Patients with
a facial nerve palsy such as Bells palsy (described in Chapter
20) frequently exhibit abnormalities in speech, resulting from
inadequate control of the lips and cheeks. The intrinsic and
extrinsic muscles of the tongue also are essential for precise
enunciation of sounds and syllables [1]. Tongue position pro¬
duces the distinction between the sounds of “d” and “g,” and
the shape of the tongue contributes to the different sounds of
“d” and “1.” Thus successful vocalization requires precise
coordination of the muscles of the larynx, pharynx, soft palate,
nose, and mouth as well as the abdominal muscles.

Common Abnormalities in Voice
Production

Diagnosis and treatment of voice problems are the purview of
speech and language specialists, but it is useful for all neuro-
musculoskeletal experts to appreciate the general form of
typical voice problems. Voice problems that are based on dys¬
functions within the voice production apparatus can be cate¬
gorized as hyperfunctional or hypofunctional voices. A
hyperfunctional voice results from overuse and is found in
individuals who participate in prolonged and violent use of
their vocal folds, such as cheerleaders. The prolonged yelling
results in repeated and forceful contact between the vocal
folds and can lead to the appearance of nodules on the folds
themselves. Individuals who frequently clear their throat sus¬
tain similar trauma to the vocal folds. During the U.S. presi¬
dential campaign, prior to the 1992 election, then-candidate
William Clinton sustained trauma to the vocal folds after a
period of sustained campaigning and little sleep. As a result,
his voice became hoarse and finally required a few days
respite from speeches.

Hypofunctional voices are characterized by a lower pitch,
an inability to sustain a constant pitch, and hoarseness.
Weakness of the laryngeal muscles produces a hypofunctional
voice and is a common finding in individuals who have sus¬
tained a cerebrovascular accident or head trauma. The speech
production in such an individual can be compromised
still further by weakness of the tongue and muscles of
facial expression.

SUMMARY

This chapter presents a brief review of the structure of the
larynx and the intrinsic muscles that control it and provides an
overview of the mechanics of voice production. Vibration of
the vocal folds produces voice in much the same way that
vibration of a guitar string produces a musical note, The
intrinsic muscles of the larynx alter the voice s pitch by modi¬
fying the aperture between the vocal folds and the rima glot¬
tis and also altering the tension within the folds. Narrowing

the rima glottis and increasing the tension in the vocal folds
increase pitch, while a wider rima glottis and decreased
tension in the vocal folds lower the pitch.

The muscles of facial expression, the tongue, the pharynx,
and the muscles of respiration also make important contribu¬
tions to the quality of speech, particularly by modifying reso¬
nance and pronunciation. The actions of the lips and tongue
allow pronunciation of specific language sounds. Resonance
of the voice is modified by changes in the air pressure passing
over the vocal folds, controlled by the abdominal muscles and
by the volume of the chambers through which the air passes,
including the mouth and nose. Muscles of facial expression
and the soft palate help control the volume of these chambers
and contribute to changes in resonance.

Thus speech results from a complex interaction of mus¬
cles throughout the head, neck, and trunk. Rehabilitation
specialists possess expertise that can assist speech and lan¬
guage specialists in improving an individuals verbal com¬
munication by facilitating the function of the contributing
musculoskeletal components, including trunk musculature,
muscles of facial expression, and muscles of the tongue, soft
palate, and larynx.

References

1. Dworkin JP, Aronson AE: Tongue strength and alternate motion
rates in normal and dysarthric subjects. J Commun Disord 1986;
115-132.

2. Farley GR: A biomechanical laryngeal model of voice F0
and glottal width control. J Acoust Soc Am 1996; 100: 3794-3812.

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4. Hunter EJ, Titze IR, Alipour F: A three-dimensional model of
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1747-1759.

5. Kuna ST, Smickley JS, Insalaco G: Posterior cricoarytenoid mus¬
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6. Kuna ST, Smickley JS, Insalaco G, Woodsen GE:
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361-369.

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muscle function by quantitative analysis of the EMG interfer¬
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9. Lindestad PA, Fritzell B, Persson A: Quantitative analysis of
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975-981.

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CHAPTER

Mechanics and Pathomechanics
of Swallowing

CHAPTER CONTENTS

FOOD PATHWAY FROM MOUTH TO STOMACH .424

MUSCLES OF THE MOUTH.424

Muscles of the Tongue .425

Muscles of the Soft Palate .427

Muscles of the Pharynx.428

Suprahyoid Muscles.430

Infrahyoid Muscles .431

Intrinsic Muscles of the Larynx.431

NORMAL SEQUENCE OF SWALLOWING.433

Oral Phase .433

Pharyngeal Phase .434

Esophageal Phase .435

COMMON ABNORMALITIES IN SWALLOWING .435

Impairments of the Oral Preparatory Phase.435

Impairments of the Oral Phase .435

Impairments of the Pharyngeal Phase.435

Impairments of the Esophageal Phase .435

Signs of Swallowing Impairment.435

SUMMARY .436

S wallowing dysfunction, known as dysphagia, is potentially a life-threatening disorder. The danger associated
with swallowing disorders is the possibility of asphyxia or aspiration, which is the introduction of a solid or
liquid, including saliva, into the airway. Aspiration introduces foreign substances including bacteria into the
lungs and often leads to aspiration pneumonia, one of the leading causes of death in elders [17,19,21]. A basic under¬
standing of the mechanics of swallowing prepares clinicians to identify individuals who may be experiencing difficulty
in swallowing and to refer these patients to professionals qualified to evaluate and implement treatment if necessary.

In the United States several different types of health care professionals may participate in the evaluation and treat¬
ment of individuals with dysphagia, including otorhinolaryngologists, gastroenterologists, speech and language spe¬
cialists, occupational therapists, and, less frequently, physical therapists. Because of the potential threat associated
with dysphagia, however, any rehabilitation specialists who come in contact with patients who may have swallowing

423

424

Part III I KINESIOLOGY OF THE HEAD AND SPINE

problems require at least rudimentary information regarding the mechanisms of swallowing, even though these clini¬
cians may not treat the dysphagia directly. The clinician's need for a basic understanding of the functional and dysfunc¬
tional swallow is made even more acute as the trends in health care continue to cause sicker patients to be followed in
a home-care setting, often by a single clinician. The treating clinician must be able to recognize the signs of swallowing
impairment to make the appropriate referrals for the patient to obtain the necessary care.

Clinical Relevance

A CASE REPORT: A family requested a special consultation for a patient who was experiencing increasing difficulties walking
and who had fallen several times. A therapist who specialized in locomotion disorders went to see the patient , arriving as he was
eating lunch. The therapist waited as the patient ate and noticed that he coughed after every swallow. The therapist questioned
the family who reported that the coughing was common when he ate but seemed to subside shortly after the meal. After the
patient finished lunch , the therapist proceeded to evaluate his locomotion and instructed the patient and family in a home pro¬
gram of exercises. After leaving the patient , the therapist called the case manager who had requested the consultation and
reported the coughing incident and requested that he receive a thorough evaluation of his swallowing function. Follow-up con¬
tact revealed that the patient had been hospitalized for dysphagia where further tests and interventions were occurring.

This case report demonstrates that knowledge of swallowing allowed the therapist to recognize an important sign of swallow¬
ing difficulty , coughing specifically related to eating. The therapist was able to inform the case manager ; who helped the family
access the appropriate services. The swallowing difficulties presented a greater threat to the patient's health than the gait prob¬
lems and took priority in the patient's treatment regimen.

The focus of this chapter is on presenting the basic mechanics of swallowing, also known as deglutition, so that the
clinician can appreciate the whole process and the structures that participate in the swallowing event. Specifically, the
purposes of this chapter are to

■ Briefly describe the parts of the alimentary canal that participate in the swallow

■ Present the muscles that are active in the swallowing process

■ Describe the series of events that comprise the swallow

■ Describe common clinical signs of swallowing problems

FOOD PATHWAY FROM
MOUTH TO STOMACH

The alimentaiy canal —the path through which food is inges¬
ted, processed, and eliminated—consists of the mouth, pharynx,
esophagus, stomach, and intestines and the associated glands.
The superior end of the alimentary tract shares many of its
structures with the superior end of the respiratory tract
(Fig. 22.1). The two tracts deviate from one another at the level
of the larynx. The challenge of the swallow is to transmit the
contents of the mouth to the esophagus and stomach without
allowing any to enter the areas unique to respiration, includ¬
ing the nose and the trachea. Skeletal muscles move the food
from the mouth to the esophagus where further propulsion
through the alimentary canal occurs by smooth muscle until
the process of elimination of any waste products begins.

Food enters the mouth, where it is prepared for transmis¬
sion through the alimentary tract. It is then propelled into the
oropharynx, but muscle activity is required to prevent any pro¬
gression into the nasopharynx. From the oropharynx the

contents are passed through the laryngopharynx to the esopha¬
gus. Skeletal muscles protect the larynx and the primary struc¬
tures of the respiratory system, and a sphincter guards the
entrance into the esophagus. The muscles that assist in the
transport of the oral contents to the stomach are discussed in
this chapter. The muscles that are specific to the larynx are dis¬
cussed in the preceding chapter. It is important to note that
most of the muscles that prepare the food for swallow and then
propel it into the alimentary canal also participate in speech.
The muscles of facial expression discussed in Chapter 20 and
the muscles of the larynx described in Chapter 21 also partici¬
pate in swallowing, and the muscles of the tongue and pharynx
described in this chapter also participate in oral speech.

MUSCLES OF THE MOUTH

The functions performed in the mouth are

• Preparation of the ingested food into a manageable, rounded
mass, or bolus, from the contents of the mouth, or ingestate

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

425

Figure 22.1: The superior end of the alimentary tract is composed
of the mouth, pharynx, and esophagus. The respiratory tract
shares the mouth and pharynx with the alimentary tract.

• Location of the bolus in a position that allows efficient
transmission into the oropharynx while preventing the
contents from leaking into the nose

• Articulation, enunciation, and modulation of vocal reso¬
nance creating the unique sounds of voice and language
(Chapter 21 describes these functions more fully).

Liquids are localized on the tongue, forming a liquid
bolus prior to transmission into the oropharynx. Solid food
is formed into a bolus by chewing, or mastication, by the
muscles of mastication (discussed in detail in Chapter 24).
These muscles elevate the mandible to grind and soften
the food. Muscles of the lips and the buccinator also par¬
ticipate in preparation of the bolus, keeping it within the
oral cavity by closing the mouth and by compressing the
cheeks to keep the food between the teeth. The muscles of
the tongue assist the buccinator in positioning the bolus of
food during mastication and then propel it into the
oropharynx while the muscles of the soft palate close off
the nasopharynx. The muscles of the tongue and soft
palate are described below.

Muscles of the Tongue

The tongue is a muscular organ that attaches to the mandible
and to the hyoid bone, a U-shaped sesamoid bone that lies

Figure 22.2: The intrinsic muscles of the tongue consist of medial
lateral bundles, longitudinal bundles, and bundles that run from
the inferior (ventral) to superior (dorsal) surface.

posterior to the angle of the mandible. The tongue consists
of intrinsic muscles that shape the tongue and extrinsic
muscles that move the tongue (Fig. 22.2). The tongue
manipulates the contents of the mouth so that the muscles of
mastication can process the food. It kneads the food and
forms it into a manageable bolus. Then the tongue forms
itself into a chute through which the food slides to the
oropharynx.

INTRINSIC MUSCLES OF THE TONGUE

The intrinsic muscles of the tongue lie in bundles that run
transversely, longitudinally, and vertically from the ventral
(underneath) surface to the dorsal (superior) surface [38]
(.Muscle Attachment Box 22.1).

426

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 22.1

ATTACHMENTS AND INNERVATION OF
THE INTRINSIC MUSCLES OF THE TONGUE

Longitudinal fibers

Attachments: From the hyoid bone and the
fibrous tissue at the root of the tongue to the
mucosal covering of the tongue anteriorly to the
sides and tip of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)

Transverse fibers

Attachments: From the fibrous septum that runs
through the center of the length of the tongue
to the mucosal covering of the tongue along the
sides of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)
Vertical fibers

Attachments: From the mucosal covering of the
tongue dorsally to the mucosal covering on the
ventral surface of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)

Palpation: Although the tongue is readily palpated,
the intrinsic muscles cannot be palpated individually.

Actions

The intrinsic muscles of the tongue change the shape of the
tongue.

MUSCLE ACTION: LONGITUDINAL FIBERS

Action

Evidence

Shorten the tongue

Supporting

Curl the tongue up or down Supporting

MUSCLE ACTION: TRANSVERSE FIBERS

Action

Evidence

Lengthen the tongue

Supporting

Narrow the tongue

Supporting

MUSCLE ACTION: VERTICAL FIBERS

Action

Evidence

Flatten the tongue

Supporting

Widen the tongue

Supporting

These muscles together allow the tongue a broad range of
shapes that are essential to the progression of the food into
the oropharynx. The diversity in shape also contributes to

word articulation by assisting in creating the differences in
sounds between the letters “d” and “1” or between “e” and “i”
[9,10,36].

EXTRINSIC MUSCLES OF THE TONGUE

The extrinsic muscles of the tongue move the tongue but also
influence its shape [36,38] (Muscle Attachment Box 22.2)
(Fig. 22.3). The extrinsic muscles of the tongue are essential
for propelling the bolus into the orophaynx. They also partici¬
pate in speech by positioning the tongue to create the distinct
sounds of vowels and consonants [27].

Actions

MUSCLE ACTION: GENIOGLOSSUS

Action

Evidence

Protrude the tongue

Supporting

Depress the center of the tongue

Supporting

Deviate the tongue to the
opposite side

Supporting

MflHL MUSCLE ATTACHMENT

BOX 22.2

ATTACHMENTS AND INNERVATION OF THE
EXTRINSIC MUSCLES OF THE TONGUE

Genioglossus

Attachments: The dorsal surface of the midline
of the mandible to the hyoid bone and middle
pharyngeal constrictor muscle and to the entire
ventral surface of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)

Hyoglossus

Attachments: The hyoid bone to the lateral
aspects of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)
Styloglossus

Attachments: The styloid process of the temporal
bone to the dorsolateral aspect of the tongue

Innervation: Hypoglossal nerve (12th cranial nerve)

Palatoglossus

Attachments: The tendons of the tensor veli
palatini muscles in the soft palate to the sides
of the tongue

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Palpation: The genioglossus can be palpated intra-
orally in the floor of the mouth under the tongue.

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

427

Figure 22.3: The extrinsic muscles of the tongue include the
genioglossus, hyoglossus, styloglossus, and palatoglossus
(not shown).

MUSCLE ACTION: HYOGLOSSUS

Action

Evidence

Depress the tongue

Supporting

Retract the tongue Inadequate

MUSCLE ACTION: STYLOGLOSSUS

Action

Evidence

Lifts and retracts tongue

Supporting

MUSCLE ACTION: PALATOGLOSSUS

Action

Evidence

Elevate the back of the tongue

Supporting

By lifting the back of the tongue the palatoglossus separates
the oral cavity from the oropharynx. The muscles of the
tongue generate large forces to move the bolus in the mouth
during mastication and then help to propel the bolus into the
oropharynx. Forces of up to 15 N (approximately 3.4 lb) are
reported in the tongue during hard swallows [28]. Although it
is impossible to test the tongue muscles in isolation, the
strength of tongue movements is readily measured. Peak pro¬
trusion forces of over 3.0 kg (6.6 lb) are reported, greater in
males than in females [10]. Reported peak forces of lateral
deviation to the left and right are approximately equal to each
other but slightly less than those of protrusion.

Muscles of the Soft Palate

The soft palate is a soft wall of tissue that drapes from the
posterior border of the hard palate and is covered by mucosal
tissue. Its inferior border contains a central projection, the
uvula, which hangs down toward the tongue. Enclosed with¬
in the soft palate are two pairs of muscles found in the lat¬
eral expanses of the soft palate, the levator and tensor veli

MUSCLE ATTACHMENT BOX 22.3

ATTACHMENTS AND INNERVATION
OF THE MUSCLES OF THE SOFT PALATE

Levator veli palatini

Attachments: The temporal bone, the carotid
sheath, and the auditory tube to the contralater¬
al levator veli palatini muscle by way of the pala¬
tine aponeurosis in the soft palate

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Tensor veli palatini

Attachments: The sphenoid bone and auditory
tube to the palatine aponeurosis of the soft
palate

Innervation: Mandibular branch of the trigeminal
nerve (5th cranial nerve)

Musculus uvulae

Attachments: The palatine bone of the hard
palate and the palatine aponeurosis of the soft
palate to the mucosal covering of the uvula

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Palatopharyngeus

Attachments: The palatine aponeurosis and
mucosal covering of the soft palate to the
posterior surface of the thyroid cartilage and
the walls of the pharynx

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Palpation: The motion of the soft palate can be
observed easily, but direct palpation is not possible.

palatini (Muscle Attachment Box 22.3) (Fig. 22.4). The mus¬
culus uvulae lies in the central portion of the soft palate,
projecting into the uvula itself.

LEVATOR VELI PALATINI

MUSCLE ACTION: LEVATOR VELI PALATINI

Action

Evidence

Elevate the soft palate

Supporting

Pull the soft palate posteriorly

Supporting

The levator veli palatini is important for closing off the
nasopharynx during a swallow (Figs. 22.5 and 22.6). The muscle
also closes the nasopharynx to varying degrees during speech.

428

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Tensor veli
palatini

Figure 22.4: The muscles of the soft palate are the levator veli
palatini, tensor veli palatini, musculus uvulae, and palatopharyn-
geus (not shown).

Figure 22.5: Contraction of the muscles of the soft palate ele¬
vates the soft palate and closes off the nasopharynx. Contraction
occurs when saying "Ah." A. Relaxed. B. Contracting. (Photo
courtesy of Arnold J. Malerman, DDS, PC, Dresher, PA.)

Palatine
tonsil

Uvula

Figure 22.6: With unilateral weakness of the muscles of the
soft palate, the soft palate is elevated and pulled toward the
strong side.

TENSOR VELI PALATINI

MUSCLE ACTION: TENSOR VELI PALATINI

Action

Evidence

Pull the soft palate taut

Supporting

When both tensor veli palatini muscles contract together,
they tighten the soft palate and may help close off the
nasopharynx.

MUSCULUS UVULAE

MUSCLE ACTION: MUSCULUS UVULAE

Action

Evidence

Retract the uvula

Supporting

By retracting the uvula the musculus uvulae assists in the
closure of the nasopharynx.

PALATOPHARYNGEUS

MUSCLE ACTION: PALATOPHARYNGEUS

Action

Evidence

Elevate the pharynx

Supporting

Elevation of the pharynx helps to shorten the pharynx, thereby
facilitating the swallow.

Muscles of the Pharynx

The pharynx is the space posterior to the nasal, oral, and
laryngeal spaces. The muscles of the pharynx are found in the

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

429

walls of the pharynx, where their primary function is to shorten
the pharynx, thereby preventing access to the larynx, and
to clear any residue of the bolus from the pharynx [16].
Muscles of the pharynx also help to elevate the pharynx dur¬
ing a swallow (Muscle Attachment Box 22.4) (Fig. 22.7).

MUSCLE ATTACHMENT BOX 22.4

ATTACHMENTS AND INNERVATION
OF THE MUSCLES OF THE PHARYNX

Superior constrictor

Attachments: The sphenoid bone, the mandible,
and the posterior portion of the lateral aspects
of the tongue and indirectly to the base of the
occiput to join the fibers of the superior constric¬
tor muscle of the other side

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Middle constrictor

Attachments: The hyoid bone and the stylohyoid
ligament to the middle constrictor muscle on the
other side by way of the posterior fibrous band
known as the median pharyngeal raphe

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Inferior constrictor

Attachments: The cricoid and thyroid cartilages
to the inferior constrictor muscle on the other
side by way of the posteriorly positioned median
pharyngeal raphe

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Stylopharyngeus

Attachments: The styloid process of the temporal
bone to the pharyngeal constrictor muscles and
the mucosal lining of the pharynx and to the thy¬
roid cartilage

Innervation: Glossopharyngeal nerve (9th cranial
nerve)

Salpingopharyngeus

Attachments: The cartilage of the auditory tube
to blend with the palatopharyngeus muscle

Innervation: Pharyngeal plexus of the vagus
nerve (10th cranial nerve)

Palpation: Not possible.

Figure 22.7: The muscles of the pharynx include the superior,
middle, and inferior constrictor muscles and the stylopharyngeus
and salpingopharyngeus muscles that lie deep to the constrictors
(not shown).

These muscles contribute to speech by closing or opening the
nasopharynx and thus altering the resonance of the voice.

SUPERIOR, MIDDLE, AND INFERIOR
CONSTRICTOR MUSCLES

The constrictor muscles of the pharynx lie in the posterior
and lateral walls of the pharynx. Constriction of the pharynx
facilitates pharyngeal clearance.

MUSCLE ACTION: CONSTRICTOR MUSCLES

Action

Evidence

Constrict the pharynx

Supporting

MUSCLE ACTION: STYLOPHARYNGEUS AND SALPIN¬
GOPHARYNGEUS

Action

Evidence

Elevate the pharynx

Supporting

430

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Suprahyoid Muscles

The suprahyoid and infrahyoid muscles are also known as the
extrinsic muscles of the larynx. The suprahyoid muscles
attach to the hyoid bone and to either the mandible or the
temporal bone and play important roles in both swallowing
and mastication (Muscle Attachment Box 22.5) (Fig. 22.8).
When the mandible is held fixed by the mandibular eleva¬
tors, the suprahyoid muscles elevate the hyoid bone, which
is how they function in swallowing [35] (Fig. 22.9).

\JS1

MUSCLE ATTACHMENT BOX 22.5

ATTACHMENTS AND INNERVATION
OF THE SUPRAHYOID MUSCLES

Digastric

Attachments: The posterior belly arises from
the mastoid process of the temporal bone, and
the anterior belly from the posterior surface
of the midline of the mandible. The bellies join
at the hyoid bone.

Innervation: The posterior belly is innervated by
the facial nerve (7th cranial nerve). The anterior
belly is innervated by a branch of the trigeminal
nerve (5th cranial nerve).

Stylohyoid

Attachments: The styloid process of the temporal
bone to the hyoid bone

Innervation: Facial nerve (7th cranial nerve)

Mylohyoid

Attachments: The posterior surface of the
mandible to the hyoid bone. It forms the floor
of the mouth.

Innervation: A branch of the trigeminal nerve
(5th cranial nerve)

Geniohyoid

Attachments: From the posterior surface of
the symphysis of the mandible to the anterior
surface of the hyoid bone. It lies superior to
the mylohyoid muscle.

Innervation: Fibers from the first cervical nerve
carried by the hypoglossal nerve (12th cranial
nerve)

Palpation: The mylohyoid and anterior digastric
muscles are palpable in the submental space, which
is the inferior surface of the chin. The geniohyoid
and stylohyoid muscles are not palpable.

Figure 22.8: The suprahyoid muscles consist of the digastric,
stylohyoid, mylohyoid, and geniohyoid muscles.

Figure 22.9: Fixation of the mandible during suprahyoid activity.
For the suprahyoid muscles to elevate the hyoid bone, their
superior attachments on the mandible must be fixed. This fixa¬
tion occurs by contraction of the mandibular elevators keeping
the jaw stabilized so the suprahyoid muscles can pull from their
inferior attachments.

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

431

Conversely, when the hyoid bone is fixed, the suprahyoid
muscles depress the mandible, producing movement at the
temporomandibular joints. In this role they participate in
chewing, which is discussed in greater detail in Chapter 24
[36]. The suprahyoid and infrahyoid muscles have received
little study individually, in part, because they are difficult to
isolate. More studies report their group actions during
swallowing.

DIGASTRIC

The anterior belly of the digastric muscle has been studied by
electromyography (EMG) more extensively than the posterior
belly, since it is accessible by surface EMG electrodes in the
submandibular space.

Actions

MUSCLE ACTION: DIGASTRIC

Action

Evidence

Elevate the hyoid bone

Supporting

Pull hyoid bone anteriorly

Supporting

Depress the mandible

Supporting

The function of the digastric muscle on the hyoid bone is
important during swallowing [35]. With the hyoid bone fixed,
the reported action of the digastric muscle is depression of
the mandible. The digastric muscles role in depressing the
mandible is relevant in opening the mouth [1,36].

STYLOHYOID

The stylohyoid is not well studied.

Actions

MUSCLE ACTION: STYLOHYOID

Action

Evidence

Elevate the hyoid bone

Supporting

Pull hyoid bone posteriorly

Supporting

This muscle is likely active in swallowing and perhaps in
speech, although additional research is needed to describe its
function in detail.

MYLOHYOID

The mylohyoid muscle forms the floor of the mouth and is
palpable with the anterior belly of the digastric in the sub¬
mandibular space.

Actions

MUSCLE ACTION: MYLOHYOID

Action

Evidence

Elevate the floor of the mouth

Supporting

Elevate the hyoid bone

Supporting

Depress the mandible

Supporting

The mylohyoid muscle is active in the early phases of the
swallow to elevate the hyoid bone [35]. With the hyoid bone
fixed, the mylohyoid muscle acts with other suprahyoid mus¬
cles in depression of the mandible. The mylohyoid is active
with the other mandibular depressors in chewing and open¬
ing the mouth wide, as in a yawn.

GENIOHYOID

The geniohyoid is deep to the mylohyoid and cannot be
palpated directly.

Actions

MUSCLE ACTION: GENIOHYOID

Action

Evidence

Elevate the hyoid bone

Supporting

Pull hyoid bone anteriorly

Supporting

Depress the mandible

Supporting

Like the other suprahyoid muscles, the geniohyoid muscle is
active in swallowing, as it contributes to the elevation of the
hyoid bone [35]. With the hyoid bone fixed, the action of the
geniohyoid muscle is mandibular depression.

Infrahyoid Muscles

The infrahyoid muscles are strap muscles that extend from
the hyoid bone to the thyroid cartilage, also known as the
Adams apple, and to the sternum and scapula (Fig. 22.10).
They include the sternohyoid, sternothyroid, thyrohyoid, and
omohyoid (Muscle Attachment Box 22.6).

ACTIONS

MUSCLE ACTION: INFRAHYOID MUSCLES

Action

Evidence

Depress the hyoid bone

Supporting

Fix the hyoid bone

Supporting

The infrahyoid muscles depress the hyoid bone at the end
of the swallow and stabilize the hyoid bone when the suprahy¬
oid muscles contract to depress the mandible [1] (Fig. 22.11).
They also are active during speech, helping to stabilize the
larynx, particularly at lower pitches [30]. The sternothyroid
and thyrohyoid muscles attach to the thyroid cartilage, which
is a component of the larynx. Thus these two muscles can
depress and elevate the larynx, respectively, motions that
occur during speech. When the infrahyoid and suprahyoid
muscles contract together, with the mandible and
hyoid bones fixed, they contribute to cervical flexion
[14] (Fig. 22.12).

Intrinsic Muscles of the Larynx

The larynx is the “voice box” and the entry into the trachea.
The intrinsic muscles of the larynx alter the size of the

432

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 22.10: The infrahyoid muscles consist of
the sternohyoid, sternothyroid, thyrohyoid,
and omohyoid muscles.

MUSCLE ATTACHMENT BOX 22.6

ATTACHMENTS AND INNERVATION
OF THE INFRAHYOID MUSCLES

Sternohyoid

Attachments: The posterior surface of the medial
clavicle and manubrium of the sternum to the
inferior aspect of the hyoid bone

Innervation: Ventral rami of C1-C3 via fibers
of the ansa cervicalis, which is a loop of nerves
from the ventral rami of C2 and C3 and the
hypoglossal nerve

Sternothyroid

Attachments: The posterior surface of the ster¬
num and cartilage of the first rib to the laminae
of the thyroid cartilage

Innervation: Ansa cervicalis, which is a loop of
nerves from the ventral rami of C2 and C3 and
the hypoglossal nerve

Thyrohyoid

Attachments: The laminae of the thyroid carti¬
lage to the hyoid bone. It is essentially the contin¬
uation of the sternothyroid muscle.

Innervation: Ventral rami of Cl carried by the
hypoglossal nerve (12th cranial nerve)

Omohyoid

Attachments: The inferior belly attaches to the
superior border of the scapula. The superior belly
attaches to the hyoid bone. The two bellies join
by an intermediate tendon at the base of the
neck, posterior to the sternocleidomastoid muscle.

Innervation: Ansa cervicalis, which is a loop of
nerves from the ventral rami of C2 and C3 and
the hypoglossal nerve

Palpation: Not palpable.

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

433

Figure 22.11: Fixation of the hyoid bone by the infrahyoid
muscles. Contraction of the infrahyoid muscles holds the hyoid
bone interiorly so that the suprahyoid muscles can contract and
depress the mandible.

opening to the trachea and modulate the tension in the
vocal folds. The intrinsic laryngeal muscles that function in
swallowing are those that close the entrance to the trachea.
They include the interarytenoid, the lateral cricoarytenoid,
and the thyroarytenoid muscles [22,26,32].

NORMAL SEQUENCE OF SWALLOWING

Swallowing is a complex series of coordinated events that
transmit the contents of the oral cavity to the stomach through
a region that includes parts of the respiratory tract. The chal¬
lenge of the swallow is to propel the oral contents past the
entries to the dedicated respiratory components, including the
nose and trachea.

The swallow consists of four phases, the oral preparatory,
oral, pharyngeal, and esophageal phases [8]. The oral

preparatory phase technically precedes the actual swallow and
is the period in which the ingested material is chewed and
formed into a bolus. This process demands rhythmic coordi¬
nation among the muscles of facial expression, the tongue, the
muscles of mastication, and the suprahyoid and infrahyoid
muscles [36]. During the oral preparatory phase the posterior
aspect of the tongue elevates to the soft palate, closing off the
pharynx and maintaining the food in the mouth. The common
admonition “Don’t speak with your mouth full” is wise advice,
since speaking alters the position of the tongue and can open
the passages to the nose and airway. The mechanisms and the
muscles that participate in chewing are described in Chapters
23 and 24.

The remaining three phases—oral, pharyngeal, and
esophageal—are described below. Although the sequence of
events described below is generally accepted, it is important to
recognize that individuals exhibit considerable variability in
their own swallow patterns, affected by the size and con¬
sistency of the bolus, and there is even more variability
across individuals [7,20,29]. Swallowing problems are
common among the elderly, but aging itself does not appear to
alter the normal sequence in a swallow, although the sequence
is slowed with age [4,18,29,33].

Oral Phase

The oral phase is under voluntary control, while the pharyn¬
geal and esophageal phases are reflexive. However, there is
evidence that some reflexive movements within the larynx
occur during the oral phase [23,32]. Once the ingestate is
adequately prepared for swallowing, during the oral prepara¬
tory phase, the process of propelling it to the stomach begins.
The oral phase is composed of the tongue movements that
thrust the bolus into the pharynx and movements of the phar¬
ynx toward the bolus. At the initiation of the swallow, the tip
of the tongue lifts the contents of the mouth up against the
hard palate [3,39]. Motion of the tongue propagates through
the rest of the tongue by contraction of the genioglossus and
then the intrinsic muscles of the tongue, creating a peristaltic
motion of the tongue that propels the contents posteriorly
[8,15]. The back of the tongue then lowers, opening the phar¬
ynx, and forms a chute that empties into the pharynx.
Contact by the bolus on the anterior arch, or faucial fold, of
the soft palate descending from the uvula triggers the swallow
reflex, beginning the pharyngeal phase of the swallow.

Concurrent with the tongue s actions in the oral phase, the
suprahyoid muscles begin to pull the pharynx superiorly,
which shortens the distance that must be traveled by the
bolus, facilitating transmission of the bolus and closure of the
vocal cords [3,5,8,23,31,35]. For the suprahyoid muscles to
pull the hyoid bone superiorly, their superior attachment on
the mandible must be fixed by the mandibular elevators, the
masseter, temporalis, and medial pterygoid muscles discussed
in Chapter 24 [37]. Thus it is impossible to swallow with the
jaw relaxed. The oral phase of a swallow lasts 0.5 to 1.0 second
[3,8]. The airway is open during the oral phase of swallowing,

434

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 22.12: Cervical flexion by the suprahyoid and
infrahyoid muscles. Simultaneous contraction of the
suprahyoid and infrahyoid muscles along with the
mandibular elevators contributes to cervical flexion.

and precise muscle control and coordination are required to
avoid aspiration.

Pharyngeal Phase

The pharyngeal phase of the swallow is under reflex control
and consists of several distinct tasks:

• Transmission of the bolus past the nasopharynx and larynx,
requiring activity of muscles that close off these spaces

• Continued elevation of the larynx by the suprahyoid mus¬
cles assisted by the palatopharyngeus, stylopharyngeus,
and salpingopharyngeus

• Action of the pharyngeal constrictor muscles to clear the
bolus from the pharynx

• Relaxation of the esophageal sphincter

As the bolus passes the walls of the soft palate, initiating
the swallow reflex, the muscles of the soft palate, the levator

and tensor veli palatini, and the musculus uvulae contract,
pulling the soft palate superiorly and posteriorly, sealing off
the nasal cavity from the bolus [24,38,39]. Data from endo¬
scopic videos and from EMG studies reveal continued adduc¬
tion of the vocal folds and progressive closure of the laryngeal
inlet [23,32,35].

Elevation of the hyoid bone and consequently the pharynx,
which begins in the oral phase, continues in the pharyngeal
phase. This elevation provides additional protection for the
airway, because as elevation proceeds, the epiglottis, the
leaflike cartilage of the larynx, folds posteriorly, creating a lid
over the larynx and entry to the trachea. Maximum elevation
of the hyoid bone during a swallow is approximately 1.0 to
1.5 cm [7]. At the same time, the constrictor muscles create a
wave of muscle contraction that pushes the bolus inferiorly
through the pharynx. The bolus slides over the epiglottis, past
the larynx, and into the esophagus [25,26]. The pharyngeal
phase of the swallow lasts less than 1 second [3,8].

Chapter 22 I MECHANICS AND PATHOMECHANICS OF SWALLOWING

435

Clinical Relevance

THE ROLE OF CORRECT POSTURE IN SWALLOWING:

Erect posture facilitates the normal swallow , and abnormal
posture contributes to impairments in swallowing [11]. An
attempt to swallow with the cervical spine hyperextended
demonstrates to the reader the direct association between
head posture and swallowing. Improving the swallowing
pattern in children with cerebral palsy requires treatment
that facilitates upright sitting position while also directly
treating the swallowing dysfunction. An interdisciplinary
approach that uses clinicians with expertise in swallowing
disorders and those who are expert in promoting upright
sitting posture can optimize a patient's swallowing
potential and minimize the dangers of aspiration
[ 1114 ].

Esophageal Phase

The bolus enters the esophagus through the esophageal
sphincter, also known as the cricopharyngeus muscle. Except
during the swallow, this sphincter maintains a low constant
level of activity to prevent regurgitation [12,13,24,26,34].
However, contact of the bolus on the sphincter relaxes the
muscle reflexively [2]. In addition, elevation of the hyoid
bone by the suprahyoid muscles during the oral and pharyn¬
geal phases applies traction to the sphincter, facilitating its
opening [8].

The bolus is transmitted through the esophagus via peri¬
staltic contractions of the smooth muscle of the esophagus.
The esophagus is a muscular tube that is about 25 cm long,
and transport through it can take several seconds [6,38]. After
the bolus is entirely within the esophagus and the upper
esophageal sphincter closes again, the infrahyoid muscles con¬
tract to pull the hyoid bone inferiorly to its resting position.

COMMON ABNORMALITIES
IN SWALLOWING

Abnormalities in swallowing may involve any of the phases,
including the oral preparatory phase, and can lead to aspira¬
tion at any time, before, during, or even after the swallow.
Common impairments in the swallow mechanism for each
phase are described below.

Impairments of the Oral
Preparatory Phase

Impairments during the oral preparatory phase impede the
ability to chew the contents of the food and form it into a
bolus. Weakness of the facial muscles of the lips and cheeks
can allow the contents to leak from the mouth or become
sequestered between the cheeks and teeth. Abnormal control

of the muscles of mastication impairs the ability to grind the
food, and inadequate tongue control makes it difficult to
locate the food between the teeth for successful chewing.
These impairments may lead to an inability to form a bolus
or the production of a bolus that is too large to be propelled
easily to the stomach.

Impairments of the Oral Phase

The oral phase requires coordinated movement of the
tongue, so impaired tongue movement can result in slowed
movement of the bolus toward the pharynx. Conversely,
inadequate tongue control also can allow the oral contents
to slip too rapidly into the pharynx. Since the airway is open
during the oral preparatory and oral phases of swallowing,
premature movement of the bolus into the pharynx can lead
to aspiration.

Impairments of the Pharyngeal Phase

Impairments of swallowing during the pharyngeal phase
result from decreased muscle function and can include inad¬
equate closure of the nasopharynx or larynx and inadequate
propulsion of the bolus through the pharynx. Weakness of the
muscles of the soft palate can allow the contents of the mouth
to enter the nose by way of the nasopharynx. Impaired laryn¬
geal protection may result directly from weakness of the
intrinsic muscles of the larynx or from inadequate elevation of
the pharynx, so that the protection provided by the tilting of
the epiglottis is absent. Inadequate relaxation of the upper
esophageal sphincter also may allow the contents to collect at
the base of the laryngeal pharynx and slip into the larynx.
Weakness of the pharyngeal constrictor muscles may impede
the progress of the bolus and again allow it to enter the airway
after the laryngeal muscles have relaxed.

Impairments of the Esophageal Phase

Inadequate peristalsis can delay the progression of the bolus
through the esophagus. Failure of the cricopharyngeus mus¬
cle to close the esophagus at the end of the swallow may
allow regurgitation, and aspiration of the regurgitated con¬
tents can result.

Signs of Swallowing Impairment

As noted at the beginning of this chapter, the goal of this dis¬
cussion is to assist a clinician in identifying individuals who
may have difficulty in swallowing, to refer the individual to the
health care providers best suited to evaluate and treat the con¬
dition. Clinicians should suspect swallowing difficulties when
an individual coughs regularly or clears the throat regularly
before, during, or after a swallow. The material ingested may
slip past the mouth before the individual has initiated the swal¬
low, producing a cough before the swallow has even begun.
The material may be transmitted appropriately but may slip

436

Part III I KINESIOLOGY OF THE HEAD AND SPINE

into an inadequately closed larynx, producing a cough just as
the individual swallows. The material may not be transported
completely through the pharynx and may become sequestered
within the pharynx only to slip later onto the larynx, producing
a cough several minutes following the swallow. Since in each
of these instances some of the ingested material arrives at the
larynx, alterations in voice quality while eating may also sug¬
gest inadequate swallow. The voice may sound “wet” or “gur-
gly,” as though the individual needs to clear his or her throat.
Quiet aspiration also occurs in which there is no coughing or
throat clearing to indicate the presence of a liquid or solid at
the larynx. Signs of silent aspiration include loss of voice, face
reddening, and eye watering.

Examination of the oral cavity to determine if food is
sequestered in the cheeks or on the hard palate is useful.
Observation of voluntary tongue movements, lip muscula¬
ture, and muscles of the soft palate is also possible. The mus¬
cles of the soft palate contract to close off the nasopharynx
when an individual says “ah,” and elevation of the soft palate
is easily observed for excursion and symmetry. The hyoid
bone and the thyroid cartilage of the larynx are palpable dur¬
ing the swallow to assess the motion of the hyoid bone and
thus the participation of the supra- and infrahyoid muscles.
The clinician must realize that these examination procedures
are screening tools to identify patients who may exhibit swal¬
lowing disorders. Because swallowing disorders have the
potential to lead to lethal sequelae including aspiration pneu¬
monia and asphyxia, the clinician must refer the patient for
further, more detailed evaluations by specially trained health
care providers who can diagnose and implement treatment
for dysphagia.

SUMMARY

This chapter provides an overview of the muscles that partici¬
pate in swallowing, specifically the muscles of the tongue and
soft palate, and the extrinsic muscles of the larynx, the
suprahyoid and infrahyoid muscles. The intrinsic muscles of
the tongue alter the tongue s shape, while its extrinsic muscles
move the tongue, helping to form the bolus and propel it into
the oropharynx. The muscles of the soft palate help close off
the nasopharynx as the bolus passes into the oropharynx. The
suprahyoid muscles elevate the larynx, and the infrahyoid
muscles assist in lowering the larynx. Elevation of the larynx
along with contraction of the intrinsic muscles of the larynx
closes the larynx during the swallow, preventing aspiration.

The swallow is a complex series of coordinated events and
consists of four phases—the oral preparatory, oral, pharyn¬
geal, and esophageal phases. Impairments and ensuing dys¬
functions can occur in any of the phases, and the chapter lists
signs that an individual with swallowing impairments may
exhibit, including coughing, a “gurgly” voice, or loss of voice.
Because aspiration and aspiration pneumonia are common
among the elderly, clinicians who deal with elders must be
able to recognize signs of impaired swallowing and refer the

patient to specially trained professionals qualified to diag¬
nose and treat individuals with swallowing disorders.

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oral-pharyngeal bolus transit and clearance during swallowing:
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171-178.

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9. Dworkin JP, Aronson AE: Tongue strength and alternate motion
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10. Dworkin JP, Aronson AE, Mulder DW: Tongue force in normals
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11. Ekberg O: Posture of the head and pharyngeal swallowing. Acta
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12. Elidan J, Gonen B: Electromyography of the inferior constrictor
and cricopharyngeal muscles during swallowing. Ann Otol
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13. Elidan J, Shochina M, Gonen B, Gay I: Manometry and
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CHAPTER

Structure and Function of the
Articular Structures of the TMJ

Z. ANNETTE IGLARSH, P.T., PH.D., M.B.A.
CAROL A. OATIS, P.T., PH.D.

CHAPTER CONTENTS

BONY STRUCTURES THAT CONSTITUTE AND INFLUENCE THE TMJ.439

Cranium.439

ARTICULAR STRUCTURES OF THE TMJ.443

Articular Disc.443

Ligaments .445

ARTICULAR FUNCTIONS OF THE TMJ.446

Static Positions of the TMJ.446

Functional Motions of the TMJ.446

Disc Movement.447

Normal Ranges of Motion at the TMJ .448

RELATIONSHIP BETWEEN HEAD AND NECK POSTURE AND THE TMJ.448

SUMMARY.450

T he temporomandibular joint (TMJ) is a potential source of acute and chronic pain in the head and neck

regions, and countless types of painful syndromes have been attributed to dysfunction of this joint and its
surrounding muscles [24]. Complaints associated with TMJ dysfunction include headaches, tinnitus (ringing
in the ears), and altered taste. An understanding of the pathomechanics of an impaired TMJ requires a thorough grasp
of the structures and function of the TMJ and surrounding tissue [30].

The joint participates in the essential functions of chewing, or mastication, and speech [15]. As a consequence, the
evaluation and treatment of the joint falls within the scope of practice of dentists and speech and hearing specialists.
As a joint of the neuromusculoskeletal system that refers pain to the upper quadrant, it also is studied and treated by
therapists and physicians. Consequently, an understanding of the structure and function of the joint can contribute to
interdisciplinary collaborations leading to more effective treatments and improved patient outcomes. The purposes of
this chapter are to

■ Review the structure and function of the articular components of the TMJ
■ Describe the motions that occur in the TMJ
■ Review the normal ranges of motion of the TMJ

■ Describe how TMJ structures and dysfunction may contribute to patient complaint
■ Briefly discuss the relationship between the TMJ and posture of the head and cervical spine

438

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

439

BONY STRUCTURES THAT CONSTITUTE
AND INFLUENCE THE TMJ

The TMJ, also known as the craniomandibular joint, is the
articulation between the mandible and the temporal bone of
the skull [18] (Fig. 23.1). The structural relationship of the
mandible to the head contributes to the impact of the TMJ on
the muscles of the upper quadrant and the cervical spine. The
postures of the mandible with respect to the head and of the
head with respect to the neck are so interdependent that it is
almost impossible to alter the position of one structure with¬
out influencing the other structures [39].

Cranium

The bones of the cranium provide an articular surface for the
TMJ and attachments for the muscles of mastication, which
are described in Chapter 24. The cranial bones that provide
articular surfaces for the TMJ are the temporal and mandibu¬
lar bones (Fig. 23.2). Other bones participate in the function
of the joint by providing muscle attachments and articulation
for the teeth. The sphenoid and zygomatic bones provide
large attachments for the muscles of mastication. The upper
teeth articulate with the maxilla, and the palatine bones attach
posteriorly to the maxilla, providing additional attachments
for the muscles of mastication.

Figure 23.1: The TMJ consists of the articulation of the head of
the mandible with the mandibular fossa and articular eminence
on the inferior surface of the temporal bone

Zygomatic process

Figure 23.2: Cranial bones particularly relevant to the TMJ are the
mandible and the temporal, sphenoid, zygomatic, maxilla, and
palatine bones. The palatine bones are not visible in this figure.

TEMPORAL BONE

The temporal bone is a large bone forming part of the lateral
wall of the cranium [40,47]. Its inferior surface provides the
rostral articular surfaces of the TMJ, including the concave
mandibular fossa, or glenoid fossa, and the articular eminence
(Fig. 23.3). The articular eminence forms the anterior limit of
the mandibular fossa and contributes the anteriormost por¬
tion of the articular surface of the temporal bone. A styloid
process lies slightly posterior to the mandibular fossa, pro¬
jecting inferiorly from the inferior surface of the temporal
bone and providing attachment for muscles of the tongue and
pharynx.

Laterally, the temporal bone provides a large relatively flat
surface, which together with the lateral aspect of the sphe¬
noid constitutes the temporal fossa that provides attachment
for the temporalis muscle. The large, prominent mastoid
process lies posterior and lateral to the styloid process. The
facial nerve exits the skull at the stylomastoid foramen that
lies between the mastoid and styloid processes. The mastoid
process is readily palpated and helps orient the clinician to
the other structures during a clinical examination.

The external acoustic meatus also is located laterally, supe¬
rior to the styloid process and slightly posterior to the
mandibular fossa. The external auditory meatus leads to the

440

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 23.3: Articulating surface on the temporal bone. The
mandibular fossa and articular eminence, which form the articu¬
lar surface for the head of the mandible, lie on the inferior sur¬
face of the temporal bone. The styloid and mastoid processes lie
posterior to the mandibular fossa.

middle and inner ears, lying within the temporal bone. The
proximity of the external, middle, and inner ears to the TMJ
may help explain why some individuals with TMJ dysfunction
also report impaired hearing [3,9,19].

Clinical Relevance

EAR SYMPTOMATOLOGY WITH TMJ DYSFUNCTION:

Patients with TMJ dysfunction can present in the clinic with
complaints of ear pain, ringing in the ears (tinnitus), or even
impaired hearing. Such complaints may result from swelling
in the area of the TMJ or from direct pressure from the head
of the mandible on the inner ear ; increasing the pressure
around the structures of the ear canal.

The zygomatic process of the temporal bone projects
anteriorly from the lateral aspect of the temporal bone,
superior to the external acoustic meatus. The inferior aspect
of the root of the zygomatic process and the lateral aspect of

the articular eminence give rise to the articular tubercle that
provides attachment for the temporomandibular ligament.
The zygomatic process of the temporal bone joins the zygo¬
matic bone anteriorly.

SPHENOID BONE

The sphenoid articulates with the anterior aspect of the tem¬
poral bone, contributing to the temporal fossa laterally
[20,47]. Along with the palatine bones, the sphenoid bone
also contributes to the hard palate of the mouth. The inferior
surface of the sphenoid bone contributes to the anterior
aspect of the base of the skull, and it is this surface that con¬
tains structures that are important to the TMJ (Fig. 23.4). The
lateral aspect of the inferior surface of the sphenoid bone
contributes to the infratemporal fossa and provides the prox¬
imal attachment for two of the four primary muscles of mas¬
tication, the medial and lateral pterygoid muscles. The medi¬
al border of the infratemporal fossa is the lateral pterygoid
plate, which projects inferiorly from the inferior surface of
the sphenoid bone and contributes an additional attachment
for the medial and lateral pterygoid muscles. The medial
pterygoid plate also projects inferiorly from the inferior sur¬
face of the sphenoid bone and lies medial to the lateral ptery¬
goid plate; it ends anteriorly as the hamulus, which can be
palpated intraorally on the hard palate.

Clinical Relevance

PALPATION OF THE HAMULUS: The hamulus of the
sphenoid bone is palpated on the hard palate by placing
the palpating finger posterior and medial to the third
molar (Fig. 23.5). The pterygomandibular raphe, which is
a fibrous band that runs from the hamulus to the
mandible, also helps identify the hamulus intraorally,
where it too is readily palpated, covered by a layer of
mucous membrane. The lateral pterygoid muscle passes
just lateral to the hamulus of the sphenoid, and its con¬
traction can be palpated by palpating the lateral aspect of
the hamulus as the individual protracts the mandible.
Tenderness in this region during contraction may indicate
tenderness in the lateral pterygoid muscle.

ZYGOMATIC, MAXILLA, AND PALATINE BONES

The zygomatic bone joins the zygomatic process of the tem¬
poral bone, completing the zygomatic arch, and is known as
the cheekbone. The zygomatic arch gives rise to the masseter
muscle, an important muscle of mastication. The arch serves
to increase the mechanical advantage of the masseter while
reinforcing the strength of the zygomatic and temporal bones
to resist the forces of powerful jaw closing. The zygomatic
bone also contributes to the lateral wall of the eye socket, or
orbit, and to the infraorbital fossa. Anteriorly, the zygomatic
bones articulate with the maxillae, the largest bones of the

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

441

Figure 23.4: Sphenoid bone. A. Posterior view reveals that the
inferior surface of the sphenoid bone is marked by the medial
and lateral pterygoid plates. The lateral pterygoid plate
provides attachment for the medial and lateral pterygoid
muscles. B. Inferior view reveals only the greater wings of
the sphenoid bone and their projections, since the body
of the sphenoid is covered by nasal bones. Interiorly, the
medial pterygoid plate contains the projection, the hamulus,
palpable inside the oral cavity.

face. The maxillae contain the upper row of teeth and form
the upper jaw (Fig. 23.6). They also compose most of the roof
of the mouth and the floor and lateral walls of the nasal cavi¬
ty and contribute to the infratemporal fossae [20,47]. The
large maxillary sinus, a cavity that lies within the maxilla, is

Figure 23.5: The hamulus of the medial pterygoid plate of the
sphenoid bone can be palpated intraorally posterior and medial
to the third molar. Pterygomandibular raphe attaches to the
hamulus and is an easily identified fibrous band, covered by
mucous membrane. (Photo courtesy of Arnold J Malerman, DDS,
PC, Dresher, PA)

anterior to the TMJ, but its proximity to the joint may explain
why some individuals with TMJ symptoms also report chronic
sinus pain or irritation. The palatine bones attach between the
maxillae and the sphenoid to contribute to the hard palate
and floor of the nasal cavity [47].

maxillary sinus

Figure 23.6: Zygomatic bone attaches to the temporal bone and
to the maxilla, which contains the upper row of teeth. Palatine
bones articulate with the maxilla anteriorly and the sphenoid
posteriorly, and together they form the hard palate. The large
maxillary sinus found within the maxilla lies anterior to the TMJ.
Its proximity to the joint may explain patients' complaints of
sinus irritation along with TMJ pain.

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Part III I KINESIOLOGY OF THE HEAD AND SPINE

Condylar processes

Figure 23.7: The mandible consists of a body and two rami, each
ending in coronoid and condylar processes.

Figure 23.8: The angle of the mandible (A) is easily palpated and
helps locate the hyoid bone (B) interiorly and the transverse
process (C) of Cl posteriorly. The coronoid process (D) of the
mandible is palpable inferior to the zygomatic arch.

MANDIBLE

The mandible, or jaw bone, consists of a U-shaped body
containing the lower row of teeth, and two rami projecting
posteriorly and superiorly from the right and left sides of
the body of the mandible [8,10] (Fig 23.7). The angle of the
mandible marks the junction of the body and ramus and is
easily palpated at the posterior aspect of the jaw on either
side of the face (Fig. 23.8). The angles of the mandible are
important landmarks, lying superior to the posterior tips of
the hyoid bone and inferior and anterior to the transverse
processes of the first cervical vertebra.

Each mandibular ramus ends superiorly in two
processes. The anterior, coronoid process provides
attachment for the temporalis muscle. The anterior
border of the coronoid process is palpable inferior to the
zygomatic arch. The posterior condylar process thickens at
its superior end to form the head of the mandible that artic¬
ulates with the temporal bone in the TMJ. The head nar¬
rows inferiorly, forming the neck of the ramus that provides
attachment for a portion of the lateral pterygoid muscle.

The condyles of the mandible are shaped like footballs
cut in half that tilt anteriorly and medially toward each
other (Fig. 23.9). The condyles are more curved in the

Head of mandible

Figure 23.9: A. Medial view of the mandible reveals that the articu¬
lar surfaces of the mandibular condyles face anteriorly. B. Posterior
view reveals that the articular condyles also face medially.

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

443

anterior—posterior direction and slightly flatter in the
medial-lateral direction, but show considerable interindi¬
vidual variability [35,47].

ARTICULAR STRUCTURES OF THE TMJ

The mandible is suspended from the temporal bones at the
TMJs, which together form a compound joint in which
both TMJs must move simultaneously whenever the
mandible moves. Although each TMJ often is described as a
hinge joint, each joint exhibits more complex motion that
occurs in the sagittal, transverse, and frontal planes [7]
(Fig. 23.10). Opening and closing of the mouth occur pri¬
marily in the sagittal plane. Protrusion and retrusion consist
primarily of forward and backward translation of the
mandible, primarily in the transverse plane, although the
shape of the articular eminence requires that the mandible
descend as it glides anteriorly and rise as it glides posteriorly.
The TMJ allows rotation of the mandible in the transverse
plane motion about an axis that projects vertically through a
mandibular head. The mandible also can swing from left to
right in the frontal plane about an anterior posterior axis.
The teeth and the shapes of the articular surfaces guide and
limit the motion of the jaw.

The articular surfaces of the mandibular condyle and
the articular eminence of the temporal bone are both
covered by articular cartilage. Unlike most synovial joints,
however, the articular cartilage consists of fibrocartilage
rather than hyaline cartilage. As in other synovial joints, the
articular cartilage lacks a vascular supply and is nourished
and lubricated by the synovial fluid supplied by the sur¬
rounding synovial tissue.

Clinical Relevance

WITHSTANDING LARGE FORCES AT THE TMJ: Most
synovial joint surfaces are covered with hyaline cartilage. In
contrast fibrocartilage is found in joints that sustain large
forces such as the intervertebral joints of the spine. The pres¬
ence of fibrocartilage in the TMJ suggests that the TMJ incurs
large forces during mastication. Hu et al. suggest that the
fibrocartilage is a more important shock absorber at the TMJ
than the articular disc [22]. Arthritis and degeneration of the
articular cartilage can lead to severe problems in mastication.

The articular surface of the mandible consists of the
superior and anterior surfaces of the mandibular head. The
anterior portion of the articular surface on the temporal
bone consists of the posterior, convex portion of the articular
eminence (Fig. 23.11). The remainder of the temporal
articular surface is the mandibular fossa that ends posteriorly
as the posterior articular ridge, immediately anterior to the
external auditory meatus. The shape of the articulating

surface of the temporal bone explains the complex motion
that occurs during opening and closing the mouth, combining
rotation about a medial lateral axis and translation along the
curved surface of the articular eminence.

The roof of the mandibular fossa is typically thin and
non-weight-bearing. Loads are borne between the mandi¬
bular condyle and intraarticular disc and between the disc
and articular eminence of the temporal bone. The layer of
fibrocartilage covering the entire articulating surface of the
temporal bone is thickest on the articular surface of the
articular eminence where the stress is the greatest, and
thinnest at the roof of the mandibular fossa where little load
bearing occurs [38].

Articular Disc

Like the knee, the TMJ contains an intraarticular disc, or
meniscus, that separates the joint into a superior joint space,
between the disc and the articular eminence, and an inferi¬
or joint space, between the disc and the mandibular head
[17,18,28,31,34] (Fig. 23.12). The disc increases the con¬
gruency between the joint surfaces, but also can be a source
of pain and dysfunction. The articular disc is concave super¬
iorly to conform to the articular eminence of the temporal
bone and concave inferiorly to mold to the convex mandibu¬
lar head [47].

The disc consists of dense fibrous connective tissue that
adheres more firmly to the mandible than to the temporal
bone. The stronger inferior connection includes medial and
lateral bands from the disc to the articular condyle, a strong
anterior connection with the fibers of the lateral pterygoid
and a loose fibrous connection posteriorly. The disc is thick¬
est peripherally and thinnest at its center. If the normal
anatomical alignment of the joint surfaces is altered, the disc
can be torn or perforated as forces compress the thin center
of the disc [2,6,17,28,41,49].

The disc continues posteriorly as two layers of loose fibrous
tissue, a fibroelastic layer that attaches to the posterior aspect
of the mandibular fossa of the temporal bone, and an inferior
inelastic layer that attaches to the condyle of the mandible.
This bilaminar region is highly vascular and rich in nerve
endings and fuses with the articular capsule posteriorly as the
retrodiscal pad. The central portion of the disc is avascular,
an indication of its stress-bearing role in TMJ function.

Clinical Relevance

DISC PATHOLOGY: Many temporomandibular joint
dysfunctions involve problems with the disc. The disc can
degenerate or tear just as the menisci within the knees do.
The bilaminar region and retrodiscal pad can become
inflamed and painful from repeated or prolonged compressive
forces. Such forces can occur from teeth clenching or grinding.
The disc itself can be subluxed (partially dislocated) or dislocated

( continued)

444

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Horizontal axis

Anterior-posterior

Figure 23.10: The TMJ exhibits three-dimensional motion that includes rotations about medial-lateral (A) f anterior-posterior (B) f and
vertical (C) axes. It also allows translation in the sagittal and transverse planes.

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

445

Figure 23.11: The articulating surface of the temporal bone
consists of the mandibular fossa and the articular eminence.
Opening and closing require rotation of the mandible about a
medial lateral axis and translation of the mandibular head along
the articular eminence, producing anterior and inferior transla¬
tion of the mandible.

(Continued)

anteriorly (internal derangement of the TMJ), producing
abnormal opening and closing patterns of movement and
even an inability to fully close the mouth. Just as a complete
assessment of the knee includes assessment of the menisci,
a thorough assessment of the TMJ includes consideration
of the articular disc

Figure 23.12: The intraarticular disc divides the TMJ into a superior
space between the temporal bone and the disc and an inferior
space between the disc and the mandibular head. A. Sagittal
plane view of left disc with the mouth closed. B. Sagittal plane
MRI of right disc with the mouth closed.

Ligaments

The primary ligamentous supporting structures of the TMJ
are the joint capsule and the temporomandibular ligament
[17,20,23,28,42,44] (Fig. 23.13). Additional ligaments include
the sphenomandibular and stylomandibular ligaments. It
is important to recognize that stability of the TMJ comes
not just from ligamentous support but also from the mus¬
cles of mastication that are discussed in detail in Chapter
24 [25].

JOINT CAPSULE

The joint capsule encloses the articular surfaces of the tem¬
poral bone and mandibular head, as well as the disc. The cap¬
sule can be traced superiorly along the rim of the mandibular
fossa, anteriorly around the articular surface of the articular
eminence, and inferiorly around the mandibular head.

The horizontal fibers of the joint capsule connect directly
to the lateral and medial parts of the disc. The capsule fuses

Figure 23.13: Primary ligaments of the TMJ are the joint capsule
and the temporomandibular ligament. The stylomandibular and
sphenomandibular (not seen in this view) ligaments are accessory
ligaments.

446

Part III I KINESIOLOGY OF THE HEAD AND SPINE

with the anterior disc, while posteriorly, the disc connects
with the capsule via the retrodiscal pad. Consequently, the
disc translates anteriorly easily as its posterior attachments to
the retrodiscal pad stretch, but glides little in a posterior
direction because of its firm inelastic anterior attachment to
the joint capsule. The capsular ligament allows joint motion
in the sagittal plane but restricts motion in the frontal and
transverse planes [20,25,27].

TEMPOROMANDIBULAR LIGAMENT

The temporomandibular ligament, also known as the lateral
temporomandibular ligament, reinforces the joint capsule lat¬
erally and consists of two layers, the wide, superficial layer
and the medial, deep portion [47]. The superficial portion of
the ligament courses downward and posteriorly from the
articular tubercle to the posterior surface of the mandibular
head, while the deep, or medial, part runs from the articular
tubercle and the temporal squama in an anterior and medial
direction. These fibers, running horizontally, join the fibers of
the joint capsule and disc.

The lateral fibers of the joint capsule limit inferior transla¬
tions of the mandible. The medial fibers, assisted by the lat¬
eral pterygoid muscle, limit posterior translation of the
mandible during retrusion or from a direct blow to the jaw.
Consequently, these fibers protect the highly vascular and
sensitive retrodiscal pad in the posterior joint. Thus the tem-
peromandibular ligament helps prevent excessive jaw opening
by checking the mandible s descent beyond the articular emi¬
nence [25,27]. It also prevents damage to the retrodiscal pad by
preventing excessive posterior translation of the mandible [20].

THE STYLOMANDIBULAR AND
SPHENOMANDIBULAR LIGAMENTS

These two ligaments are accessory ligaments and appear to have
minor effects on movement of the mandible. Their names
reflect their sites of attachment: the stylomandibular ligament
starts at the apex of the styloid process and ends on the ramus
of the mandible and the sphenomandibular ligament courses
from the spine of the sphenoid bone to the mandibular foramen.
The stylomandibular ligament may limit forward glide of the
mandible during protrusion, but the sphenomandibular liga¬
ment appears to have little effect on TMJ motion or stability [47].

ARTICULAR FUNCTIONS OF THE TMJ

The TMJs are unique because they are mechanically linked
by the mandible and must work synchronously for the
mandible to move normally [17,21,36,39,46].

Clinical Relevance

IMPACT OF SYNCHRONOUS MOVEMENT AND
DYNAMIC EVALUATION OF THE TMJ: When a patient
complains of TMJ pain or dysfunction , it is not possible to

isolate the cause in one joint without recognizing the impact
on the opposite joint. The impairment identified in each
joint can be opposite from one another. For example; the
practitioner may determine that one joint is hypomobile and
then find that the opposite joint is hypermobile, a likely
compensation for the joint restrictions found in the hypomo¬
bile TMJ.

Static Positions of the TMJ

The rest position of the mandible is a natural position in
which there is a balance between the weight of the mandible
and the forces that support the TMJs in the upright posture.
In the erect position it is impossible to unload the joint com¬
pletely or eliminate all muscle tension, since the muscles of
mastication must contract to keep the mouth closed against
the pull of gravity. (Anyone who doubts this need only observe
the open-mouthed posture of a student who has fallen asleep
in class.) In the normal rest position the tongue is maintained
against the hard palate by negative air pressure within
the mouth, forming an area referred to as Donder’s space.
The negative air pressure decreases the amount of muscle
force needed to support the jaw. The two rows of teeth do not
touch in the rest position, but the lips are in gentle contact
with each other. In this position, the mandibular head faces
the articular eminence of the temporal bone and the disc is
seated anteriorly on the mandibular head, between the two
articulating surfaces. This combination of disc position and
limited muscle activity mechanically unloads the soft tissue of
the TMJ. In contrast, the occlusal position is defined as the
posture in which the two rows of teeth are in gentle contact.

Functional Motions of the TMJ

As noted earlier, the TMJs allow complex three-dimensional
motion. The functional movements that allow the jaw to move
during mastication and speech are opening and closing the
mouth, as well as protrusion, retrusion, and lateral deviation
of the jaw.

OPENING AND CLOSING THE MOUTH

Mouth opening, also known as mandibular depression,
combines rotation about a medial-lateral axis with protrusion
in the transverse and sagittal planes. Closing, or mandibular
elevation, consists of upward rotation of the mandible and
retrusion. Most of the rotation and translation occur simulta¬
neously throughout the range of motion, although the rela¬
tive contribution of rotation and translation at initial opening
is controversial [5,11]. Some investigators suggest that open¬
ing begins with rotation, others suggest it begins with trans¬
lation, and still others report that the contribution is equally
distributed [5,11,29]. These differences very well may repre¬
sent normal individual variation and require additional
research to resolve. The rotary motion of the joint occurs

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

447

mostly in the inferior joint space between the disc and
mandibular head. The translation occurs predomi¬
nantly in the superior joint space as the disc moves to
seat itself in the mandibular fossa.

Anterior translation of the disc is necessary to keep the
disc in contact with the mandibular head. Forward transla¬
tion stretches the retrodiscal tissue that contains collagen
and elastin fibers. The loose tissue of the retrodiscal pad per¬
mits movement of the disc in the mandibular fossa of the
temporal bone, and its recoil helps relocate the disc posteri¬
orly. This movement occurs in the superior portion of the
joint capsule.

Mouth opening occurs by the following combination of
events [32,47]:

• The opening motion, as the mandible descends and the
chin lowers, begins with mandibular rotation in the inferior
joint space, the space between the disc and the mandibular
head. (Slight anterior translation, or protrusion, which
occurs in the superior joint space between the disc and
temporal fossa may accompany or even precede the
mandibular rotation.)

• As the mandible rotates downward, the disc moves poster¬
iorly, relative to the mandibular head, to become seated on
top of the mandibular head.

• Ligaments attached to the disc become taut, and the disc
is held firmly against the mandibular head.

• Thus this motion is rotation between the disc and the
mandibular condyle.

• The disc and mandibular head complex move as a single
unit, translating anteriorly and inferiorly along the surface
of the articular eminence, producing protrusion and addi¬
tional depression.

Closing the mouth involves a reversal of the movements of
opening, the mandible rotates upward and retrudes,
although the mandible s path during closing differs slightly
from its opening path [11,43,47,48]. Thus the simple motion
of mouth opening or mandibular depression is not a simple
hinge action in which the muscles of mastication relax and
allow the mandible to rotate downward by gravity. The
actions of mouth opening and closing are a complex series of
controlled rotations and translations. The role of muscles in
depressing and elevating the mandible is presented in
Chapter 24.

PROTRUSION, RETRUSION, AND LATERAL MOTION

Protrusion, the motion of “jutting” the jaw forward, is accom¬
plished as the mandibular condyles and the articular disc glide
anteriorly and inferiorly along the articular eminence.
Retrusion occurs in the opposite direction and is limited by
taut anterior ligamentous and muscle fibers and the mass of
the retrodiscal pad of the disc. Because the retrodiscal tissue is
very vascular and well innervated, compression or irritation of
the retrodiscal pad by excessive or sustained retrusion may
produce TMJ pain.

Clinical Relevance

IMPACT OF CLENCHING ON TMJ FUNCTION: Chronic
clenching of the teeth may produce compression and exces¬
sive forces on the retrodiscal pad as a result of excessive or
prolonged retrusion , impairing the blood flow and produc¬
ing an inflammatory response in the area. Because the
region is rich in nerve endings , severe TMJ pain may result.

Lateral deviation of the mandible also involves complex
motions of both TMJs. It occurs by protrusion of the mandible
on one side while the opposite side rotates about a vertical axis
(Fig. 23.14). Consequently, one condyle and its disc move
anteriorly, inferiorly, and medially along the articular emi¬
nence, causing deviation of the mandible toward the opposite
side as the opposite condyle rotates laterally around a vertical
axis. As in opening and closing, protrusion and lateral motions
of the mandible occur by delicately coordinated muscle activ¬
ity at both TMJs. Mastication is a complex series of motions
that includes all the directions of motion discussed above and
the corresponding synchronized contraction of the muscles of
mastication. This activity is discussed in Chapter 24.

As noted earlier in the chapter, the TMJs are capable of
complex three-dimensional motion. Although not specifically
measured, the tilting motion of the mandible in the frontal
plane, about the anterior-posterior axis, is also an essential
movement of the TMJ. This motion allows chewing of a bolus
of food on one side of the mouth, critical for controlling the
bolus and preparing it for swallow. The tilt increases the joint
reaction force on the side to which the mandible tilts.

Disc Movement

Normal movement of the TMJ requires precise synchroniza¬
tion between the movement of the mandible and the move¬
ment of the intraarticular disc. Abnormal movements of
either can alter the mechanics of the whole TMJ complex

Figure 23.14: Lateral deviation of the mandible to the left
requires protrusion of the right TMJ and rotation of the left TMJ
about a vertical axis.

448

Part III I KINESIOLOGY OF THE HEAD AND SPINE

and contribute to patients’ signs and symptoms. As the
muscles of mastication contract or relax and the mouth closes
or opens, the disc remains in contact with the mandibular
head and the disc-mandibular head complex moves together
in translation [7].

When the mouth is fully open, the intermediate zone of the
articular disc is between the articular tubercle of the temporal
bone and the dorsal convexity of the condylar process of the
mandible, increasing the congruency between the bony sur¬
faces. The posterior retrodiscal cushion is stretched, and the
anterior connective tissue is compressed in this position. When
the mouth is closed, the retrodiscal cushion is moderately
enlarged and the anterior cushion is smaller than when the
teeth are in full contact. During protrusion of the mandible, the
intermediate zone of the articular disc is between the convexi¬
ties of the condylar process and the articular eminence. The
disc is compressed between the mandible and the temporal
bone on the lateral side of the joint. In lateral deviation, the disc
is stabilized between the bony joint elements on the side
toward which the mandible is deviating. On the opposite side,
the articular disc protrudes, with the retrodiscal tissue filling
the posterior, lateral half of the mandibular fossa.

Clinical Relevance

SOUNDS ELICITED DURING OPENING AND CLOSING:

The TMJ may emit a variety of sounds during movements;
including popping; clicking, or grinding. Some sounds are
consistent with normal function such as the popping sound
associated with bubbles forming in the synovial fluid. In
contrast clicking and grinding sounds are often associated
with joint pathology or impairment and serve as clues to the
examining clinician [37]. Grinding, or crepitus , suggests
increased friction between the articular surfaces and may
reflect damage to the articular cartilage. Clicking sounds are
often associated with abnormal movement of the intraarticu-
lar disc. Some clinicians contend that a clicking sound dur¬
ing opening and early in the phase of closing is heard as the
condyle catches up with an anteriorly displaced disc
| [4,13,16]. A click later in closing may indicate that
XX the mandible has glided posteriorly beyond the disc.

Normal Ranges of Motion at the TMJ

Table 23.1 presents the reported normal excursion of the tem¬
poromandibular [32]. There are no known studies that
demonstrate the variability of joint excursions available at the
TMJ among subjects without joint pathology. Consequently,
practitioners disagree about the preferred goals for range of
motion to be achieved in the rehabilitation of the TMJ. While
20-25 mm appears sufficient for functional opening, some
practitioners suggest that a patient should achieve a range of
motion in excess of 45 mm. Total opening is greater in men
than women [11,12]. This difference seems to be primarily the
effect of the size of the mandible.

TABLE 23.1: Normal Range of Motion of the Mandible

Depression of the mandible/opening of the mouth:
Functional active motion

Minimal opening for functional activity

35-55 mm
25-35 mm

Elevation of the mandible/closing of the mouth

The mandible returns from depression until
the teeth of the mandible and maxilla
come into contact

Protrusion of the mandible

Functional active motion

3-6 mm

Retrusion of the mandible

Functional active motion

3-4 mm

Lateral deviation of the mandible

Functional active motion

10-15 mm

Clinical Relevance

MEASURING OPENING: Because mouth opening is
dependent upon the size of the mandible, clinicians may
have difficulty judging whether a patient's opening ROM is
"normal." Opening can be measured linearly using a ruler,
or specialized insert. It can also be assessed functionally by
determining the number of knuckles that the patient can
comfortably insert into the open mouth (Fig. 23.15). This
technique helps adjust for size since the patient's finger size
should be proportional to his or her mandible size.

RELATIONSHIP BETWEEN HEAD
AND NECK POSTURE AND THE TMJ

The position of the mandible on the head is inseparably related
to the position of the head on the neck [1,7,14,33]. Head
position alters the direction of pull of many of the muscles
that open the jaw [26]. In the occlusal position, anterior and
posterior changes in head and neck postures move the point
of contact between the teeth and also alter the joint space of
the TMJ [45]. Side bending in the cervical spine reduces the
joint space in the ipsilateral TMJ. Reduced joint space
may increase the joint reaction forces on the TMJ,
contributing to increased joint pain.

Clinical Relevance

HEAD AND NECK POSTURE IN INDIVIDUALS WITH
TMJ PAIN: Head and neck posture may contribute signifi¬
cantly to the pathomechanics of the TMJ, and patients with
TMJ pain must undergo a thorough postural evaluation.

A forward head posture tends to stretch the soft tissue on the
anterior aspect of the cervical spine, including the suprahyoid
muscles. Tension in these structures tends to pull the mandible
posteriorly, retruding the mandible (Fig. 23.16). A chronically
retruded mandible may produce inflammation of the retrodis¬
cal pad, resulting from sustained compression, and also may
apply pressure on the middle and inner ears. Thus a forward
head posture may produce or contribute to TMJ pain.

Chapter 23 I STRUCTURE AND FUNCTION OF THE ARTICULAR STRUCTURES OF THE TMJ

449

Figure 23.15: Measurement of mouth opening by (A) linear measurement of TMJ opening and (B) functional assessment of opening by
inserting knuckles into the open mouth.

Figure 23.16: A forward head posture stretches
the suprahyoid muscles, some of which attach to
the mandible. The stretch on these muscles pulls
the mandible posteriorly into retrusion, which
may lead to compression and irritation of the
retrodiscal pad and, consequently, to temporo¬
mandibular pain.

450

Part III I KINESIOLOGY OF THE HEAD AND SPINE

SUMMARY

This chapter describes the structure of the bones and liga¬
ments of the TMJ and the motions available to these struc¬
tures. The two TMJs constitute a compound joint in which
both TMJs must move whenever one joint moves. A joint cap¬
sule and a temporomandibular ligament support each TMJ,
and the articular surfaces are protected by an intraarticular
disc. The posterior aspect of the joint space contains highly
vascularized, sensitive loose connective tissue, the retrodiscal
pad, which helps protect the posterior joint space and sup¬
ports the intraarticular disc.

Each TMJ exhibits complex three-dimensional motion as
the mandible elevates, depresses, or deviates laterally.
Opening the mouth requires mandibular depression and pro¬
trusion; closing consists of elevation and retrusion. Lateral
deviation consists of asymmetrical movement of both TMJs,
in which one side protrudes and the opposite side rotates.
The disc moves to maintain a cushion between the head of
the mandible and the articulating surface of the temporal
bone. Impairments of the joint can alter the movement of
either the mandible or the disc, contributing to a patients
complaints.

The chapter demonstrates how head and neck posture
affects the TMJ. Head and neck posture can alter the area of
contact between the teeth as well as the orientation of the
mandible on the temporal bone. A careful postural evaluation
is an essential component of a thorough TMJ evaluation. The
following chapter presents the muscles that provide the coor¬
dinated movement of both TMJs essential to mastication and
speech.

References

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3. Bubon MS: Documented instance of restored conductive hear¬
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16. Gossi DB, Gallo LM, Bahr E, Palla S: Dynamic intra-articular
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17. Guttman GD: Animating functional anatomy for the web. Anat
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18. Hanthorne G: Craniomandibular dysfunctions. ASHA 1998; 30:
69-69.

19. Henderson DH, Cooper JC Jr, Bryan GW, et al.: Otologic com¬
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20. Hiatt J, Gartner L: Textbook of Head and Neck Anatomy. New
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21. Howard RP, Bowles AP, Guzman HM, Krenrich SW: Head,
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22. Hu K, Qiguo R, Fang J, Mao JJ: Effects of condylar fibrocarti-
lage on the biomechanical loading of the human temporo¬
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ment model. Med Eng Phys 2003; 25: 107-113.

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mandibular pain and dysfunction. J Mass Dent Soc 1995; 44:
21-25.

25. Koolstra JH: Dynamics of the human masticatory system. Crit
Rev Oral Biol Med 2002; 13: 366-376.

26. Koolstra JH, van Eijden TMGJ: Functional significance of the
coupling between head and jaw movements. J Biomech 2004; 37:
1387-1392.

27. Koolstra JH, van Eijden TMGJ: Three-dimensional dynamical
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28. Kordass B: The temporomandibular joint in video motion—
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451

31. Loughner BA, Gremillion HA, Mahan PE, et al.: The medial
capsule of the human temporomandibular joint. J Oral
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32. Magee DA: Orthopedic Physical Assessment. Philadelphia: WB
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38. Rainer B, Mall G, Landgraf J, Scheck R: Biomechanical analysis
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Kinematics of the human mandible for different head postures.
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46. Weinberg S, Lapointe H: Cervical extension-flexion injury
(whiplash) and internal derangement of the temporomandibular
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Anatomical Basis of Medicine and Surgery, Br. ed. London:
Churchill Livingstone, 1995.

48. Yatabe M, Zwijnenburg AJ, Megens CC, Naeije M: Movements
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1999; 2: 45-49.

CHAPTER

Mechanics and Pathomechanics
of the Muscles of the TMJ

NEAL PRATT , P.T., PH.D. AND
CAROL A. OATIS, P.T., PH.D.

CHAPTER CONTENTS

MUSCLES OF MASTICATION .452

Masseter.453

Temporalis.455

Medial Pterygoid.457

Lateral Pterygoid.458

Accessory Muscles.459

MASTICATION.459

Mandibular Motion during Chewing .459

Muscle Activity during Mastication.461

SUMMARY .463

T he preceding chapter presents the bones and connective tissue structures of the temporomandibular joint
(TMJ). It also describes the mechanics of movement occurring at the joint. The purpose of this chapter is to
review the anatomy of the muscles of mastication and to describe their individual actions and their roles in
mastication. As noted in Chapter 23, the two TMJ joints function together, creating a compound joint that permits
motion of the mandible. Consequently, the muscles of mastication produce different motions of the mandible, depend¬
ing on whether they contract unilaterally or bilaterally. The specific purposes of this chapter are to

■ Review the structure of the primary muscles of mastication

■ Discuss the motions produced by each muscle when contracting unilaterally and bilaterally
■ Present the current understanding of the muscles' roles in mastication

■ Demonstrate the relationships between the behavior of muscles of mastication and some of the signs and symp¬
toms of patients with TMJ dysfunction

MUSCLES OF MASTICATION

The primary muscles of the TMJ are the masseter and
temporalis, which are superficially positioned, and the
medial and lateral pterygoids, which occupy the infratem¬
poral fossa (Fig. 24.1). Accessory muscles include the

buccinator, a muscle of facial expression; the muscles of
the tongue; and the suprahyoid muscles, which are the
muscles that form the floor of the mouth. These accessory
muscles are discussed in detail in Chapters 20 and 21.
Their role in chewing, or mastication, is presented later
in this chapter.

452

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

453

Figure 24.1: The primary muscles of mastication are the mas-
seter, temporalis, and medial and lateral pterygoid muscles.

A. The masseter and temporalis lie on the lateral surface of the
joint. B. The medial and lateral pterygoid lie on the medial
surface of the joint.

The four primary muscles of mastication share several
anatomical and functional characteristics. The mandibular
elevators, the masseter, temporalis, and medial pterygoid
muscles, have large cross-sectional areas indicating their spe¬
cialization for force production, a necessity for grinding hard,
tough foods. The muscles of the TMJ appear to provide
the primary stabilizing support to the TMJs [15,17]. Only in
extreme mediolateral movements do the ligamentous struc¬
tures play a primary role.

Sagittal plane or midline motion of the mandible occurs
only when both the left and right muscles of a pair contract.

Albeit to different degrees, all of these muscles are oriented
obliquely with respect to the axes of the TMJs, so when con¬
tracting unilaterally, each produces combinations of motions
simultaneously For example, the lateral pterygoid causes pro¬
trusion and deviation of the mandible to the opposite side; the
left lateral pterygoid causes deviation of the mandible to the
right, and the right lateral pterygoid causes deviation to the left
(Fig. 24.2). Each lateral pterygoid also produces protrusion of
the mandible along with the lateral deviation. Consequently,
protrusion in the sagittal plane results only when both right
and left lateral pterygoid muscles contract together, counter¬
acting the lateral deviation pull of each muscle individually.

Most skeletal muscles, particularly those in the extremi¬
ties, can produce motion of either the bone serving as the
origin or the bone serving as the insertion. The muscles of
mastication arise from the skull and insert on the mandible.
Since the skull is fixed relative to the mandible, only the
mandible moves with activity of these muscles.

All of the muscles of mastication are innervated by
the mandibular division of the trigeminal (5th cranial) nerve
(Fig. 24.3). The mandibular division branches from the main
trunk of the trigeminal nerve in the middle cranial fossa and
passes through the foramen ovale into the infratemporal fossa.
It branches high in the fossa and, in addition to the muscles,
supplies general sensation (not taste) to the mandibular denti¬
tion, the mucosa of the cheek, the anterior two thirds of the
tongue and the skin superficial to the mandible and posterior
temporal region.

Clinical Relevance

TRIGEMINAL NEURALGIA: Trigeminal neuralgia

(tic douloureux) is a syndrome characterized by brief but
severe episodes of pain in regions of the head correspon¬
ding to the distribution of one or more of the divisions of
the trigeminal nerve , most commonly the mandibular
nerve. Because the mandibular division supplies the mus¬
cles of mastication as well as cutaneous areas , chewing
can trigger the onset of pain.

Masseter

The masseter is positioned superficial to the mandibular
ramus, extends from the zygomatic arch to the angle of the
mandible, is readily palpable, and is composed of superficial
and deep parts (Fig. 24.4; Muscle Attachment Box 24.1). The
superficial part lies more anteriorly than the deep part and is
composed of fibers that pass slightly posteriorly from above
downward. The deep part is positioned more posteriorly, and
consists of more vertically oriented fibers.

ACTIONS

The actions of the masseter are considered unilaterally and
bilaterally [1,32,33].

454

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 24.2: Superior view of the motion of the mandible with unilateral and bilateral contractions of the lateral pterygoid muscle.

A. Unilateral contraction of the lateral pterygoid pulls the ipsilateral ramus of the mandible anteriorly, causing the mandible to deviate
toward the contralateral side. B. Bilateral and symmetrical contraction of the lateral pterygoid muscles produces protrusion of the
mandible, with no lateral deviation.

(5th cranial) innervates the muscles of mastication and provides
sensation to the mandibular teeth, the mucosal lining of the
cheek, the anterior two thirds of the tongue, and the skin over-
lying the masseter and posterior temporal region.

Figure 24.4: Alignment of the superficial and deep portions of
the masseter muscle. The fibers of the superficial portion of the
masseter run interiorly and posteriorly, while the fibers of the
deep portion are more vertically oriented.

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

455

MUSCLE ATTACHMENT BOX 24.1

ATTACHMENTS AND INNERVATION
OF THE MASSETER MUSCLE

Cranial attachment:

Superficial part: Lower border of the anterior
aspect of the zygomatic arch

Deep part: Deep and lower aspects of the
zygomatic arch

Mandibular attachments:

Superficial part: Lateral inferior aspect of the
ramus of the mandible

Deep part: Lateral superior aspect of the ramus
of the mandible

Innervation: Mandibular division of the trigeminal
nerve (5th cranial nerve)

Palpation: The superficial portion of the masseter is
easily palpated at the angle of the mandible as the
subject gently clenches the teeth.

MUSCLE ACTION: MASSETER UNILATERAL ACTIVITY

Action

Evidence

Mandibular elevation

Supporting

Ipsilateral deviation of the mandible Supporting

MUSCLE ACTION: MASSETER BILATERAL ACTIVITY

Action

Evidence

Mandibular elevation

Supporting

Forceful occlusion

Supporting

Figure 24.5: Lateral deviation of the mandible with contraction
of the masseter muscle. Unilateral contraction of the masseter
produces ipsilateral deviation of the mandible.

producing ipsilateral deviation (Fig. 24.5 ) [17,18]. Direct
electrical stimulation of the masseter produces elevation, ipsi¬
lateral deviation, and slight protrusion [44]. Biomechanical
analysis of the masseter s action supports its role in only ele¬
vation and ipsilateral deviation [17].

In the upright posture the weight of the mandible tends to
depress it, producing an open mouth. Full opening is prevented
in the relaxed upright posture by a low level of activity in the
mandibular elevators. However, electromyographic (EMG)
studies suggest that the masseter muscles are only minimally
active in maintaining resting mandibular posture in the
upright position [38,39]. In contrast, the masseter muscles are
responsible for producing a powerful bite [26,33]. EMG data
reveal that activity in the masseter muscles increases with bite
force [3,11,27,29]. The role of the masseter in providing
forceful bite is consistent with its large cross-sectional area. In
addition, it has the largest moment arm of the mandibular
elevators, allowing it to generate the large elevation moments
at the TMJ necessary for chewing uncooked carrots or a
tough piece of meat [29].

The masseter attaches to the lateral surface of the ramus
of the mandible, so contraction pulls the mandible laterally,

Temporalis

The temporalis muscle is the largest of the masticatory
muscles. It is fan-shaped and positioned superficially on the
lateral aspect of the skull so it is readily palpable. (Muscle
Attachment Box 24.2 ) Since the size of its origin greatly
exceeds that of its insertion, the orientation of its fibers varies
widely across the whole muscle, so that individual segments
of the muscle are capable of distinctly different actions
(Fig. 24.6). Even though it is commonly divided into anterior,
middle and posterior parts, the anterior and middle parts
(vertical fibers) and the posterior part (horizontal fibers) form
two functional units [3,17,35,40].

ACTIONS

The actions of the temporalis as a whole are considered uni¬
laterally and bilaterally.

456

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 24.2

ATTACHMENTS AND INNERVATION
OF THE TEMPORALIS MUSCLE

Cranial attachment: Temporal fossa of the skull

Mandibular attachments: Coronoid process and
deep surface of the anterior aspect of the ramus of
the mandible

Innervation: Mandibular division of the trigeminal
nerve (5th cranial nerve)

Palpation: The anterior portion of the temporalis is
palpated on the skull superior and slightly anterior
to the ear during gentle teeth clenching. The poste¬
rior portion may be palpated just posterior to the
superior tip of the ear during retrusion.

MUSCLE ACTION: TEMPORALIS UNILATERAL ACTIVITY

Action

Evidence

Mandibular elevation

Supporting

Mandibular retrusion

Supporting

Ipsilateral deviation of the mandible

Supporting

Figure 24.6: Horizontal and vertical fibers of the temporalis
muscle. The temporalis is divided functionally into a group of vertical
fibers that produce elevation of the mandible and more horizontal
fibers that produce elevation and retrusion of the mandible.

MUSCLE ACTION: TEMPORALIS BILATERAL ACTIVITY

Action

Evidence

Mandibular elevation

Supporting

Mandibular retrusion

Supporting

The temporalis is considered the primary postural muscle of
the mandible in that it maintains mandibular posture in the
upright resting position, and it has been described as the most
important muscle during both incisor bite and molar occlu¬
sion [38,39]. During maximal mandibular depression, as in
opening the mouth very wide, it may help to counteract or
prevent dislocation of the TMJ by limiting anterior translation
of the mandibular condyle [29,37]. Like the masseter, the
temporalis pulls the mandible laterally, producing ipsilateral
deviation (Fig. 24.7 ) [17,18].

Separate actions of the vertical and horizontal fibers also
are reported [3,17,35,40,43]. The horizontal fibers contribute
to retrusion, elevation, and lateral deviation of the mandible,
while the vertical fibers elevate and deviate the mandible
laterally and may provide slight protrusion [40,43]. EMG

Figure 24.7: Lateral deviation of the mandible with contraction
of the temporalis muscle. The attachment of the temporalis
on the cranium is lateral to its attachment on the mandible, which
is why unilateral contraction of the temporalis muscle produces
ipsilateral deviation of the mandible.

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

457

studies demonstrate that both portions of the temporalis are
active during bite, regardless of whether the bite occurs
between the incisor teeth or between the molars [3,24,35].
However, the anterior fibers appear to contribute more than
the posterior fibers during an incisal bite [24].

Medial Pterygoid

The medial pterygoid is the deepest of the muscles of masti¬
cation and is oriented obliquely in both the sagittal and
frontal (coronal) planes. (Muscle Attachment Box 24.3) Its
sagittal orientation is similar to that of the superficial part of
the masseter, so that it inclines posteriorly from superior to
inferior. It is more oblique in the frontal plane and inclines
considerably laterally as it projects from the skull to the
mandible (Fig. 24.8).

ACTIONS

The actions of the medial pterygoid muscle are considered
unilaterally and bilaterally.

MUSCLE ACTION: MEDIAL PTERYGOID UNILATERAL
ACTIVITY

Action

Evidence

Mandibular elevation

Supporting

Contralateral deviation of the mandible

Supporting

MUSCLE ACTION: MEDIAL PTERYGOID BILATERAL
ACTIVITY

Figure 24.8: Alignment of the medial pterygoid muscle in the
frontal plane. The fibers of the medial pterygoid muscle run inter¬
iorly and laterally to attach on the medial side of the mandible.

Action

Evidence

Mandibular elevation

Supporting

Slight mandibular protrusion

Supporting

MUSCLE ATTACHMENT BOX 24.3

ATTACHMENTS AND INNERVATION
OF THE MEDIAL PTERYGOID MUSCLE

Cranial attachment: Deep surface of the lateral
pterygoid plate of the sphenoid bone

Mandibular attachments: posterior aspect of the
medial surface of the mandibular ramus

Innervation: Mandibular division of the trigeminal
nerve (5th cranial nerve)

Palpation: The medial pterygoid can be palpated
intraorally with care between the medial surface of
the ramus of the mandible and the lateral side of
the molars.

The location of the medial pterygoid on the deep surface of
the mandible explains why it, like the lateral pterygoid, pro¬
duces contralateral deviation. It pulls the ramus of the
mandible medially, shifting the whole mandible toward the
contralateral side (Fig. 24.9) [17,18]. EMG and biomechani¬
cal evidence suggests that the medial pterygoid also is capa¬
ble of slight protrusion, which is consistent with the sagittal
orientation of its fibers [8,17].

Clinical Relevance

BRUXING: Grinding one's teeth is known as bruxing and is
produced by overactivity of the mandibular elevators. It
often occurs while an individual sleeps (nocturnal bruxing).
Tenderness in the mandibular elevators and even chronic
headaches are associated with greater intensity , frequency ,
and duration of activity of these muscles than in those of
nonbruxing healthy control subjects [7,13]. Muscle tender¬
ness may be the direct result of overuse of these muscles [2].
Compression of the retrodiscal tissue of the joint , resulting
from retrusion produced by overactivity of the posterior

(continued)

458

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 24.9: Lateral deviation of the mandible with contraction
of the medial pterygoid muscle. The alignment of the medial
pterygoid produces a medial pull on the mandible during unilat¬
eral contraction, producing contralateral deviation of the
mandible.

(Continued)

portion of the temporalis muscle ’ may also contribute to the
patient's complaints. The retrodiscal tissue is highly vascu¬
lar ; and compression may produce inflammation and even
ischemic pain.

Treatment of the symptoms associated with bruxing
includes relaxation exercises, stress management strategies ,
and oral splints that increase the space between the teeth i,
preventing contact between the upper and lower teeth [32].
The splints may also position the TMJs to reduce the pres¬
sure on the retrodiscal tissue.

Lateral Pterygoid

The lateral pterygoid muscle is oriented horizontally and has
distinct superior and inferior parts. (Muscle Attachment Box
24.4) From the cranium, the fibers of the two parts converge
and pass more obliquely laterally than the medial pterygoid.
As a result, balanced bilateral activity of the two lateral ptery¬
goids is necessary if the mandibular and maxillary teeth are to
be aligned normally.

MUSCLE ATTACHMENT BOX 24.4

ATTACHMENTS AND INNERVATION
OF THE LATERAL PTERYGOID MUSCLE

Cranial attachment: The superior head attaches to the
infratemporal surface of the greater wing of the
sphenoid bone. The inferior head attaches to
the lateral aspect of the lateral pterygoid plate.

Mandibular attachments: The superior head attaches
to the articular capsule and intraarticular disc of the
TMJ. The inferior head attaches to the pterygoid
fovea on the neck of the mandible.

Innervation: Mandibular division of the trigeminal
nerve (5th cranial nerve)

Palpation: The lateral pterygoid can be palpated
intraorally along the lateral aspect of the hamulus
during protrusion (see Fig. 23.5).

ACTIONS

The actions of the lateral pterygoid muscle are considered
unilaterally and bilaterally.

MUSCLE ACTION: LATERAL PTERYGOID UNILATERAL

ACTIVITY

Action

Evidence

Mandibular protrusion

Supporting

Contralateral deviation of the mandible

Supporting

MUSCLE ACTION: PTERYGOID BILATERAL
ACTIVITY

Action

Evidence

Mandibular protrusion

Supporting

EMG and biomechanical studies provide evidence for
the lateral pterygoids role in protrusion and contralateral
deviation [17,22] (Fig. 24.2). It is the primary force in pro¬
trusion and deviation to the contralateral side [22]. This
muscle is particularly important in maintaining continuity
between the intraarticular disc and the mandible as the
mandible depresses during opening of the mouth. The
superior head attaches directly on the intraarticular disc
and produces the anterior translation of the disc that occurs
in the early stages of mandibular depression. Both heads of
the lateral pterygoid muscle also pull the mandibular
condyle anteriorly during opening [14,42]. The lateral
pterygoid muscle and the posterior fibers of the temporalis,
together, control the anterior and posterior translation of
the mandible.

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

459

Clinical Relevance

HYPERACTIVITY OF THE LATERAL PTERYGOID
MUSCLE: Excessive activity , or spasm , of the superior head
of the iaterai pterygoid has been associated with anterior
subluxation of the intraarticular disc with respect to the
head of the mandible. Conversely , overactivity of the inferior
head has been associated with anterior subluxation of the
mandible with respect to the disc or even subluxation of the
head of the mandible on the articular eminence of the tem¬
poral bone [42]. Asynchronous movement of the intraarticu¬
lar disc and head of the mandible can produce audible
clicks with opening or closing of the mouth as the disc and
mandible suddenly and forcefully separate or reunite [30].

Protrusion of the mandible combines anterior and inferior
translation of the mandible as the head of the mandible
glides along the surface of the glenoid fossa of the temporal
bone, which slopes anteriorly and inferiorly (Fig. 24.10).
Because the lateral pterygoid is the most oblique of the mas¬
ticatory muscles, it is thought to be responsible for the devi¬
ation of the mandible that results from a mandibular nerve
injury. With such an injury the mandible is deviated to the
side opposite the nerve injury. This deviation may be appar¬
ent at rest but is accentuated when the mouth is opened
against resistance.

Figure 24.10: Movement of the mandible during protrusion.
As the mandible glides anteriorly in protrusion it also glides
inferiorly as it follows the surface of the glenoid fossa of the
temporal bone.

Accessory Muscles

The accessory muscles of the TMJ include the suprahyoid
muscles and the tongue muscles. Although their individual
attachments and functions are discussed in Chapter 22, it is
useful to review their effects on the TMJ. The suprahyoid
muscles form the floor of the mouth and play an important
role in mouth opening and during chewing. They function as
mandibular depressors when the hyoid bone is fixed by the
infrahyoid muscles (Fig. 24.11 ) [17-19]. Thus while the
suprahyoid muscles are considered muscles of the TMJ, their
effect on the joint requires that they contract in unison with
the infrahyoid muscles.

The muscles of the floor of the mouth and the tongue mus¬
cles also participate in lateral movement of the mandible.
EMG and biomechanical data reveal that the mylohyoid mus¬
cle, contracting unilaterally, produces significant lateral devi¬
ation of the mandible to the contralateral side [17,18] (Fig.
24.12). Bilateral contraction of the tongue muscles helps
establish a symmetrical alignment of both TMJs.

Clinical Relevance

TONGUE POSITION DURING ACTIVE EXERCISE OF
THE TMJs: Lateral deviation of the TM1 frequently occurs
during mouth opening in the presence of asymmetrical
muscle control. A typical goal of intervention in patients
with TM1 dysfunction is to reestablish symmetrical mouth
opening. Careful control of tongue position during opening
can facilitate symmetrical motion. One useful strategy is to
instruct the patient to maintain the tip of the tongue on the
highest point of the hard palate whenever performing open¬
ing and dosing exercises of the mouth (Fig. 24.13).
Maintenance of this position requires the contraction of the
intrinsic and extrinsic muscles of the tonguewhich
stabilizes the mandible in the transverse plane and
limits the unwanted lateral deviation.

MASTICATION

Chewing is a complex rhythm of mandibular movement,
powered by coordinated activity of the muscles of mastica¬
tion, facial expression, and tongue. The following describes
the sequence of mandibular movements that constitute chew¬
ing, or mastication, and then discusses the role of the muscles
that participate in the function.

Mandibular Motion during Chewing

A single chewing stroke consists of one loop of mandibular
depression, lateral deviation, and elevation [5,28]. A frontal
plane view reveals that the mandible typically follows a path
along the midline of the body during depression (Fig. 24.14).

460

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 24.11: Depression of the
mandible by the suprahyoid mus¬
cles. Contraction of the suprahyoid
muscles with simultaneous con¬
traction of the infrahyoid muscles
fixes the hyoid bone and allows
the suprahyoid muscles to depress
the mandible.

contraction of the muscles of the tongue and floor of the mouth
assist contralateral deviation of the mandible.

Figure 24.13: Location of the tongue on the hard palate during
mouth opening or closing. By locating the tip of the tongue on
the hard palate, the individual exhibits symmetrical activity of
the tongue muscles and helps maintain symmetrical alignment
of the mandible during opening and closing of the mouth.

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

461

Figure 24.14: Frontal plane view of the path taken by the
mandible during a single chewing stroke. Observation of the
mandible during chewing reveals that the mandible moves in the
sagittal plane as it depresses in the opening phase. As closing
begins, the mandible elevates and deviates laterally during the
crushing phase. When the two rows of teeth contact each other,
mandibular elevation ceases, and the mandible returns to the
midline during the grinding phase.

As elevation begins, the path of the mandible devi¬
ates laterally and returns to midline as mandibular
depression begins again.

In the rest position, the upper row of teeth typically does
not contact the lower teeth. When the mandible is elevated
in the sagittal plane from this position, the teeth of the lower
row make only slight contact with the upper row because the
mandibular teeth are arranged in a narrower arc than the
maxillary teeth (Fig. 24 . 15 ). To maximize contact between
upper and lower teeth, a necessity to grind food, the
mandible deviates laterally as it elevates to the maxillary
teeth. This explains the small loop that the mandible makes
in the frontal plane as it depresses then laterally deviates and
elevates to crush the oral contents.

Within the chewing stroke, there are two distinct phases of
food preparation by the teeth. The crushing phase occurs as
the food is compressed between the maxillary teeth and the
teeth on the elevating mandible. This phase ends with maxi¬
mum mandibular elevation. When elevation is complete, con¬
tact between the rows of teeth persists as the teeth slide on
each other to achieve the intercuspal position in which con¬
tact between the molars on one side of the mouth is maximal.
The gliding between the rows of teeth constitutes the grind¬
ing phase of mastication. This phase is characterized by
transverse plane motion of the mandible, with little or no
additional elevation.

Figure 24.15: Arcs formed by the maxillary and mandibular teeth.
The bottom row of teeth (mandibular teeth) forms a smaller arc
than the row of teeth on the maxilla. Maximum contact between
mandibular and maxillary molars requires lateral deviation of the
mandible to one side.

Muscle Activity during Mastication

The act of chewing typically occurs on one side of the mouth
at a time. The side on which the actual chewing occurs is
known as the working side, while the opposite side is known
as the balancing side. EMG studies consistently demon¬
strate considerable muscle activity on both the working and
balancing sides [4,23,31,33].

Several discrete roles of muscle activity during mastication
can be described. These functions are to

• Move the mandible in the masticatory path

• Stabilize the balancing side of the mandible

• Maintain appropriate alignment between the disc and the
mandibular condyle

• Control the location of the food to optimize mastication

The muscles’ participation in these functions is described
below.

MUSCLES THAT MOVE THE MANDIBLE DURING
MASTICATION

Chapter 23 describes in more detail the movement of the
mandible during elevation and depression. Depression of the
mandible, or mouth opening, includes rotation about a medial
lateral axis and protrusion of the mandible in the transverse
plane. As the mandible depresses during the chewing stroke,
the suprahyoid muscles contract [18,20,21,34,36,41]. At the

462

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 24.16: Motion on the balancing side of the
mandible during bite. As the mandible deviates and
rotates to the working (chewing) side, the TMJ on the
balancing side undergoes distraction.

same time, the infrahyoid muscles contract, fixing the hyoid
bone. As a result, the suprahyoid muscles contribute to
mouth opening. Also during the opening phase of mastica¬
tion, the lateral pterygoid muscle contracts, particularly
the inferior head, producing the anterior translation of the
mandible that accompanies mandibular depression
[6,14,18,21,36].

Mandibular depression and protrusion are followed by
lateral deviation, elevation, and retrusion of the mandible for
crushing and grinding. These motions occur with the
mandibular elevators, the masseter, medial pterygoid, and
temporalis muscles as well as the lateral pterygoid [10,18].
Lateral deviation occurs with contraction of the ipsilateral
masseter and temporalis and the contralateral medial and
lateral pterygoid [5,12]. The temporalis also produces retru-
sion [4,23,33]. The crushing phase consists of active
mandibular elevation and, therefore, the muscle contractions
within this phase are primarily concentric. Grinding occurs
with little or no additional elevation, so contraction of the
mandibular elevators during this phase is primarily isometric.
The moment arms for the mandibular elevators increase as
the mouth moves from the open toward the closed position
[10,16]. The moment arms are maximum at approximately
the point at which the mandible is positioned to grind the
food, thus optimizing the moments the muscles can generate
to chew the food.

The following summarizes the motions of the TMJs during
mastication and the muscles primarily responsible for these
motions:

• Depression: digastric, mylohyoid, and geniohyoid muscles

• Protrusion: lateral pterygoid muscle

• Elevation: masseter, temporalis, and medial pterygoid

muscles

• Lateral deviation: masseter and temporalis on the ipsilat¬
eral side and medial and lateral pterygoid on the con¬
tralateral side

• Retrusion: temporalis muscle

STABILIZATION OF THE BALANCING SIDE
OF THE MANDIBLE

Forceful contraction of the mandibular elevators on the work¬
ing side produces lateral deviation toward the working side
and tends to produce a rotation of the mandible toward the
chewing side about an anterior posterior axis [25] (Fig. 24 . 16 ).
This rotation tends to distract the TMJ on the balancing side
and to compress the TMJ on the working side. The mandibu¬
lar elevators on the balancing side of the mandible contract
with the contralateral elevators to stabilize the mandible dur¬
ing the crushing and grinding phases [17]. The activity of the
muscles on the balancing side adds to the bite force and also
stabilizes the mandible to maintain the bite location on the
teeth [35]. At the same time the bolus on the teeth of the
chewing side tends to distract the TMJ on the chewing (work¬
ing) side and narrows the joint space on the opposite (balanc¬
ing) side [9]. This tilt of the mandible helps to explain the large
compressive forces that occur on the balancing side. The harder
the food is to chew, the more compression occurs on the bal¬
ancing side [10].

MAINTAIN APPROPRIATE ALIGNMENT BETWEEN
THE DISC AND MANDIBULAR CONDYLE

In chewing, the mandible is cyclically opening and closing,
requiring repeated anterior and posterior gliding of the
mandibular condyle. The intraarticular disc also translates to
stay with the head of the mandible and maximize congruency

Chapter 24 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE TMJ

463

Figure 24.17: Anterior rotation of the intraarticular disc during
bite. During the crushing and grinding phase of the chewing
stroke, contraction of the superior head of the lateral pterygoid
muscle rotates the intraarticular disc anteriorly, so that it pro¬
vides cushioning to the anterior surface of the head of the
mandible.

between mandible and temporal bone. The lateral pterygoid
plays a critical role in stabilizing the disc and maintaining its
alignment on the mandible as well as in protruding the
mandible. The inferior head of the lateral pterygoid muscle is
active during jaw opening, apparently assisting the anterior
translation of the mandible. In contrast, EMG studies reveal
that the superior head of the lateral pterygoid muscle is active
in the mandibular elevation phase of bite [5,14]. This activity
appears to stabilize the disc and mandible against the retru-
sive pull of the mandibular elevators, particularly the tempo¬
ralis. It also rotates the disc anteriorly to provide cushioning
between the mandible and articular eminence of the tempo¬
ral bone (Fig. 24.17).

CONTROL FOOD LOCATION

Regardless of the integrity of the primary muscles of mastica¬
tion, effective chewing requires that the food be located
appropriately between the teeth during the crushing and
grinding phases. In addition, the food, moistened by saliva,
requires kneading to form a bolus that can be swallowed safely
The buccinator and the intrinsic and extrinsic muscles of the
tongue perform this function in mastication. The buccinator
is the only muscle that can regulate the lateral part of the
cheek between the mandible and maxilla (Chapter 20). This
muscle, along with the tongue, is responsible for maintaining
the position of a bolus of food between the maxillary and
mandibular teeth. The buccinator is essential in preventing
food from becoming trapped in the buccal space, between
the cheek and the teeth. It must be remembered that the
buccinator is paralyzed with an injury of the facial nerve,

making the mucosa of the cheek vulnerable to laceration
between the teeth and producing difficulty in chewing.

The tongue muscles also manipulate the food, keeping
it between the teeth even while the working side of the
mandible is alternated from side to side. As noted in
Chapter 22, once the food is prepared thoroughly, the
tongue forms a chute and propels the food as the swallow is
initiated. Thus mastication is a complex, cyclical movement
requiring precise coordination of several muscle groups
using concentric, eccentric, and isometric contractions.
Mastication is the beginning of the normal digestive
process, and pain or incoordination that limits an individ¬
uals ability to chew can have profound effects on the
persons diet and nutritional status. Restoration of normal
muscle balance in patients with temporomandibular dys¬
function can provide substantial relief of pain.

Clinical Relevance

MUSCLE DYSFUNCTION IN TMJ DISORDERS: Just like
the muscles of the upper or lower extremity , muscles of
mastication and the TMJ's accessory muscles , including the
buccinator and tongue muscles, can undergo disuse atro¬
phy and loss of coordination resulting from inactivity.
Individuals with severe pain in a TMJ often resort to soft
diets because chewing food of typical consistency is too
painful. Consequently the individual may lose strength not
only in the mandibular elevators; but in the tongue muscles
as well. Atrophy and incoordination of the tongue are com¬
mon impairments seen in individuals with TMJ dysfunction.
Happily , the muscles of the tongue appear to be as
amenable to exercise and rehabilitation as the muscles of
the knee or elbow. Patients with TMJ pain are likely to ben¬
efit from tongue exercises as well as from direct interven¬
tion at the TMJ.

SUMMARY

This chapter presents the structure and actions of the four
primary muscles of mastication. The three elevators of the
mandible—temporalis, masseter, and medial pterygoid
muscles—work together to raise and deviate the mandible to
produce forceful grinding of food. The lateral pterygoid pro¬
trudes the mandible and participates in opening the mouth. It
also helps maintain continuity between the intraarticular disc
and the mandible. The accessory muscles of mastication
include the suprahyoid muscles as well as muscles of the
tongue and face. They play an important part in chewing by
helping to manipulate the food during chewing and to mold
the food into a manageable bolus. These muscles are
reviewed briefly since they are covered in greater detail in
preceding chapters.

464

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Chewing requires coordinated activity of several muscles
to produce the rhythmic opening and closing, protrusion and
retrusion, and side-to-side translation that produces the
chewing stroke. The following chapter discusses the loads
that the TMJ sustains during normal function as well as how
those loads may contribute to a patients complaints.

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36. Uchida S, Inoue H, Maeda T: Electromyographic study of the
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465

39. Vitti M, Basmajian JV: Integrated actions of masticatory mus¬
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Anat Rec 1977; 187: 173-189.

40. Williams P, Bannister L, Berry M, et al.: Grays Anatomy, The
Anatomical Basis of Medicine and Surgery, Br. ed. London:
Churchill Livingstone, 1995.

41. Yoshida K: Masticatory muscle responses associated with
unloading of biting force during food crushing. I Oral Rehabil
1998; 24: 830-837.

42. Zijun L, Huiyun W, Weiya P: A comparative electromyographic
study of the lateral pterygoid muscle and arthrography in

patients with temporomandibular joint disturbance syndrome
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43. Zwijnenburg AJ, Kroon GW, Verbeeten B Jr, Naeije M: Jaw
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1798-1803.

44. Zwijnenburg AJ, Lobbezoo F, Kroon GW, Naeije M:
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CHAPTER

Analysis of the Forces
on the TMJ during Activity

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE FORCES IN THE TMJ COMPLEX .466

RESULTS FROM SOPHISTICATED MODELING OF THE TMJ.469

Bite Force.469

Joint Reaction Forces .469

SUMMARY .471

T he temporomandibular joints (TMJs) are the articulation sites for the mandible, a rather small bone of the
face with no other additional appendage. The joints serve no function in normal weight bearing and would
seem to bear small loads. However, the joints are equipped with intraarticular discs, usually a sign that the
joint sustains large stresses. In addition, the muscles that move the TMJs are large and powerful, generating large
chewing forces to grind food into a manageable bolus. It is useful for the clinician to investigate the loads sustained
by the joint and to consider their contributions to the relatively common complaint of TMJ pain.

The purpose of this chapter is to examine the loads sustained by the TMJs and to review simple analytical tools useful
in calculating the loads on the TMJs. Specifically, the objectives of this chapter are to

■ Demonstrate a two-dimensional analysis of the forces on the TMJ
■ Examine the loads on the structures of the TMJ

■ Consider the role that loading may have on the etiology of TMJ dysfunction

TWO-DIMENSIONAL ANALYSIS
OF THE FORCES IN THE TMJ COMPLEX

Although the TMJ exhibits motion through three planes,
most of the motion of the mandible occurs in the sagittal
plane. Thus a two-dimensional model of the joint is an accept¬
able first approximation of the joints performance. The free-
body diagram of a simplified model is presented in
Examining the Forces Box 25.1 along with an analysis of the
forces in the mandibular elevators and the joint reaction force
during forceful bite. This example uses a peak bite force on

the molars of 500 N (112 lb) [20], although peak bite forces
up to 1,000 N (225 lb) are reported in adults [24,27].
Calculations in this example reveal a load of 1013 N (228 lb)
in the muscle that elevates the mandible and a joint reaction
force on the head of the mandible of 877 N (197 lb) at an
angle of 60° from the horizontal.

The example presented in Examining the Forces Box 25.1
uses dimensions reported in the literature but also makes use
of an important simplifying assumption [9,21]. The mandibu¬
lar elevators are represented by a single vertically aligned
muscle, the temporalis, despite abundant evidence demon¬
strating co-contraction of the masseter, medial pterygoid,

466

Chapter 25 I ANALYSIS OF THE FORCES ON THE TMJ DURING ACTIVITY

467

The following data are taken from the literature
[9,20,21 ].

Moment arm of the temporalis (T): 0.037 m
Angle of application of the temporalis:

90° Bite force (F): 500 N

Distance along the x axis from the point of app¬
lication of the bite force to the joint: 0.063 m
Angle of application of the bite force: 30° from
the occlusal plane, which lies on the horizontal

J x + B x = 0 where B x = the bite force X

(cos 30°) in the -x direction

J x - 433 N = 0
J x = 433 N

Sf y

J Y + B y + T = 0 B Y = the bite force x (sin 30°)
in the -y direction
J Y - 250 N + 1013 N = 0
J Y = -763 N

Solve for the temporalis force (T):

Using the Pythagorean theorem:

SM = 0

(T X 0.037 m) - (500 N X sin 30° X 0.063 m)
- (500 N X cos 30° X 0.05 m) = 0
(T X 0.037 m) = 37.5 Nm

T = 1013 N

J 2 = J 2 + J 2

J » 877 N

Using trigonometry, the direction of J can be deter¬
mined:

Calculate the joint reaction forces (J) on the head
of the mandible.

sin 0 = J Y /J

0 » 60° from the horizontal

both parts of the temporalis, and even the superior head of
the lateral pterygoid muscle during bite [4,13,19,25]. This
simplification is necessary to solve for the muscle force directly,
since inclusion of all of these muscles produces a state of static
indeterminacy that allows an infinite number of solutions and
requires more sophisticated analysis for a final solution. (See
Chapter 1 for more details on static indeterminacy.)

The assumption that only one vertical muscle provides all
of the force of mandibular elevation produces an artificially
small muscle force and, consequently, underestimates the

joint reaction force. The mandibular elevators and superior
head of the lateral pterygoid muscle pull either anteriorly or
posteriorly, producing a force couple that rotates the
mandible in elevation, while the anterior and posterior pulls
counteract each other, producing only slight translation.
However, the co-contractions produce large compressive forces
on the joint itself. The model also assumes that the mandibu¬
lar elevator pulls vertically with an optimal 90° angle of appli¬
cation, although analyses reveal that the actual direction
of pull of the muscles varies widely, from approximately

468

Part III I KINESIOLOGY OF THE HEAD AND SPINE

30° to 150° [9,24,26]. Angles of application less or greater
than 90° require larger muscle forces, since the moment arm
of the muscle is smaller when the angle of application is
greater or less than 90°. (See Chapter 4 for details of muscle
mechanics.) Thus this simplification also produces unrealisti¬
cally small muscle and joint reaction forces [26]. Despite
these simplifications and consequent underestimations, the
model reveals substantial muscle loads during forceful bite,
which result in large joint reaction forces.

The data presented above are based on a bite force located
on the second molar. Anyone who has bitten a raw carrot
knows that the jaw also can generate large forces during an
incisor bite. Examining the Forces Box 25.2 examines some
of the mechanical alterations produced by an incisal bite.

The free-body diagram in Examining the Forces Box 25.2
demonstrates that an important effect of an incisal bite is an
increase in the moment arm of the bite force. The model
also uses a smaller peak bite force on the incisors than on
the molars. Measurements of peak bite forces between the
incisors reveal loads ranging from 150 to almost 400 N
(34 to 90 lb). Incisal bite requires protrusion of the
mandible, so a lower incisal bite force than a molar bite
force may be the result of inhibition of the muscles that
retrude the mandible, particularly the horizontal fibers of
the temporalis muscle [27,30]. In addition, protrusion alters
the angles of application of the elevators, decreasing their
mechanical advantage, although these alterations are
ignored in the current example.

Chapter 25 I ANALYSIS OF THE FORCES ON THE TMJ DURING ACTIVITY

469

The analysis in Examining the Forces Box 25.2
demonstrates that the elevators generate large forces
during incisal bite, even though the bite force is small¬
er than that in the molar bite. The large muscle force is need¬
ed because the bite force acts farther from the point of rota¬
tion and produces a larger moment. Despite the smaller bite
force and large muscle force, the joint reaction force during
incisal bite is large although smaller than the joint reaction
force during bite on the molars.

The analyses in Examining the Forces Boxes 25.1 and 25.2,
albeit oversimplified, reveal substantial loads in the mastica¬
tory muscles and on the TMJ. More-accurate calculations
require more-sophisticated analyses, and such approaches are
less commonly applied to the TMJ than to joints of the upper
and lower limbs.

RESULTS FROM SOPHISTICATED
MODELING OF THE TMJ

Although the TMJ appears to be a relatively simple mechan¬
ical system, it has eluded definitive biomechanical characte¬
rization. Several factors help explain the lack of consensus
on the forces sustained by the joints components. Although
most of the motion at both joints occurs in the sagittal plane,
each joint does exhibit three-dimensional motion. The four
primary muscles of each TMJ are large with complex archi¬
tectures, so the direction of pull and physiological cross-
sectional areas are difficult to determine. Consequently,
their effects on the joint also are disputed.

The compound nature of the two TMJs also increases the
difficulty of analysis. The muscles on one side of the mandible
affect both ipsilateral and contralateral joints, although the rel¬
ative effect on each joint is impossible to measure. As noted in
Chapters 23 and 24, in chewing, the side where the bolus is
located is known as the working side and the opposite side
is referred to as the balancing side. During mastication
muscles at both TMJs contract simultaneously to move and
stabilize the joints as they alternate between working and
balancing. Consequently, both joints are loaded substantially
regardless of which side is actually grinding the food. Finally,
the location of the bite force has a significant influence on the
muscle forces, as indicated in Examining the Forces Box 25.2.
As a result of these challenges to a biomechanical analysis of
the TMJ, the literature offers widely varying estimates of the
loads applied to the TMJs. The results presented here offer
clinicians a perspective on the forces sustained by the joint
complex and a framework by which to consider the signs and
symptoms reported by patients. Additional research is
required to obtain more-precise estimates of the forces to
which the structures of the TMJ are subjected.

Bite Force

Peak bite force and forces generated during functional bite
are both reported [5,12,21-24,28]. The bite force is greatly
influenced by the location of the bite. It is generally agreed
that bite forces are greatest when the bite occurs close to the

first molar and are least when it occurs at the incisors
[23,24,28]. The decrease in bite force during an incisal bite
appears to result from a decrease in the muscles’ mechanical
advantage and probable inhibition of the temporalis muscle
needed to maintain the protruded position [23,28].

The magnitude of reported peak bite forces varies widely
because the location and positioning of the measurement
device differ substantially among the studies. The position of
the mouth during measurement also affects the results and
contributes to the diversity in reported bite forces [23].
Beported peak bite forces on the molars range from approxi¬
mately 500 N to almost 1,000 N (112-225 lb) [1,20,23,24].
Using a load transducer implanted in the crown of a maxillary
molar, Kawaguchi et al. report a load of approximately 173 N
(39 lb) on a single molar during a maximum contraction of the
mandibular elevators [14].

Unlike maximum strengths in the appendicular skeleton,
maximum bite force appears less affected by gender and more
by physical maturity and by the shape of the cranium and the
angles of applications of the muscles [5,21]. Maximum bite
strength is less than 100 N (22.5 lb) in children aged 6 to
8 years and appears to increase steadily during maturation.

Despite the variety in reported peak bite forces, there is
agreement that peak bite forces are quite large, well over
100 lb in adults. Such loads would seem to have the potential
to injure the teeth. However, the arrangement and structure
of the teeth appears to provide a protective mechanism [12].
The area of tooth contact, occlusal contact area, increases
with increasing bite force. As a result, as the bite force
increases, so does the area over which the bite force is
applied. As the bite force increases, the stress (force/area)
decreases, thus decreasing the risk of injury to any tooth.

Although most functional chewing requires submaximal
bite forces, the magnitude of a functional bite force is still sig¬
nificant. Measures of the bite forces during mastication of
various types of food range from approximately 54 to 88 N
(12-20 lb) [22]. However, investigators disagree as to whether
there is an actual increase in muscle force with harder foods
or whether there is a change in chewing rhythm [4,22].

Clinical Relevance

CHANGES IN DIET IN PATIENTS WITH TMJ
DYSFUNCTION: Because mastication requires large muscle
forces , many individuals with chronic TMJ dysfunction find
that only a diet of soft foods can be eaten without an
increase in symptoms. Instructing an individual to avoid
hard , tough food may help control the person's symptoms
while normal joint function is being restored.

Joint Reaction Forces

Although there are several studies that consider the joint reac¬
tion forces of the TMJ, most emphasize the factors that influ¬
ence the validity of the calculations, including the location,
magnitude, and direction of the bite force as well as the

470

Part III I KINESIOLOGY OF THE HEAD AND SPINE

assumptions made regarding the moment arm and cross-
sectional diameters of the active muscles [2,23,27,29]. Actual
calculations of peak joint reaction forces on a mandibular
condyle are available from only a few studies and range from
approximately 400 N to approximately 1,100 N (90-250 lb)
[16,18,24]. Although it is generally accepted that the balanc¬
ing side of the mandible sustains significant loads during
bite, only one known study compares the loads on the bal¬
ancing and working sides, suggesting that the balancing side
of the temporomandibular complex sustains approximately
twice the load sustained by the working or chewing side [9].
Additional studies report that the joint space is narrower on
the balancing side during mastication, supporting the
view that the balancing side sustains more compres-
vSV sion during chewing than the working side [10,11].

Clinical Relevance

1 CAN'T EVEN CHEW ON THE OPPOSITE SIDE!: Indi¬
viduals with acute TMJ pain often assume that chewing on the
opposite side of the mouth will help reduce their symptoms. So
they continue to bite large tough rolls or chew tough meat. A
clinician can help convince a patient to avoid hard\ tough
foods through the acute phase of a TMJ disorder by helping
the patient to understand that the opposite', or balancing; side
sustains even larger loads than the chewing; or working; side.

Some studies examine the stress (force/area) applied to
the mandible and intraarticular disc and report that during
bite the anterior aspect of the mandibular condyle and neck
sustain compressive loads while the posterior aspect and the
articular surface of the temporal bone sustain both compres¬
sive and tensile loads [7,8] (Fig. 25.1). The intraarticular disc

Figure 25.1: Stresses in the TMJ. During bite, the head of the
mandible sustains compressive loads (F c ) while the articular sur¬
face on the temporal bone sustains both compressive and tensile
loads (F T ).

reportedly sustains large stresses in the lateral aspect of the
intermediate zone [3]. The chin-cup apparatus used in
orthodontic appliances also apparently applies significant
stresses to the mandible and joint [8]. Although additional
research is needed to evaluate the loads within the TMJ, the
available studies consistently demonstrate that the articular
structures sustain substantial loads. The magnitude and
repetitive nature of these loads may help explain why the
TMJ is a frequent site of pain and degeneration.

Clinical Relevance

TRACTION OF THE CERVICAL SPINE: Traction of the
cervical spine is a useful diagnostic procedure as well as a
common intervention for pain in the head, neck , or shoulder
[17]. Many cervical traction procedures apply a tensile force
to the cervical spine through the occiput and mandible
(Fig. 25.2). The clinician must exercise considerable care to
avoid applying too much force on the mandible, which
could produce excessive compression of the TMJ.

Figure 25.2: Loads on the TMJ during traction of the cervical
spine. To apply manual traction to the cervical spine, the thera¬
pist must be careful to minimize the load on the mandible,
applying most of the force through the occiput, to avoid apply¬
ing excessive compressive loads to the TMJs.

Chapter 25 I ANALYSIS OF THE FORCES ON THE TMJ DURING ACTIVITY

471

SUMMARY

This chapter provides an overview of the loads sustained
by the TMJ during forceful bite and during chewing. While
there is no consensus regarding the magnitude and direction
of the loads on the TMJ, the joint sustains loads of over 100 lb.
Such high loads may help explain why pain in the TMJ is a
common complaint.

A simple two-dimensional model was used to examine the
mechanics of loading and the effect of bite position on the
muscles of mastication and the joint reaction forces. Biting
with the incisors differs from chewing with the molars by
altering the moment arm of the bite force as well as changing
the participation of the muscles of mastication. Instructing an
individual to avoid hard foods may help to reduce the muscle
and joint reaction forces exerted at the TMJ. Even interven¬
tions to treat cervical spine pain may inadvertently produce
large forces on the TMJ, and clinicians are cautioned to con¬
sider how their treatments may have unintended conse¬
quences on the TMJ and to identify ways to protect the joint
from excessive loads.

References

1. Bakke M, Michler L, Han K, Moller E: Clinical significance of
isometric bite force versus electrical activity in temporal and
masseter muscles. Scand J Dent Res 1989; 97: 539-551.

2. Barbenel J: The biomechanics of the temporomandibular joint:
a theoretical study. J Biomech 1972; 5: 251-256.

3. Beek M: Three-dimensional finite element analysis of the
human temporomandibular joint disc. J Biomech 2000; 33:
307-316.

4. Bishop B, Plesh O, McCall WD: Effects of chewing frequency
and bolus hardness on human incisor trajectory and masseter
muscle activity. Arch Oral Biol. 1990; 35: 311-318.

5. Braun S: A study of maximum bite force during growth and
development. Angle Orthod 1996; 66: 261-264.

6. Chadwick EKJ, Nicol AC: Elbow and wrist joint contact forces
during occupational pick and place activities. J Biomech 2000;
33: 591-600.

7. Chen J, Akyuz U, Xu L, Pidaparti RM: Stress analysis of the
human temporomandibular joint. Med Eng Phys 1998; 20:
565-572.

8. Deguchi T: Force distribution of the temporomandibular joint
and temporal bone surface subjected to the head-chin-up force.
Am J Orthod Dentofac Orthop 1998; 114: 277-282.

9. Faulkner MG, Hatcher DC, Hay A: A three-dimensional inves¬
tigation of temporomandibular joint loading. J Biomech 1987;
20: 997-1002.

10. Fushima K, Gallo LM, Krebs M, Palla S: Analysis of the TMJ
intraarticular space variation: a non-invasive insight during mas¬
tication. Med Eng Phys 2003; 25: 181-190.

11. Gallo LM: Modeling of temporomandibular joint function using
MRI and jaw-tracking technologies—mechanics. Cells Tissues
Organs 2005; 180: 54-68.

12. Hidaka O, Iwasaki M, Saito M, Morimoto T: Influence of
clenching intensity on bite force balance, occlusal contact area,
and average bite pressure. J Dent Res 1999; 78: 1336-1344.

13. Hiraba K, Hibino K, Hiranuma K, Negoro T: EMG activities of
two heads of the human lateral pterygoid muscle in relation to
mandibular condyle movement and biting force. J Neurophysiol
2000; 83: 2120-2137.

14. Kawaguchi T, Kawata T, Kuriyagawa T, Sasaki K: In vivo
3-dimensional measurement of the force exerted on a tooth
during clenching. J Biomech 2007; 40: 244-251.

15. Koh TJ, Herzog W: Increasing the moment arm of the tibialis
anterior induces structural and functional adaptation: implica¬
tions for tendon transfer. J Biomech 1998; 31: 593-599.

16. Koolstra JH, van Eijden TMGJ, Weijs WA, Naeije M: A three-
dimensional mathematical model of the human masticatory sys¬
tem predicting maximum possible bite forces. J Biomech 1988;
21: 563-576.

17. Magee DA: Orthopedic Physical Assessment. Philadelphia: WB
Saunders, 1998.

18. May B, Saha S, Saltzman M: A three-dimensional mathematical
model of temporomandibular joint loading. Clin Biomech 2001;
16; 489-495.

19. McCarroll RS, Naeije M, Hansson TL: Balance on masticatory
muscle activity during natural chewing and submaximal clench¬
ing. J Oral Rehabil 1989; 16: 441^46.

20. Meyer C, Kahn JL, Boutemy P, Wilk A: Determination of the
external forces applied to the mandible during various static
chewing tasks. J Craniomaxillofac Surg 1998; 26: 331-341.

21. Moriya Y, Tuchida K, Sawada T, et al: The influence of craniofa¬
cial form on bite force and EMG activity of masticatory muscles.
VIII-1. Bite force of complete denture wearers. J Oral Sci 1999;
41: 19-27.

22. Neill DJ, Kydd WL, Naim RI, Wilson J: Functional loading of the
dentition during mastication. J Prosthet Dent 1989; 62: 218-228.

23. Osborn J: Features of human jaw design which maximize the
bite force. J Biomech 1996; 29: 589-595.

24. Pruim GJ, de Jongh HJ, ten Bosch JJ: Forces acting on the
mandible during bilateral static bite at different bite force levels.
J Biomechan 1980; 13: 755-763.

25. Spencer MA: Force production in the primate masticatory
system: electromyographic tests of biomechanical hypotheses.
J Hum Evol 1998; 34: 25-54.

26. Throckmorton G: Quantitative calculations of temporoman¬
dibular joint reaction forces-II. The importance of the direction
of the jaw muscle forces. J Biomech 1985; 18: 453-461.

27. Throckmorton G: Sensitivity of temporomandibular joint force
calculations to errors in muscle force measurements. J Biomech
1989; 22: 455-468.

28. Throckmorton G, Groshan GJ, Boyd SB: Muscle activity pat¬
terns and control of temporomandibular joint loads. J Prosthet
Dent 1990; 63: 685-695.

29. Throckmorton GS, Throckmorton LS: Quantitative calculations
of temporomandibular joint reaction forces- I. The importance
of the magnitude of the jaw muscle forces. J Biomech 1985; 18:
445-452.

30. Zwijnenburg AJ, Kroon GW, Verbeeten B Jr, Naeije M: Jaw
movement responses to electrical stimulation of different parts
of the human temporalis muscle. J Dent Res 1996; 75:
1798-1803.

UNIT 5

SPINE UNIT

T he spine unit consists of 12 chapters examining the structure and function of the four regions of the spine:

cervical, thoracic, lumbar, and pelvic. Each region is described in three chapters, the first discussing the struc¬
ture of the bones and joints and the factors that influence mobility and stability in each region. The second
chapter on each region presents the muscles that support and move the spine as well as those that perform special
functions such as the muscles of respiration in the thoracic region. The third chapter of each spinal region examines the
forces sustained by the region during daily activities or as a result of trauma commonly associated with the region. By
the end of this unit the reader will have an understanding of the features common to the whole spine as well as the
unique features that distinguish one region from another.

The purposes of this unit are to

■ Relate the structure of the bones and joints of each spinal region to the mobility and stability available in that
region

■ Discuss the role of the muscles of a spinal region in moving and supporting the region as well as their contributions
to special functions

■ Consider the effects of joint or muscle impairments on the function of the spinal region
■ Examine the loads normally applied to the spinal region and discuss the mechanical factors that contribute
to injuries in the spinal regions

472

CHAPTER

Structure and Function of the Bones
and Joints of the Cervical Spine

SUSAN R. MERCER , PH.DB.PHTY., F.N.Z.C.P.

CHAPTER CONTENTS

STRUCTURE OF THE BONES OF THE CERVICAL SPINE .473

Craniovertebral Vertebrae .474

Lower Column C3-C7 Vertebrae .475

JOINTS OF THE CERVICAL SPINE .477

Craniovertebral Joints.477

Joints of the Lower Cervical Spine.480

NORMAL RANGE OF MOTION.482

Total Motion of the Cervical Spine .482

Segmental Motion of the Craniovertebral Joints.483

Segmental Motion of the Lower Cervical Region .486

SUMMARY .489

T he cervical spine supports the head, provides attachment for muscles of the neck and upper extremity, and,

along with the rest of the spine, protects the spinal cord. It must meet the demand of providing a large range
of motion (ROM) to ensure optimal functioning of the special senses such as vision, smell, and hearing that
are housed in the head. Yet, it must also serve the contradictory demands of balancing and supporting the head, pro¬
tecting neural and vascular structures, and providing muscle and ligament attachment. The mechanism of meeting
these disparate needs is reflected in the morphology of the bones and joints of the cervical spine. The specific pur¬
poses of this chapter are to

■ Describe the structure of the individual vertebrae that compose the cervical vertebral column

■ Describe the articulations joining the bony elements

■ Describe the factors contributing to stability and instability in the cervical spine

■ Review the normal ROM of the head and neck

STRUCTURE OF THE BONES
OF THE CERVICAL SPINE

The morphology of the cervical spine is complex, yet, com¬
pared with the lumbar spine, has been sparsely studied.
Consequently, much of what appears as definitive descrip¬
tions of the structure and function of the bones and joints of
the cervical spine is extrapolation from other areas of the spine.

Throughout this chapter attention is drawn to these problems
in the literature. Finally, fundamental to developing an
understanding of the cervical spine is an appreciation that
each cervical vertebra contributes to the complexities of neck
function neither equally nor regularly.

The cervical vertebral column consists of seven vertebrae,
of which the first two are morphologically distinct, while the
third through seventh vertebrae follow a typical morphology
with minor variations. For ease of study, two distinct units

473

474

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Alar ligaments

Figure 26.1: Coronal section through the craniovertebral region
reveals articulations of the atlas with the occiput and axis. The
posterior portions of the bone are removed, leaving a posterior
view of the anterior ligaments.

within the cervical vertebral column may be described. These
are the craniovertebral, or suboccipital, region, comprising
the atlas and axis, and the lower cervical vertebral column,
comprising vertebrae C3 through C7. Together they con¬
tribute to the function of the neck.

Craniovertebral Vertebrae

ATLAS

The atlas sits like a washer between the skull and the lower
cervical spine (Fig. 26.1). It functions to cradle the occiput
and to transmit forces from the head to the cervical spine.
Secondarily, it is adapted for attachment of ligaments and
muscles. Its distinctive morphology of two large lateral
masses vertically aligned below the occipital condyles
reflects these functions. Slender arches join the lateral
masses anteriorly and posteriorly, transforming the atlas into
a ring and allowing the lateral masses to act in parallel [59]
(Fig. 26.2).

Anterior arch

Figure 26.2: Superior view of the lateral masses of the atlas. The
superior surface of the lateral masses contain kidney-shaped
articular facets for the occiput and form a ring by anterior and
posterior arches.

The superior aspect of each lateral mass exhibits a deep
socket that is concave anteroposteriorly and mediolaterally,
consistent with the curvature of the occipital condyles so
that the skull rests securely on the atlas. The size and shape
of the sockets vary greatly, but in general, the articular
surfaces of these superior facets are directed upward
and medially, with their outer margins projecting more
superiorly [93]. In long, deeply concave sockets the anterior
wall may face backward, and the posterior wall forward. In
most Cl vertebrae, each socket is completely or incom¬
pletely divided into two facets or into a dumbbell-shaped
facet having a nonarticular waist. The atlantal sockets
typically exhibit right-left asymmetry [37,86].

ATLANTO-OCCIPITAL RANGE OF MOTION: The large
variation in normal morphology of the atlantal sockets
means that the apparent differences in motion between right
and left atlanto-occipital joints or among individuals may
be due to normal differences in the joint structure and may
not indicate joint impairment. Clinicians must identify rela¬
tionships between ROM measures and other signs and
symptoms to suspect that differences in motion reflect true
impairments.

The occipital condyles transmit the weight of the head to
the axis (C2) via the large lateral masses of the atlas (Cl). This
is achieved by an articulation between the apparently flat,
broad inferior articular surfaces of the lateral masses, which
are directed inferiorly and medially to the wide shoulders of
the superior articular facets of the axis below (Fig. 26.1).

The robust transverse processes of the atlas are the pri¬
mary site of muscle attachment for this vertebra. The size of
each transverse process accommodates the loading associated
with suspension of the scapula through the attachment of the
levator scapulae muscle. Consequently, any movement of the
upper limb exerts compressive forces on the entire cervical
spine. The length of each transverse process increases the
moment arms of the muscles attached to it, but also allows the
vertebral artery to clear the large lateral masses of the axis
below (Fig. 26.2).

The anterior arch that joins the lateral masses of the atlas
is short and slender, since it is uninvolved in transmitting
large forces. A small, smooth facet lies centrally on the pos¬
terior aspect of the anterior arch for articulation with the
odontoid process of the axis. The position of the anterior arch
against the odontoid process ensures that there is a bony
block to posterior translation of the atlas. Enclosed by the
anterior and posterior arches and lateral masses, the central
foramen of the atlas has two distinct parts. The smaller ante¬
rior part partially encircles the odontoid process, or dens,
while the larger posterior portion is the vertebral foramen
proper (Fig. 26.2).

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

475

AXIS

The axis accepts the load of the head and atlas and transmits
that load to the remainder of the cervical spine. It also pro¬
vides axial rotation of the head and atlas. The broad, laterally
placed, superior articular facets of the axis accept and trans¬
mit loads from the head and atlas, while the centrally placed
odontoid process, or dens, acts as a pivot around which the
anterior arch of the atlas spins and glides to produce axial
rotation (Fig. 26.3).

The superior articular facets of the axis are lateral to the
dens and face upward and laterally and slope inferiorly and
laterally. They support the lateral masses of the atlas and
transmit the load of the head and atlas inferiorly and anteri¬
orly to the C2-3 intervertebral disc and inferiorly and posteri¬
orly to the C2-3 zygapophysial, or facet, joints. The inferior
articular facet is located posterior to the superior facet in a
position similar to the articular processes of the lower cervical
vertebrae (Fig. 26.3B).

The laminae of the axis are broad and robust, meeting at a
broad, roughened spinous process. The size and strength of the
spinous process reflect the number, size, and direction of pull
of the attaching muscles. Like the transverse processes of the
lower cervical vertebrae, each transverse process of the atlas
and axis contains a transverse foramen that, with the other
foramina on the same side, form a canal through which the
vertebral artery travels on its way to the foramen magnum.
Each transverse process of the axis is short, ending in a single
tubercle, while each transverse process of the atlas is long.
Consequently, as the canal for the vertebral artery approaches
the inferior surface of the superior articular mass, it turns
sharply laterally to exit beneath the lateral margin of the
superior articular facet.

Figure 26.3: Axis viewed anteriorly (A) and laterally (B). The axis
includes the dens, short transverse processes, and a long spinous
process. The superior articular processes slope inferiorly and lat¬
erally and face upwardly and posteriorly. The inferior facets lie
posterior to the superior facets.

Clinical Relevance

VERTEBRAL ARTERY TEST: The vertebral artery takes a
circuitous route with sharp bends as it travels superiorly to
the foramen magnum , and movement of the cervical spine
can produce additional bends or crimps in the artery. If the
artery is already narrowed by atherosclerosis , additional
crimping or stretching may reduce blood flow through the
artery. A variety of vertebral artery tests examine the effects
of head and neck movements on blood flow.

Lower Column C3-C7 Vertebrae

The lower five cervical vertebrae must support the axial load
of the head and vertebrae above, keep the head upright, sup¬
port the reactive forces of muscles, yet provide for mobility of
the head. The vertebrae, therefore, exhibit features that
reflect these load-bearing, stability, and mobility functions.
Together, the lower five vertebrae may be considered as a tri¬
angular column consisting of an anterior pillar composed of
the vertebral bodies and two posterior columns composed of
the right and left articular pillars of the articulating superior
and inferior articular processes (Fig. 26.4).

The vertebral bodies exhibit a modified blocklike quality
reflecting their ability to bear and transmit axial loads
(Fig. 26.5). Due to the presence of the uncinate processes
along the posterolateral margins, the superior surface of the
body of a lower cervical vertebra is concave transversely,
while in the sagittal plane, the superior surface slopes forward

B C

Figure 26.4: Anterior view of the cervical vertebral column.

The cervical vertebral column consists of a central anterior pillar
(A) and the right (B) and left (C) articular pillars, forming a trian¬
gular column.

476

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Uncinate process

Figure 26.5: Typical cervical vertebra viewed superiorly (A) and
laterally (B). The superior surface of the body of a typical cervical
vertebra is concave from side to side and slopes interiorly and
anteriorly. The superior facets face posteriorly and superiorly; the
inferior facets face anteriorly and interiorly.

Figure 26.6: The superior facets of C3 face slightly medially as
well as superiorly and posteriorly. The superior facets of C3 and
C7 are more steeply oriented than those of C5.

and downward. Inferiorly, the surface of the body is concave
anteroposteriorly, with an anterior lip that projects anteroin-
feriorly toward the anterior superior edge of the vertebra
below [10]. Appreciation of such detail regarding the geome¬
try of the articular surfaces is vital to understanding the pat¬
terns of segmental motion.

Posteriorly, the articular processes bear the superior and
inferior articular facets. Generally, the superior facets are
directed superiorly and posteriorly, while the inferior facets
are directed anteriorly and inferiorly (Fig. 26.5). The orienta¬
tion of the facets contributes to the function of each vertebra.
In the upright posture, the superior facet lies between the
transverse and frontal planes, and as a consequence, it helps
support the weight of the head and stabilizes the vertebra
above against forward translation. However, at each level
there are subtle differences in orientation of the facets
[56, 70,94] (Fig. 26.6). In addition to facing superiorly and
posteriorly, the superior facet of C3 also faces medially by
about 40° [61,94]. Consequently, the superior articular
processes of C3 form a socket into which the inferior articular
processes of C2 nestle [10]. The superior articular facets
change from a posteromedial orientation at the C2/C3 level to
posterolateral orientation at C7/T1. The transition typically
occurs at the C5/C6 level [71].

Descending the column, the superior facets sit higher rela¬
tive to the superior vertebral endplate, and the C3 and C7
facets are steeper [66]. Knowledge of the height of the articu¬
lar processes is important, as it has been demonstrated that the
height of the articular processes is perfectly related to the loca¬
tion of the instantaneous axes of rotation. It is articular height,
not slope, that is the major determinant of the patterns of
motion of the cervical vertebrae [66].

The unique morphology exhibited by the seventh cervi¬
cal vertebra reflects its load-bearing function (Fig. 26.7). It
is the point where the neck is cantilevered off the more

Spinous

process

Figure 26.7: Seventh cervical vertebra: superior view (A) and lat¬
eral view (B). The spinous process of C7 is long and robust. The
superior articular facets sit high relative to the superior surface of
the vertebral body and are steeply inclined in the coronal plane.

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

477

rigid thoracic spine [14,43,44]. It also is the site of attach¬
ment of several structures, including the raphe of the liga-
mentum nuchae, the large, middle portion of the trapezius,
the rhomboid minor, and the muscles of respiration,
scalenus medius, scalenus posterior, and levator costae.
Consequently, the spinous process and posterior tubercles
are long and robust. The superior articular facets sit high
relative to the superior surface of the vertebral body and
are steeply inclined in the coronal plane. Such geometry
appears to provide optimal stability, guarding
against forward translation at this transition from
the cervical to the thoracic spine [66].

JOINTS OF THE CERVICAL SPINE

Just as the bones of the cervical region are organized into two
distinct regions, the joints of the cervical spine also are
described in two regions. The craniovertebral joints exhibit
specialized characteristics that dictate the mobility and stabil¬
ity of the upper cervical region. The joints of the lower cervi¬
cal segments exhibit modified interbody and facet joints,
reflecting the stability, mobility, and load-bearing roles of the
lower neck.

Craniovertebral Joints

The two atlanto-occipital joints are found between the supe¬
rior concave sockets of the atlas and the occipital condyles of
the skull (Fig. 26.1). Being typical synovial joints, they are
enclosed by a joint capsule and contain intra-articular inclu¬
sions. These are fat pads that sit in the nonarticular waist of
the bean-shaped articular surface of the atlas and act as
deformable space fillers [57].

The atlantoaxial joints consist of three synovial joints: the
left and right lateral atlantoaxial joints and the median
atlantoaxial joint. Together these joints allow axial rotation
of the head and atlas where the centrally placed odontoid
process acts as a pivot around which the anterior arch of the
atlas spins. This movement is accommodated anteriorly by
the median atlantoaxial joint and inferiorly by the lateral
atlantoaxial joints. The median atlantoaxial joint lies
between the odontoid process and osseoligamentous ring
made by the anterior arch of the atlas and the transverse lig¬
ament (Fig. 26.8).

At the lateral atlantoaxial joints, the superior articular sur¬
faces of the axis and the corresponding inferior articular sur¬
faces of the atlas appear flat (Fig. 26.3). In vivo, however, they
are covered by articular cartilage that is convex in the sagittal
plane [48] (Fig. 26.9). The apex of this convexity lies along a
ridge passing downward and laterally across the articular facet
so that each cartilaginous facet presents a curved posterior
and anterior slope. In the neutral position, the apex of the car¬
tilage of the inferior articular facet sits on the apex of the
superior articular cartilage of the axis. Large intraarticular
meniscoids fill the spaces between the articular spaces

Figure 26.8: Superior view of a transverse section through the
atlas and dens reveals the median atlantoaxial joint and support¬
ing ligaments.

anteriorly and posteriorly [57]. These meniscoids act not only
as moveable space fillers, but also protect those articular sur¬
faces that are not in contact with one another by ensuring that
a film of synovial fluid coats them.

Little research has been undertaken regarding the struc¬
ture of the joint capsule of the lateral atlantoaxial joints,
although it is described as loose and thin [96]. The capsule
must be lax to allow approximately 45° of axial rotation in
each direction, yet it contributes to stability of the joint at the
end range of these movements [20].

Anterior atlantoaxial
Lateral membrane

Figure 26.9: Lateral atlantoaxial joints: anterior view (A) and lat¬
eral view (B). Covered by articular cartilage, the articular surfaces
of the lateral atlantoaxial joints are convex.

478

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

THE MENISCOIDS AS A SOURCE OF PAIN: Bruising of
meniscoids has been identified following cervical trauma [84].
As these structures are innervated and composed of fibroadi-
pose tissue', it has been postulated that they may become a
source of pain and/or act as the source for intraarticular
fibrofatty tissue proliferation [35,57].

LIGAMENTS OF THE CRANIOVERTEBRAL JOINTS

Many supposedly definitive descriptions of the structure
and function of the ligaments of the cervical spine are
extrapolations from other areas of the spine or are impres¬
sions, rather than results of systematic anatomical studies.
In particular, many structures have been described as liga¬
ments that are in fact only fascial membranes. True or
proper ligaments are composed predominately of strong
collagen fibers oriented in the direction of the movement
that they are designed to resist, and they attach bone to
bone. This chapter distinguishes between such proper liga¬
ments and fascial membranes, or false ligaments, which
differ by consisting of collagen that is loosely arranged and
therefore not strong.

Alar ligaments

Cruciform
ligament:

Superior
fibers

Transverse
ligament

Inferior
fibers

Occiput

Atlas

Axis

Figure 26.10: Coronal section of the occiput, atlas, and axis, with
the posterior section removed, reveals the transverse and alar
ligaments. The transverse ligament combines with the longitudi¬
nally oriented superior and inferior fibers to form the cruciform
ligament.

The cruciform or cruciate ligament is formed by the trans¬
verse ligament, with associated superior and variably present
inferior bands that together make a cross-shaped structure
(Fig. 26.10). The functional significance of these longitudi¬
nally directed median bands, which cannot be classed as
proper ligaments because of their attachment sites, has not
been determined.

Transverse Ligament

The transverse ligament is classified as a proper ligament,
being a well-defined and strong structure consisting almost
exclusively of collagen fibers. It spans the anterior portion of
the central foramen, attaching on the inner surface of each
lateral mass of the atlas, and so completes the osseoligamentous
ring of the median atlantoaxial joint (Figs. 26.1, 26.8, 26.10).
The transverse ligament resists forward translation of the
atlas relative to the axis and is integral to the stability of the
atlantoaxial joint [31,34].

Disruption of the transverse ligament does not totally dis¬
able the atlantoaxial joint complex. Transection of the trans¬
verse ligament results in about 4 mm of forward translation of
the median atlantoaxial joint, after which the joint is stabilized
by the alar ligaments (described later), which prevent the
head from moving relative to the axis and restrict the motion
of the interposed atlas [27].

Clinical Relevance

FRACTURE OF THE DENS: Following a fracture through
the base of the dens, the atlas is no longer restrained by
either the median atlantoaxial joint or the alar ligament.
These fractures are, consequently, considered unstable,
with movement of the head and atlas potentially having
disastrous neurological consequences [21].

Alar Ligaments

The anatomy of the two alar ligaments appears to differ
from the descriptions provided in the traditional textbooks
of anatomy. The morphology of these proper ligaments has
been reexamined by Dvorak and Panjabi [29]. Textbooks
tend to depict each alar ligament as passing steeply upward
and laterally from the odontoid process to the margins of the
foramen magnum. In fact, the orientation of the alar liga¬
ments is closer to being horizontal, running from the lateral
aspect of the odontoid process to the margins of the fora¬
men magnum (Figs. 26.1, 26.8, 26.10). In some specimens,
a small portion of the alar ligament has been observed to run
between the dens and the lateral masses of the atlas.
However, the functional significance of this small portion
has not been described further than reinforcing the intimate
relationship of the atlas interposed between the occiput
and axis [29].

The absence of elastic fibers and the strictly parallel orien¬
tation of the collagen fibers in the alar ligaments mean that
elongation of these ligaments is almost impossible [83]. Apart
from stabilizing the atlantoaxial joint with respect to anterior
translation, flexion, and lateral bending, the alar ligaments are
of critical importance in limiting rotation of the head and atlas
on the axis. Because the odontoid attachment of the alar liga¬
ment lies posteriorly on the dens, as the neck rotates, the con¬
tralateral alar ligament wraps around the circumference of
the dens, thereby increasing tension in the ligament. The
length between the origin and insertion of the ligament is not

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

479

a straight line during rotation but a curve around the perime¬
ter of the odontoid process, and therefore, tension develops
quickly. Following this model, Dvorak et al. report that axial
rotation slackens the ipsilateral ligament [31].

Disagreement exists regarding the role of both alar liga¬
ments in limiting rotation. Some authors report that both alar
ligaments are involved in the control of axial rotation to one
side. A model of upper cervical axial rotation predicts that
both alar ligaments must be intact for axial rotation to be
checked [20]. Transecting cadaver alar ligaments reveals that
axial rotation increases in both directions when a single alar
ligament is cut [72]. Using computed tomography (CT) scan¬
ning, Dvorak et al. show that axial rotation increases by about
11° (30%) following transection of a contralateral alar liga¬
ment [30]. While agreement exists that these paired liga¬
ments play a vital role in stabilizing and limiting the motion in
the craniovertebral region, the precise role played by each lig¬
ament remains debatable.

Clinical Relevance

REAR-END MOTOR VEHICLE ACCIDENTS: In an unex¬
pected rear-end collision , the neck may be slightly rotated
when undergoing a flexion-extension injury [30]. In this
position the aiar ligament is particularly susceptible to strain
or rupture. It has been suggested that subluxation or dislo¬
cation of the atlantoaxial joint implies destruction of both
transverse and alar ligaments [34].

Membrana Tectoria

The membrana tectoria, or tectorial membrane, is a wide
sheet of collagen fibers that covers the atlantoaxial ligament
complex (Fig. 26.11). It extends from the posterior surface of

Tectorial membrane

Figure 26.11: Tectorial membrane, posterior view. The tectorial
membrane is the superior extension of the posterior longitudinal
ligament onto the anterior and lateral margins of the foramen
magnum. It lies posterior to the transverse and alar ligaments.

atlanto-occipital

membrane

Posterior
atlantoaxial
ligament
Transverse
ligament of atlas"

Apical

ligament

atlanto-

occipital

ligament

Dens

"Anterior

atlantoaxial

ligament

Figure 26.12: Median sagittal section through the craniovertebral
region. Ligaments and membranes associated with the craniover¬
tebral region include the transverse ligament, the apical liga¬
ment, the anterior and posterior atlanto-occipital membranes,
the anterior and posterior atlantoaxial membranes, and the tec¬
torial membrane.

the vertebral body of the axis up to the margins of the fora¬
men magnum and is the direct proximal continuation of the
posterior longitudinal ligament (Fig. 26.12). It may, there¬
fore, be classified as a proper ligament. Little research has
been undertaken to determine the role of the membrana tec¬
toria in craniovertebral stability [67,95]. However, following
transection studies on cadavers, Oda et al. state that the
membrana tectoria plays a role in multidirectional stability of
the upper spine, particularly in upper cervical flexion and
axial rotation [67].

Atlanto-Occipital and Atlantoaxial Membranes

The anterior and posterior atlanto-occipital membranes and
anterior and posterior atlantoaxial membranes are often
described as ligaments associated with the craniovertebral
joints. Interestingly, although the atlanto-occipital mem¬
branes are consistently described in anatomical textbooks, the
atlantoaxial membranes are often not mentioned [82,96], or a
picture may be presented but a description of the structure
not supplied [64]. Such variability in presentation raises ques¬
tions regarding their structural and functional significance.
The anterior and posterior atlanto-occipital membranes are
found spanning the space between the upper border of the
anterior arch of the atlas and the basiocciput and the poste¬
rior arch and posterior margin of the foramen magnum
(Fig. 26.12). They appear to consist of dense areolar tissue
that is not particularly organized.

Ramsey sectioned these posterior membranes and found
some elastic fibers, although fewer than typically seen in the
ligamentum flavum [79]. He feels that these structures
should be considered being “in series” with the ligamentum
flavum. As these membranes are found in the anterior and
posterior spaces between the occiput and atlas and between

480

Part III I KINESIOLOGY OF THE HEAD AND SPINE

the atlas and axis, they may also be considered nothing
more than fascial curtains between the external space occupied
by the posterior vertebral or prevertebral muscles and the
internal epidural space. As such, they may be classified as
false ligaments.

Apical Ligament

The apical ligament is trivial in size, very thin, and missing in
20% of persons. The ligament has no known biomechanical
importance. Rather, it represents the vestigial remains of the
cranial end of the notochord passing from the posterior supe¬
rior aspect of the odontoid process to the anterior rim of the
foramen magnum (Fig. 26.12) [74,89]. This ligament should,
therefore, be considered a false ligament.

Joints of the Lower Cervical Spine

INTERBODY JOINTS

As elsewhere in the vertebral column, the vertebral bodies
below C2 are joined via intervertebral discs. These discs pro¬
vide separation of adjacent vertebral bodies, thereby allow¬
ing the superior vertebra to move on the lower vertebra. The
interposed disc must be able to accommodate the motion
occurring between vertebrae, be strong enough to transfer
loads, and not be injured during movement [11]. The form
and function of the cervical intervertebral disc is, however,
distinctly different from those of the lumbar intervertebral
disc [58]. In the adult, the anulus fibrosus in the cervical
region is a discontinuous structure surrounding a fibrocarti¬
laginous core, instead of being a fibrous ring enclosing a
gelatinous nucleus pulposus like the anulus fibrosus in the
lumbar region. Anteriorly, the anulus fibrosus in the cervical
spine is a thick crescent of oblique fibers joining the verte¬
bral bodies to constitute a strong interosseous ligament
located at the pivot point of axial rotation [59]. Posteriorly,
the anulus is a thin, narrow, vertically oriented band of fibers
joining the vertebral bodies. Laterally, there is no distinct
anulus, only flimsy fascial tissue that is continuous with the
periosteum (Fig. 26.13).

Penetrating the fibrocartilaginous core to a greater or
lesser extent are uncovertebral clefts, which are considered
normal features of cervical intervertebral discs. Found oppo¬
site the uncinate processes, they have been shown to develop
following the maturation of the uncinate processes at approx¬
imately 9 years of age. With age, the clefts progress medially
into the disc to form transverse clefts, which may completely
transect the posterior two thirds of the disc [58,68,76,88]. It
is these normally developing clefts or fissures that effectively
form a joint cavity between the vertebral bodies and allow
the swinging movement of the posterior inferior surface of
the upper vertebral body within the concavity of the uncinate
processes. The clefts, therefore, enable the interbody joint to
accommodate the coupling of lateral flexion and axial rota¬
tion that is determined by the geometry of the zygapophysial
joints [77].

Anulus fibrosus

cleft

Figure 26.13: Cervical disc: superior view (A) f lateral view (B) f ante¬
rior view (C). A typical cervical disc contains a distinct, strong anulus
fibrosus anteriorly, which is discontinuous with the thin vertically
oriented fibers of the posterior anulus fibrosus. Uncovertebral clefts
form opposite the unciform processes and may eventually form
clefts that transect the posterior two thirds of the disc.

Clinical Relevance

DISCOGENIC PAIN: The recently described morphology of
the adult cervical intervertebral disc must raise questions for
clinicians concerning the etiology and mechanism of cervi¬
cal discogenic pain. This pain cannot arise from posterolat¬
eral fissures in the anulus fibrosus, as occurs in the lumbar
disc, since there is no posterolateral anulus fibrosus in a cer¬
vical disc [63]. Given the morphology of the cervical inter¬
vertebral disc, possible sources of disc-related pain in the
cervical spine are strain or tears of the anterior anulus fibro¬
sus, especially following hyperextension trauma, and strain
of the lateral (alar) portions of the posterior longitudinal lig¬
ament by a bulging disc [58].

The nucleus pulposus of the cervical intervertebral disc is also
distinctly different from the lumbar nucleus. At birth, the
nucleus comprises less than 25% of the discs, whereas in the
lumbar disc, it comprises at least 50% [87]. The adult cervical
nucleus is characterized by fibrocartilage, with no gelatinous
component [10,58,68,88].

Clinical Relevance

CERVICAL DISC STRUCTURE: The absence of a gelati¬
nous nucleus pulposus has implications for assessment and
treatment techniques that assume that a cervical intervertebral

(continued )

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

481

(Continued)

disc is composed of a gelatinous nucleus pulposus encircled
by an anulus fibrosus [60]. Although the validity and
effectiveness of such techniques require direct assessment
the biological explanations for these techniques appear
implausible.

ZYGAPOPHYSEAL JOINTS

The cervical zygapophyseal, or facet, joints are formed by
the articulation of the inferior articular cervical vertebra
with the ipsilateral superior articular process of the
vertebra below. As typical synovial joints, the articular
surfaces are lined by articular cartilage and enclosed by a
joint capsule. A variety of intraarticular inclusions are
found within the joint, with fibroadipose meniscoids
always present along the ventral aspect of the joint and
frequently also present along the dorsal aspect [57]. The
articular facets may be round or oval, and there is often
right-left asymmetry [71].

Clinical Relevance

WHIPLASH INJURIES: Rear-impact motor vehicle colli¬
sions produce hyperextension movements of the head and
neck. It has been postulated that these movements cause
impingement of the meniscoids, which could become
inflamed and so be a source of undiagnosed neck pain fol¬
lowing whiplash injuries [47].

The capsules of the zygapophysial joints consist of well-
orientated collagen and elastic fibers. The medial, anterior,
and lateral parts of the joint capsule have been described as
thicker than the thinner posterior part [35,74,90], although
Johnson et al. state that the posterior portion is thick [45].
The elastic fibers of the medial aspect are oriented like those
of the ligamentum flavum, projecting vertically from one
articular process to the other, and may join with the ligamen¬
tum flavum. Anterolaterally, the elastic fibers are less concen¬
trated, are oriented obliquely in the transverse and sagittal
planes, and appear to provide an important barrier to antero¬
posterior shear [90].

In the neutral position, the capsules of the zygapophysial
joints are lax. This laxity is the large range of gliding that
occurs between the articular facets during the normal move¬
ments of flexion-extension and rotation of the cervical motion
segments. However, at the extremes of these motions, the
capsules are taut and hence function as stabilizing or resisting
ligaments. It is for this reason that some persons refer to these
structures as the capsular ligaments.

LIGAMENTS OF THE LOWER CERVICAL SPINE
Longitudinal Ligaments

A variety of descriptions of the structure and function of the
anterior longitudinal ligament exists, but few studies have
specifically examined the cervical ligaments. Most descrip¬
tions extrapolate structure and function from the lumbar por¬
tion of the ligament. Being arranged essentially in a uniaxial
manner, these ligaments resist tension [98].

Traditionally, descriptions of the anterior longitudinal liga¬
ment suggest that it is a multilayered ligament firmly adher¬
ent to the intervertebral discs and to the margins of adjacent
vertebral margins and, therefore, may be considered a proper
ligament [96]. The posterior longitudinal ligament is a broad,
thick ligament that blends with the posterior surface of the
intervertebral discs and attaches to the vertebral bodies near
their upper and lower margins and somewhat over their pos¬
terior surfaces [96]. It, too, may be considered a proper liga¬
ment. Cranially, the ligament expands to form the membrana
tectoria, which attaches to the anterior and lateral margins of
the foramen magnum (Fig. 26.11).

More-recent descriptions of the morphology of the cervi¬
cal longitudinal ligaments reveal that the anterior longitudinal
ligament is a centrally placed, thin structure composed of four
distinct layers of fibers. This ligament, therefore, provides a
thin covering to the front of the disc. The thicker posterior
longitudinal ligament covers the entire floor of the cervical
vertebral canal and is also distinctly multilayered. It rein¬
forces the deficient posterior anulus fibrosus with longitudi¬
nal and alar fibers. This geometry also allows the ligament to
resist tensile forces in a range of directions [58]. The mor¬
phology of the longitudinal ligaments and anulus fibrosus of
the adult cervical disc suggests that the posterior longitudinal
ligament and anterior anulus fibrosus are the important stabi¬
lizers of each interbody segment.

Ligamentum Flavum

The ligamentum flavum lacks studies that examine the form
and function of its cervical portion. The ligamentum flavum in
the neck is considered to be considerably thinner than in the
lumbar region, but it maintains similar attachments and so may
be classed as a proper ligament. This elastic ligament passes
from the border of the lamina of one vertebra to the anterior
surface of the lower edge of the vertebra above, leaving a space
between the dorsal surface of ligamentum flavum and the infe¬
rior margin of the lamina of the upper vertebra (Fig. 26.14).
The space is filled with fascia and some fat. As in the lumbar
region, the ligamentum flavum seems to serve to provide a
smooth, somewhat elastic posterior wall to the vertebral canal,
thereby protecting the spinal cord against any buckling of the
ligament that might occur if the ligament were fibrous.

Ligamentum Nuchae

The literature offers three opposing descriptions of the
ligamentum nuchae. The most common description is that
the ligamentum nuchae is a median fibrous septum, triangular

482

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 26.14: The ligamentum nuchae in the cervical region can
be described with two distant parts, a dorsal raphe and a ventral
midline septum.

in shape, which divides the muscles of the posterior neck into
right and left compartments and provides attachment for the
upper fibers of trapezius, rhomboid minor, splenius capitis,
and serratus posterior superior. It is composed of a free pos¬
terior border that extends between the external occipital pro¬
tuberance and the spinous process of the seventh cervical
vertebrae, an anterior border that is firmly attached to the
cervical spinous processes, and a short superior border that
extends along the external occipital crest [64,96]. The clinical
literature portrays this ligament as a substantial midline struc¬
ture that is important in control of head posture [7,78].

A second description found in a small number of texts
characterizes the nuchal ligament as being no more than a
thin fibrous intermuscular septum [1,38,80,101]. The third
description of the ligamentum nuchae, favored by this author,
presents the ligament with two distinct parts: a dorsal raphe
and a ventral, midline septum (Fig. 26.14). The dorsal raphe
is firmly attached to the external occipital protuberance and
spans the cervical spine to attach to the spinous process of C7
and C6. It is formed by the interlacing of the tendons of the
cervical portion of trapezius, splenius capitis, and rhomboid
minor muscles. The midline septum, which consists of unori¬
ented fascia embedded with fat and blood vessels, extends
from the ventral aspect of the dorsal raphe attaching to the
external occipital protuberance, to the external occipital crest,
and to the tips of the cervical spinous processes. This fascial

tissue is continuous between the spinous processes, so no def¬
inite interspinous ligaments are found. In the suboccipital
region, the fascia is confluent with the posterior atlanto-
occipital and atlantoaxial membranes [33,40]. Consequently, in
the cervical spine, there is no classically defined supraspinous
ligament, nor is the ligamentum nuchae a proper ligament.

Clinical Relevance

LIGAMENTUM NUCHAE: The absence of firm tendinous
attachments of the ligamentum nuchae to the cervical spine
raises questions regarding the importance of this structure
in stabilizing the head or as the source of pain from tendini¬
tis of the ligament's insertion at the tips of the cervical spin¬
ous processes.

NORMAL RANGE OF MOTION

Total Motion of the Cervical Spine

Because the American Medical Association (AMA) Guides for
the Assessment of Impairment stipulate that ROM of the head
be used to determine impairment of the neck, the clinician
must appreciate the limitations in the current knowledge of
ROM measures for the neck [4]. Total motion of the cervical
spine is typically determined by describing head motion rela¬
tive to the thorax or shoulder girdle. The wide variety of instru¬
ments and lack of standardized procedures that have been
used in both reliability and descriptive studies have contributed
to the wide range of published norms for active (Table 26.1)
and passive neck ROM [3,8,36,50,51,55,69, 85,99] (Table 26.2).
In addition, age and sex have been associated with variations
in neck ROM [17,18,26,51,53,65,91,99]. Normal variation in
ROM in subjects suggests that when measuring
individual patients, a clinician should allow for natural
variation of 12-20° [19].

Clinical Relevance

CERVICAL RANGE OF MOTION: Clinicians must recognize
the wide range of reported values for normal neck motion
in addition to the normal variation reported for individual
subjects when using neck motion to determine response to
treatment or making disability ratings. Small changes in
cervical motion may be attributable to normal variability and
may have little to do with the impairment or intervention.

TABLE 26.1: Reported Extremes of Active Range of Motion

R Axial

L Axial

R Lateral

L Lateral

ROM

Rotation

Flexion

Extension

Minimum

Maximum

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

483

TABLE 26.2: Reported Extremes of Passive Range of Motion

R Axial

L Axial

R Lateral

L Lateral

ROM

Rotation

Flexion

Extension

Minimum

Maximum

Because of the complex anatomy of the cervical spine,
global ranges of neck motion are unable to differentiate
movement occurring within the functional units of the
upper and lower cervical spine and, therefore, do not reflect
motion occurring at the segmental level. It has been demon¬
strated that in full extension, the entire cervical spine is in
lordosis. However, during flexion the degree of kyphosis
achieved in the upper and lower regions of the cervical
spine varies, depending upon the posture adopted by the
upper cervical spine. During head and neck flexion and
extension, motion occurring in both the upper and lower
regions of the cervical spine must be observed if the full
potential of cervical flexion is to be assessed [97]. To estab¬
lish full range of flexion of both the upper and lower cervi¬
cal spine, upper cervical flexion should be examined with
the lower cervical region in neutral, and then lower cervical
flexion should be examined with the upper cervical region in
slight extension [24]. This method ensures that total ROM
in both functional units is assessed.

The traditional descriptions of neck ROM have been of
total ROM. Yet the total ROM of the neck is not the arith¬
metic sum of the segmental ROMs. Total ROM appears to
be as much as 10-30° less than the sum of the maximum
segmental ROMs [92]. Segmental ROM in normals varies
from day to day and depends on whether the motion is
measured from an initial starting position of flexion or
extension [92]. Further, in individuals reporting neck pain,
dysfunctional segments have been demonstrated at levels
other than those responsible for the pain [5]. These findings
challenge the clinical relevance of considering the neck as a
single entity and determining impairment on the basis of the
ROM of the head and neck as a whole. Because global
ROMs do not fully describe what is occurring in the neck,
attempts to determine segmental mobility in both cadavers
and in vivo are important.

Segmental Motion of the
Craniovertebral Joints

ATLANTO-OCCIPITAL JOINTS

The functional challenge for the atlanto-occipital joints is to
provide stability for the balance of the head on the cervical
spine yet allow mobility. The geometry of the atlantal sockets,
designed primarily for stability, determines the pattern of
motion. The deep walls of each atlantal socket prevent trans¬
lation of the occipital condyle laterally, anteriorly, or posteri¬
orly, but the concave shape permits nodding movements of
the head [13].

The nodding motion that occurs during flexion of the
head is the result of rolling and sliding of the occipital
condyles in their sockets. As the head nods forward, the
occipital condyles roll forward in the atlantal sockets,
tending to roll up the anterior wall of the socket. Because
of the compression loading exerted by the mass of the
head, the flexor musculature, or tension in the joint cap¬
sules, the occipital condyles concomitantly translate
downward and backward [13]. As a result, anterior rota¬
tion is coupled with downward and posterior sliding, and
the condyles effectively stay nestled in the floor of the
atlantal sockets, ensuring maximum stability of the head
on the neck. The converse occurs during extension of the
head on the atlas.

The results of studies that have described range of flexion
and extension at the atlanto-occipital joints are reported in
Table 26.3. Examination of this table reveals the large varia¬
tion in reported normal range for this joint. The total range of
flexion-extension observed in vivo varies between a mean
value of 14 and 35°. Brocher observes a range from 0 to 25°
(mean, 14.3°), while Lind et al. find a mean value of 14°, but
with a standard deviation of 15° [15,53].

TABLE 26.3: Range of Flexion-Extension at the Atlanto-Occipital Joint

Source

Subject

Mean ROM (°)

Range

Brocher [15]

In vivo

14.3

0-25

Lewit & Krausova [52]

In vivo

Markuske [54]

In vivo

14.5

Fielding [34]

In vivo

Kottke & Mundale [49]

In vivo

0-22

Lind et al. [53]

In vivo

Werne [95]

Cadaver

Worth & Selvik [97]

Cadaver

18.6

0.6

484

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

DISTINGUISHING NORMAL AND ABNORMAL
RANGE OF MOTION: The wide variation in reported ROM
has important impiications for clinicians who are attempting
to differentiate normal from abnormal ROMs at the atlanto-
occipital joint. In view of normal variation in morphology
and ROM , the clinician must use additional information ,
including the patient's symptoms; to identify real impair¬
ments in ROM in the cervical region.

Although commonly described, axial rotation (about a vertical
axis) is not a true physiological movement of the atlanto-
occipital joint. For true axial rotation to occur, the contralat¬
eral occipital condyle must translate posteriorly while the
ipsilateral condyle translates anteriorly. As these translations
are prevented by the steep walls of the atlantal sockets, axial
rotation can only occur if sufficient torque is applied to the
head. This would force the occipital condyles to rise up the
walls of the sockets, which are wider at their mouths than at
their depths. Axial rotation may, therefore, occur only if
accompanied by upward vertical motion of the occiput. The
alar ligaments and tension within the capsules of the atlanto-
occipital joints resist this vertical displacement. As depicted in
Table 26.4, the ROM that has been reported is small (-2-7°),
and only one study has measured ROM in vivo.

Clinical Relevance

ATLANTO-OCCIPITAL ROTATION: Although clinicians
often report restrictions in atlanto-occipital rotation , the
small values reported in the literature cast doubt on the
validity of such clinical observations.

Atlanto-occipital lateral flexion in vivo has not been system¬
atically studied, although it has been examined in cadavers,
with a reported ROM from 2.3 to 11° [73,97] (Table 26.4).
Recause of the geometry of the atlantal socket, either the
contralateral occipital condyle must slide up and out of its
deep atlantal socket while pivoting on the ipsilateral condyle,
or both condyles must slide in parallel up the contralateral
walls of their respective sockets. These articular surface

movements are not physiological but may be induced during
manual examination. When induced, lateral flexion is coupled
with flexion, extension, or axial rotation [97]. Since the pat¬
tern of coupling depends on the shape of the joint surfaces,
and asymmetry of these joint surfaces has been extensively
documented, no single rule for a pattern of coupling can be
applied [93].

ATLANTOAXIAL JOINTS

Few studies are available that have fully investigated the avail¬
able range and patterns of motion of the atlantoaxial joints.
Most studies have used plain radiography and have only
reported ranges of flexion and extension. Axial rotation has
been either inferred from these plain films or biplanar radi¬
ography, although more recently, functional CT scanning has
been undertaken. The clinician should note the methodology
used when interpreting each study, because the measurement
methodology may affect the results.

Axial rotation at the atlantoaxial level is extremely impor¬
tant functionally, for movement at this level accounts for 50%
of the total range of axial rotation of the neck. Indeed, the first
45° of rotation of the head to either side occurs at the
C1-C2 level before any lower cervical segments
move in this plane.

Axial rotation of the atlas to the left requires anterior dis¬
placement of the right lateral mass and a reciprocal posterior
displacement of the left lateral mass. The inferior articular
cartilages of the atlas must therefore slide down the respec¬
tive slopes of the convex superior articular cartilages of the
axis (Fig. 26.15). The atlas, accordingly, screws down on the
axis as it rotates [48]. Any asymmetry between the articular
cartilages results in coupling of ipsilateral or contralateral
side-bending with the axial rotation, the side of coupling
depending upon the direction of the asymmetry [75]. Here,
as at the atlanto-occipital joint, normal asymmetry of the
articular surfaces has been documented [81]. At the limits of
axial rotation, the lateral joints are almost subluxed. The alar
ligaments are ideally located to act as the principal structures
to restrain axial rotation, with the lateral atlantoaxial joint cap¬
sules playing a secondary role [20,30]. Limitation of axial rota¬
tion is essential, as the spinal cord and vertebral arteries cross
this joint [20]. The reported normal range of axial rotation to
one side in living subjects is between 39 and 49° (Table 26.5).

The shape of the odontoid process allows the anterior arch
of the atlas to slide upward and slightly backward, thereby

TABLE 26.4: Range

of Motion for Lateral Flexion and Axial Rotation at the Atlanto-Occipital

Joint

Source

Subjects

Total

Lateral Flexion

Axial

Rotation

Panjabi et al. [73]

Cadaver

3.9 ± 1.6

Penning [75]

Cadaver

Penning & Wilmink [77]

In vivo

1 (-2-5)

Werne [95]

Cadaver

Worth & Selvik [97]

Cadaver

11.0

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

485

Figure 26.15: In axial rotation of the atlantoaxial joint from the
neutral position (A) f the inferior articular facet of the atlas slides
down the anterior slope of the convex superior facet of the axis
during contralateral rotation (B) or down the posterior slope of
the convex superior facet of the axis during ipsilateral rotation (C).

producing extension of the atlas on the axis [95]. Flexion takes
place by a downward and forward glide, with an additional
slight anterior translation of the anterior arch on the odontoid
process. The total range of flexion-extension reported in
vivo varies between 11 and 21° (Table 26.5). Panjabi et al.
report 11.5° of flexion and 10.9° of extension in cadavers [73].

The reported ROM for side-bending at the atlantoaxial
joint in cadavers ranges between 5° and 10° [22,75]. Side¬
bending does not result from pure lateral translation. As the
superior articular facets of the axis slope down and laterally,
lateral translation would produce impaction of the contralat¬
eral lateral mass of the atlas on the superior lateral mass of the
axis (Fig. 26.16). Consequently, the inferior facet must ride
down the superior facet while the contralateral inferior facet
must ride up the contralateral superior facet, thereby impart¬
ing a lateral tilt to the atlas. The contralateral alar ligament
offers primary resistance to this motion, but ultimately, the
motion is resisted by impaction of the contralateral lateral
mass onto the lateral aspect of the odontoid process [12,72].

SEGMENTAL CRANIOVERTEBRAL MOTION

The characteristic movements of the atlanto-occipital and
atlantoaxial joints that have been described do not occur in
isolation. Rather, these joints of the head, atlas, and axis nor¬
mally function as a composite unit. As noted previously, the
atlas acts essentially as a passive washer, structurally tied to
the occipital condyles by joint geometry and soft tissues.

Consequently, when the head moves during axial rotation,
the head and atlas move in concert on the axis. During flex¬
ion and extension of the head and neck, the atlas exhibits what
is known as paradoxical motion. For example, during flexion,
the atlas may flex or it may extend, and during extension of
the neck, the atlas may also flex or it may extend [75,92]. This
incongruity occurs because the convexities of the inferior
facets of the atlas rest on the convexities of the superior facets
of the axis (Fig. 26.9). The equilibrium of the resting position
is thus susceptible to small variations in the position of com¬
pression forces passing through the lateral masses. If the
compression load is exerted anterior to the fulcrum of the
articular surfaces of the lateral atlantoaxial joint, the atlas tilts
into flexion. If the compression load is exerted posterior to
the fulcrum, the atlas tilts into extension [59].

TABLE 26.5: Average Motion at the Atlantoaxial Joint Complex

Source

Subjects

Axial Rotation
One-Sided ROM

Axial Rotation

Total ROM

Flexion-

Extension

Brocher [15]

In vivo

18(2-16)

Dvorak et al. [30]

Cadaver

Dvorak et al. [28]

In vivo

43.3 ± (5.5)

Fielding [32]

In vivo

Hohl & Baker [39]

Cadaver

Kottke & Mundale [49]

In vivo

Lewit & Krausova [52]

In vivo

Lind et al. [53]

In vivo

13 ± 5

Markuske [54]

In vivo

Panjabi et al. [73]

Cadaver

38.9

Penning & Wilmink [77]

In vivo

40.5 (29-46)

Werne [95]

Cadaver

47 (22-58)

486

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 26.16: Lateral translation and side-bending at the lateral
atlantoaxial joints. A. The superior articulating facets of the axis
slope laterally and interiorly. B. As the atlas translates laterally,
its inferior facet impacts on the superior facet on the axis. C. As
the atlas translates, one inferior facet slides up on the underlying
superior facet as the other rides down, imparting a lateral tilt to
the atlas.

Clinical Relevance

HEAD POSTURE AFFECTS CERVICAL RANGE OF
MOTION: Posture of the head and neck influences whether
flexion or extension of the atlas occurs during cervical spine
motion. If the chin is protruded as the neck flexes, the atlas
flexes in accord with the other cervical vertebrae as the
compression force on the atlas is displaced anteriorly. If the
chin is tucked, the atlas extends when the other cervical ver¬
tebrae flex as the compression load moves posteriorly within
the lateral atlantoaxial joint. Initial head posture may, there¬
fore, influence craniovertebral movement patterns.

During side-bending of the head to the left, the atlas rotates
to the right while the axis rotates to the left [41,42]. This com¬
bination of movements occurs because side-bending exerts a
downward load on the ipsilateral articular pillar. The com¬
pressive load of the head passes from the ipsilateral lateral
mass of the atlas down to the C2/3 zygapophysial joint and

zygapophysial joints below. Due to the slope of the articular
facets, the inferior process of C2 moves down and backward
along the superior articular facet of C3. This backward
motion causes the axis to rotate toward the direction of side¬
bending. However, to ensure that the face is directed forward
during side-bending, the atlas undergoes contralateral axial
rotation. If, however, the patient is not asked to look forward
or the therapist does not maintain the forward head position,
the patient s head naturally rotates to the same side as
the side-bend of the neck because of the coupled
motion in the lower cervical region.

Segmental Motion of the Lower
Cervical Region

Because of the technical difficulties involved in studying seg¬
mental motion, studies of the lower cervical spine have con¬
centrated on flexion-extension movements. The nature and
range of segmental motion in the lower cervical spine are
influenced by both the geometry of the zygapophysial joints
and the morphology of the interbody joints. The orientation
and height of the articular processes oblige coupling of cer¬
tain movements. Pure anterior translation cannot occur
because the inferior articular processes of the upper vertebra
impact against the superior articular processes of the lower
vertebra. Further, translation occurs if the upper vertebra tilts
forward, drawing its inferior articular processes up the superior
articular processes below. Flexion in the lower cervical spine,
therefore, is always a combination of anterior translation and
anterior rotation in the sagittal plane. The reverse occurs in
extension, with coupling of posterior sagittal rotation and pos¬
terior translation (Fig. 26.17).

It is the height, not the slope, of the caudal adjacent supe¬
rior articular processes that dictates the relative amounts of
sagittal translation and sagittal rotation that occur at any level
[66]. In the cervical spine, the superior articular processes
become progressively taller from C3 down to C7. At more cra¬
nial levels, therefore, a greater amount of sagittal translation
can be achieved with less sagittal rotation, because of the
smaller height of the superior articular processes.

Figure 26.17: Flexion in the lower cervical spine combines
anterior translation and sagittal plane rotation of the superior
vertebra.

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

487

TABLE 26.6: Normal

Ranges

of Segmental Motion

during Cervical

Spine Flexion and Extension

Source

Number C2-3

C3-4

C4-5

C5-6

C6-7

Aho et al. [2]

12 ± 5

15 ± 7

22 ± 4

28 ± 4

15 ± 4

Bakke [6]

13 (3-22)

16 (8-23)

17 (11-24)

20 (12-29)

18 (11-26)

Bhalla & Simmons [9]

9 ± 1

15 ± 2

23 ± 1

19 ± 1

18 ± 3

de Seze [23]

Dvorak et al. [26]

10 ± 3

15 ± 3

19 ± 4

20 ± 4

19 ± 4

Buetti-Bauml [16]

11 (5-18)

17 (13-23)

21 (16-28)

23 (18-28)

17 (13-15)

Kottke & Mundale [49]

Lind et al. [53]

10 ± 4

14 ± 6

16 ± 6

15 ± 8

11 ± 7

Zietler & Markuske [100]

16 (4-23)

23 (13-38)

26 (10-39)

25 (10-43)

22 (13-29)

Mestdagh [61]

Johnson et al. [46]

Dunsker et al. [25]

10 (7-16)

13 (8-18)

13 (10-16)

20 (10-30)

12 (6-15)

A wide range of measurements is reported for normal
ranges of segmental motion during cervical spine flexion and
extension (Table 26.6). Despite the variability in reported
ROMs, the data consistently show progressively larger contri¬
butions to flexion and extension from the C2-C3 segment to
the C5-C6 segment, followed by a decrease in motion occur¬
ring at C6-C7. However, those studies report mean values
and standard deviations that highlight the huge variation seen
in normal data [2,9,26,53].

Using CT scanning, Penning and Wilmink estimate the
range of axial rotation for each segment of the lower cervical
spine (Table 26.7 ) [77]. The only other study examining seg¬
mental motion in detail was undertaken by Mimura et al.,
who used trigonometric reconstructions of motion recorded
via biplanar radiography (Table 26.8) [62]. These authors also
report ranges of coupled motions, and it is interesting to note
that axial rotation is coupled with side-bending of essentially
the same magnitude [13].

Traditionally it has been taught that side-bending of a seg¬
ment is coupled with axial rotation and axial rotation is cou¬
pled with side-bending, the basis for this coupling lying in the
morphology of the articular processes. During axial rotation
the contralateral inferior articular process impacts the supe¬
rior articular process of the vertebra below, and axial rotation
only continues if the inferior articular process glides up the
superior facet, resulting in an ipsilateral side-bend of the

TABLE 26.7: Mean Values and Ranges of Segmental
Axial Rotation

Segment

Mean (°)

Range (°)

C2/C3

3.0

0-10

C3/C4

6.5

3-10

C4/C5

6.8

1-12

C5/C6

6.9

2-12

C6/C7

2.1

2-10

C7/T1

2.1

-2-7

moving vertebra above. Therefore axial rotation is always cou¬
pled with ipsilateral side-bending (Fig. 26.18).

Reciprocally, during side-bending, as the ipsilateral infe¬
rior articular process moves down the slope of the superior
articular process of the vertebra below, the inferior process is
driven into the superior process. The inferior articular
process must, therefore, move backward, and it is this back¬
ward movement that results in the vertebra rotating toward
the side of the side-bending. Side-bending is, therefore,
always coupled with ipsilateral axial rotation (Fig. 26.19).

Examination of the structure of the cervical joints reveals
that the movements of side-bending and horizontal rotation

TABLE 26.8: Normal Range of Axial Rotation with Coupled Flexion-Extension and Side-Bending

Segment

Axial Rotation

Flexion/Extension

Side-Bending

C2/C3

7 ± 6

0 ± 3

-2 ± 8

C3/C4

6 ± 5

-3 ± 5

6 ± 7

C4/C5

+ 1

-2 ± 4

6 ± 7

C5/C6

5 ± 4

2 ± 3

1 +
00

C6/C7

6 ± 3

3 ± 3

3 ± 7

488

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 26.18: Coupling of motion during rotation in the lower
cervical spine. Traditionally, axial rotation to the left is described
as coupled with ipsilateral side-bending resulting from the right
inferior facet gliding superiorly on the underlying superior facet.

are an artificial construct, and motion should be considered as
occurring in the plane of the zygapophysial joints [13,76].
Since side-bending and rotation cannot occur independently,
they can never be considered separate movements. In fact,
each is only a partial manifestation of a single gliding motion

Figure 26.19: Traditionally side-bending to the left in the lower
cervical spine is described as coupled with ipsilateral rotation
resulting from the posterior glide of the left inferior facet on the
underlying superior facet.

Figure 26.20: The interbody joint of the lower cervical region can
be described as a saddle joint, with the convex inferior surface of
the superior vertebra cradled in the concave superior surface of
the inferior vertebra.

in the plane of the zygapophysial joint. When viewed in this
plane, the interbody joint emerges as a saddle joint, and the
functional implications of the specialized morphology of the
cervical intervertebral disc become apparent (Fig. 26.20).

The substantial anterior anulus fibrosus and the gentle
curving of the vertebral bodies in the sagittal plane make
flexion and extension the predominant motion in the lower
cervical spine. If the profile of the vertebral bodies is consid¬
ered parallel to the plane of the facet joints, the posterior
aspect of the superior vertebra is convex and the reciprocal
posterior aspect of the inferior vertebra is concave. This
structure suggests that the superior vertebral body can rock
side to side within the concavity of the uncinate processes,
pivoting about the anterior anulus fibrosus while the facets
slide freely upon one another (Fig. 26.21) [13]. This second
form of pure motion available is, therefore, rotation about an
axis perpendicular to the facets. Since the facets are oriented
at about 45° to the transverse plane of the vertebrae, the
axis of rotation is 45° from the conventional axes of both
horizontal rotation and side-bending [13,66]. Therefore, as

Figure 26.21: The motion of the saddle joint between adjacent
lower cervical vertebrae occurs in the plane of the facet joints,
about an axis perpendicular to the plane.

Chapter 26 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE CERVICAL SPINE

489

horizontal rotation and side-bending are always coupled, the
rules commonly learned for coupled motion are unnecessary
if motion is considered in the plane of the facet rather than
in the coronal or transverse plane.

SUMMARY

The morphology of each of the cervical vertebrae reflects the
function of the neck. The deep atlantal sockets of the atlas
cradle the occipital condyles of the skull. The pivot joint of
the median atlantoaxial joint and broad facets of the lateral
atlantoaxial joints ensure stability of the atlas and head while
allowing for a large range of axial rotation. The lower cervical
vertebrae must continue the transmission of the axial load of
the head and vertebrae above but also provide for mobility yet
stability of the neck. This is achieved through a saddlelike
joint between the vertebral bodies, and zygapophysial joints
that permit predominately flexion and extension and rotation
in the plane of the facet yet ensure stability. Research describ¬
ing the functional morphology and kinematics of the cervical
spine is sparse, yet the studies available indicate that the cer¬
vical spine cannot be considered similar to the lumbar spine.
It has a complex structure reflecting its role in orienting the
head in three-dimensional space. The following chapter
examines the muscles that move the head and neck and con¬
tribute to the region s stability.

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72. Panjabi M, Dvorak J, Crisco J, et al.: Flexion, extension and lat¬
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CHAPTER

Mechanics and Pathomechanics
of the Cervical Musculature

PETER PIDCOE P.T ., D.P.T., PH.D.

AND THOMAS MAYHEW P.T., PH.D.

CHAPTER CONTENTS

EXTENSORS OF THE HEAD AND NECK.493

Deep Plane .493

Semispinalis Plane.495

Splenius and Levator Scapulae Plane .497

Superficial Plane.500

FLEXORS OF THE HEAD AND NECK.502

MUSCLE FUNCTION IN THE CERVICAL SPINE.506

Muscle Interactions and Activation Patterns .507

Effects of Posture on Cervical Muscles .508

SUMMARY.509

T he musculature of the human cervical region has developed in response to two major functional demands.

With the development of bipedal gait and upright posture, the position of the skull has moved more directly
over the cervical spine. Large posterior muscles that support the weight of the head in animals in the
quadruped position have become much smaller in humans, because the skull is balanced on the atlas with a larger
bony brain case and smaller facial skeleton. In the standing position, however, the center of gravity of the human skull
does fall in front of the articular condyles of the occiput and, therefore, creates a flexion moment on the neck; the
mass of the cervical posterior/extensor muscles continues to be larger than that of the anterior/flexor muscles to offset
this tendency for the skull to fall forward. A study comparing the relative force-generating capability of the neck flex¬
ors and extensors found an extension-flexion ratio of 1.7 to 1.0 in both men and women [8].

Another major function of the cervical vertebrae and surrounding musculature, in addition to supporting the weight of
the skull, is to position the special sense organs located in the skull optimally to respond to stimuli. The need to move
the skull in response to auditory or visual stimuli to place the ears or eyes in a favorable position may be very rapid,
requires precision, and is often mediated reflexively. Patients' chief complaints are often related to the pain associated
with these rapid movements, or the inability to move the head and neck appropriately. The proximity of the small and
large muscles located in the cervical region to the head and associated sense organs can lead to a number of debilitat¬
ing conditions when there are movement problems.

There are many muscles located in the neck region, and it would seem logical to organize them according to move¬
ments produced at the head and/or neck. Many of these muscles, however, have several actions on both the head and
neck and have very different actions when considered ipsilaterally or in combination with their counterparts on the
contralateral side. Another problem related to categorizing muscle function in the cervical region is the depth at which

492

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

493

many of the muscles lie and the vital structures located in the area. Because many of the deeper muscles are covered
with three or four layers of muscle and fascia, surface electromyographic (EMG) evidence is lacking, and palpation is
difficult. Use of fine wire electrodes is risky because of the number of important vessels and nerves located so close to
these muscles. Anything more than general movement categorizations are therefore artificial. Commonly, however, the
muscles can be categorized according to their bilateral function and region with accompanying descriptions of second¬
ary actions. This is the categorization scheme used in this chapter.

The purposes of this chapter are to

■ Present the structure and function of the muscles of the cervical spine

■ Discuss the literature concerning the activity patterns of these muscles

■ Discuss the contributions of these muscles to complaints of head and neck pain in individuals

EXTENSORS OF THE HEAP AND NECK

This group includes muscles that extend the head on the neck
(atlanto-occipital joint) and muscles that extend the cervical
spine. Kapandji provides a useful description of this region by
dividing it into four planes [10]. The deep plane consists of
the suboccipital and segmentally located transversospinal
muscles. The semispinalis plane contains the semispinalis
capitis and semispinalis cervicis. The plane of the splenius
and levator scapulae includes the splenius capitis, splenius
cervicis, levator scapulae, and longissimus capitis. The super¬
ficial plane is composed of the trapezius (Kapandji includes
the posterior part of the sternocleidomastoid, but this muscle
is discussed with the anterolateral group).

Deep Plane

SUBOCCIPITAL MUSCLES

The suboccipital muscles are deeply situated in the posterior
cervical area below the occipital region of the head (Fig. 27.1).
These muscles are a group of four deeply placed muscles
spanning the distance from the axis (C2) to either the atlas
(Cl) or the skull. Consequently, based upon their attach¬
ments and direction of muscle fibers, their combined actions
are to extend the head on the upper cervical spine while ipsi-
laterally they produce rotation and lateral flexion of the head
[20]. These muscles include the rectus capitis posterior major,
obliquus capitis inferior (inferior oblique), obliquus capitis
superior (superior oblique), and rectus capitis posterior
minor. The first three of the above participate in a significant
anatomical landmark, the suboccipital triangle. Located with¬
in this triangle are two important structures: the vertebral
artery and the suboccipital nerve (dorsal ramus of Cl).

Actions

MUSCLE ACTION: RECTUS CAPITIS POSTERIOR MAJOR
UNILATERAL ACTIVITY

Action

Evidence

Ipsilateral rotation

Supporting

Lateral bending

Supporting

MUSCLE ACTION: RECTUS CAPITIS POSTERIOR
MAJOR BILATERAL ACTIVITY

Action

Evidence

Extension of the head on the atlas

Supporting

MUSCLE ACTION: RECTUS CAPITIS POSTERIOR MINOR
UNILATERAL ACTIVITY

Action

Evidence

Ipsilateral rotation

Supporting

MUSCLE ACTION: RECTUS CAPITIS POSTERIOR MINOR
BILATERAL ACTIVITY

Action

Evidence

Extension of the head on the atlas

Supporting

Figure 27.1: Suboccipital muscles include the rectus capitis
posterior major and minor and the obliquus capitis superior
and inferior.

494

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ACTION: SUPERIOR OBLIQUE UNILATERAL
ACTIVITY

Action Evidence

Ipsilateral rotation Supporting

MUSCLE ACTION: SUPERIOR OBLIQUE BILATERAL
ACTIVITY

Action Evidence

Extension of the head on the atlas Supporting

MUSCLE ACTION: INFERIOR OBLIQUE UNILATERAL
ACTIVITY

Action

Evidence

Ipsilateral rotation

Supporting

Two of the four suboccipital muscles, the rectus capitis
posterior minor (Muscle Attachment Box 27.1 ) and the supe¬
rior oblique (Muscle Attachment Box 27.2), only extend from
the atlas to the skull, and their line of action produces atlanto-
occipital extension or lateral flexion, respectively. The rectus
capitis posterior major (Muscle Attachment Box 27.3 ) extends
from the spinous process of the axis to the occiput, and
the inferior oblique (Muscle Attachment Box 27.4 ) from the
spine of the axis to the transverse process of the atlas. The
transverse process of the atlas and the spine of the axis are
prominent and, therefore, the moment arms for these two
muscles are good for the production of atlanto-occipital
extension (rectus capitis posterior major) and rotation of the
atlas on the axis (inferior oblique).

The size of these muscles must be taken into consider¬
ation when evaluating their ability to produce force and
contribute in movements of the head and neck. They are
quite small compared with the large posterior muscles
superficial to them. It has been suggested that the suboc¬
cipital muscles may be active in “fine-tuning” head and
neck movements in response to the needs of the special
sense organs, while the larger muscles are the prime
movers and postural stabilizers over these joints. The find¬
ing that there is a large concentration of muscle spindles
located in small muscles such as the suboccipital muscles
supports this theory [2].

Clinical Relevance

CERVICAL HEADACHES: All of these muscles are innervated
by the dorsal ramus of Cl (suboccipital nerve), which exits
within the suboccipital triangle; superior to the arch of the
atlas. It is primarily a motor nerve but can have cutaneous
branches [33] that may result in pain if stretched or
trapped. More often , headaches of cervical origin have

MUSCLE ATTACHMENT BOX 27.1

ATTACHMENTS AND INNERVATION OF
THE RECTUS CAPITIS POSTERIOR MINOR

Proximal attachment: Posterior tubercle on
the posterior arch of Cl (atlas)

Distal attachment: Occipital bone inferior to
the inferior nuchal line

Innervation: Dorsal ramus of Cl (suboccipital nerve)
Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.2

ATTACHMENTS AND INNERVATION
OF THE SUPERIOR OBLIQUE

Proximal attachment: Superior surface of the
transverse process of Cl (atlas)

Distal attachment: Smaller lateral impression
between the superior and inferior nuchal lines
on the posterior aspect of the occipital bone

Innervation: Dorsal ramus of Cl (suboccipital nerve)

Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.3

ATTACHMENTS AND INNERVATION OF
THE RECTUS CAPITIS POSTERIOR MAJOR

Proximal attachment: Posterior edge of the spinous
process of C2 (axis)

Distal attachment: Occipital bone inferior to the
inferior nuchal line

Innervation: Dorsal ramus of Cl (suboccipital nerve)
Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.4

ATTACHMENTS AND INNERVATION
OF THE INFERIOR OBLIQUE

Proximal attachment: Lateral surface of the spinous
process of the C2 vertebra (axis)

Distal attachment: Inferior surface of the transverse
process of the Cl vertebra (atlas)

Innervation: Dorsal ramus of Cl (suboccipital nerve)

Palpation: Not palpable.

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

495

been attributed to the greater occipital nerve (dorsal ramus
of C2), which innervates much of the posterior aspect of the
head up to the vertex. This nerve exits below the inferior
oblique (external to the suboccipital triangle) and curves supe¬
riorly to pierce the semispinalis capitis (Fig. 27.1). It has been
suggested that entrapment or stretching of the nerve as it
passes between the lamina of the axis and the inferior oblique
muscle may result in headaches or posterior neck pain [3].

The suboccipital muscles are deep and difficult to pal¬
pate. Several layers of large muscles and dense fascia are
interposed between the skin and this muscle group.
Empirically , then , it would be difficult to isolate pain result¬
ing from muscular tightness or trigger points as coming
from these muscles. Kendall describes pain associated with
muscle tightness in this area as a result of postural prob¬
lems [11]. She observes that patients with a marked for¬
ward head and kyphotic upper thoracic region have a
compensatory hyperextension of the cervical spine and
head (Fig. 27.2). This position may lead to shortening of
the suboccipital muscles and "stretch weakness" of the
anterior neck muscles. The mechanism of pain would be
an abnormally large compression force on the articular
facets due to the altered and sustained pull of the short¬
ened muscles. However ; the specific association between
impairments of the suboccipital muscles and patient symp¬
toms remains theoretical.

TRANSVERSOSPINAL MUSCLES

This group of muscles occupies the space between the trans¬
verse and spinous processes of the very short fibers (extend¬
ing a few segments) and a relatively small moment arm for
joint movement. Anatomy texts describe two layers of muscle
located in this gutter area. The deeper of the two layers is

Figure 27.2: Forward head position shows that the hyperexten¬
sion at the cervical spine could result in shortening of the neck
extensor musculature.

MUSCLE ATTACHMENT BOX 27.5

ATTACHMENTS AND INNERVATION
OF THE MULTIFIDUS

Proximal attachment: Spinous processes and
laminae of C2-C7 vertebrae, spanning one to
three vertebrae

Distal attachment: Transverse processes of upper
thoracic vertebrae and articular processes of C7-T2

Innervation: Dorsal rami of cervical spinal nerves

Palpation: Not palpable.

made up of the rotatores muscle, but this layer is only well
developed in the thoracic region and is not relevant to the
cervical region [17,26]. The multifidus muscle makes up the
more superficial layer (Muscle Attachment Box 27.5).
Anatomy texts describe this muscle quite simply as fibers that
arise from transverse processes and extend two to four
segments to attach on the spinous processes above.

Actions

MUSCLE ACTION: MULTIFIDUS UNILATERAL ACTIVITY

Action

Evidence

Lateral flexion

Insufficient

Contralateral rotation

Insufficient

MUSCLE ACTION: MULTIFIDUS BILATERAL ACTIVITY

Action

Evidence

Extension of the spine

Insufficient

Further examination of this muscle group demonstrates a
much more complicated picture of fiber arrangement and
innervation pattern [13] (Fig. 27.3). Macintosh demonstrates,
based on innervation, that the multifidus runs in a spino-
transverse direction rather than a transversospinal orientation
and has many important functions in spinal stabilization. The
multifidus, however, is much more developed in the lumbar
region, and studies describing its function usually are con¬
fined to effects on the lumbar spine. On the basis of its small
size, deep location, and relatively poor moment arm, it can be
hypothesized that it may act more as an organ of propriocep¬
tion than as a prime mover in the cervical region.

Semispinalis Plane

SEMISPINALIS CAPITIS AND CERVICIS

The semispinalis capitis and cervicis constitute a large
group of muscle fibers that originate from the transverse
processes of the upper thoracic vertebrae. The capitis

496

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 27.3: Fibers in the multifidus run obliquely superiorly and
medially.

Figure 27.4: This figure demonstrates the excellent line of pull of
the semispinalis capitis for neck extension. The convergence of
semispinalis cervicis on the spinous process of C2 is also evident.

(.Muscle Attachment Box 27.6) inserts centrally on the occi¬
pital bone between the superior and inferior nuchal lines
(Fig. 27.4). The cervicis fibers converge on the spinous
processes of C2 through C5 with the largest concentration
on C2 (Muscle Attachment Box 27.7). This muscle is bulky
and easily palpable just lateral to the ligamentum nuchae in
the upper cervical region.

Actions

The arrangement of fibers in these muscles makes them
important extensors of the head and neck, but EMG studies
report conflicting evidence of action during functional activi¬
ties [4,12,23,27].

MUSCLE ATTACHMENT BOX 27.6

ATTACHMENTS AND INNERVATION
OF THE SEMISPINALIS CAPITIS

Proximal attachment: Transverse processes of C7 and
T1-T6 vertebrae

Distal attachment: Medial half of the area between
the superior and inferior nuchal lines on the occipi¬
tal bone

Innervation: Dorsal rami of cervical spinal nerves

Palpation: Deep to the upper trapezius and levator
scapulae and not palpable.

MUSCLE ACTION:
ACTIVITY

SEMISPINALIS CAPITIS UNILATERAL

Action

Evidence

Extension with slight
lateral flexion

Supporting

MUSCLE ACTION: SEMISPINALIS CAPITIS BILATERAL
ACTIVITY

Action

Evidence

Extension of the head

Supporting

Extends the cervical spine

Supporting

Accentuation of
cervical lordosis

Supporting

MUSCLE ACTION: SEMISPINALIS CERVICIS UNILATERAL
ACTIVITY

Action

Evidence

Extension of cervical spine

Supporting

Lateral flexion of lower
cervical spine

Supporting

MUSCLE ACTION: SEMISPINALIS CERVICIS BILATERAL
ACTIVITY

Action

Evidence

Extension of the lower
cervical spine

Supporting

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

497

MUSCLE ATTACHMENT BOX 27.7

ATTACHMENTS AND INNERVATION
OF THE SEMISPINALIS CERVICIS

Proximal attachment: Transverse processes of T1-T6

Distal attachment: Cervical spinous processes of
C2-C5

Innervation: Dorsal rami of cervical spinal nerves

Palpation: Deep to the upper trapezius and levator
scapulae and not palpable.

These two muscles are perhaps the prime movers for cer¬
vical spine and head extension. Their line of pull, from the
upper thoracic area to the occiput, is well positioned to pro¬
duce pure extension and maintenance of the cervical lordosis
[18]. In the cadaver, the semispinalis cervicis is striking for its
large convergence of fibers on the spinous process of C2.
Indeed, this is a landmark for locating this structure (C2) and
the associated suboccipital muscles arising from it. This con¬
vergence suggests that the semispinalis cervicis has an impor¬
tant stabilizing function on the axis that improves the ability
of the rectus capitis posterior major and the inferior oblique
to carry out their functions.

As straightforward as the actions of these two muscles
appear to be from their anatomical positions, controversy
exists in the literature regarding their activity during various
activities. The location of the semispinalis group is superficial
enough to allow investigators to record activity using fine wire
EMG electrodes. Pauley reports that the semispinalis capitis
and cervicis are continually active during upright posture to
help support the head [21]. A later study verifies that the
major function of this muscle group is extension of the head
on the neck but notes that the semispinalis muscles are silent
during quiet standing when the head is balanced over the cer¬
vical spine [27]. In contrast to the usual description in anato¬
my texts, EMG data suggest that the semispinalis group does
not participate in rotation of the head or cervical spine [27].

Clinical Relevance

IMPAIRMENTS OF THE SEMISPINALIS CAPITIS
MUSCLE: The previously described course of the greater
occipital nerve included the fact that it pierces the
semispinalis capitis on its way to the vertex of the head
(Fig. 27.1). Entrapment or tension on the nerve can occur
within the semispinalis capitis. Travell describes a condition
in which pain and burning occur in the distribution of the
greater occipital nerve in response to spasms in the semi¬
spinalis capitis [28]. As this muscle group is often activated
during normal upright activities , continued irritation might
occur during ordinary activities of daily living.

No known studies directly address weakness in these mus¬
cles, but it can be hypothesized that maintenance of upright
head posture would be compromised by weakness of the
semispinalis muscles. Because the semispinalis cervicis may
stabilize the axis and potentiate the function of two of the
suboccipital muscles , weakness in the semispinalis cervicis
could affect the ability of these suboccipital muscles to fine-
tune head movements in response to stimuli.

Splenitis and Levator Scapulae Plane

SPLENIUS CAPITIS AND CERVICIS

The splenius muscles are a large flat group that covers
the superior-medial aspect of the posterior neck (Fig. 27.5)
(.Muscle Attachment Box 27.8). This muscle group is consid¬
ered a spinotransverse muscle group because it originates
medially on spinous processes and passes laterally and supe¬
riorly to attach to cervical transverse processes and the skull.
The lower fibers insert into the posterior tubercles of the
upper two or three cervical vertebrae posterior to the attach¬
ment of the levator scapulae and are, therefore, named sple¬
nius cervicis. The remainder of the muscle fibers course
superolaterally to the lateral half of the superior nuchal line
and the mastoid process. This part is called the splenius
capitis.

Actions

This muscle group exerts force over both the cervical spine
and the atlanto-occipital joint (head on neck).

direction to attach to the skull and transverse processes of cervi¬
cal vertebrae, respectively.

498

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 27.8

ATTACHMENTS AND INNERVATION OF
THE SPLENIUS CAPITIS AND CERVICIS

Proximal attachment: Inferior one half of mastoid
process of the temporal bone, the ligamentum
nuchae and the spinous processes of T1-T6 vertebrae

Distal attachment: Capitis—lateral aspect of the
mastoid process and the lateral one third of the
superior nuchal line of the occipital bone (deep
to the sternocleidomastoid m.); cervicis—posterior
tubercles of the transverse processes of C1-C4
vertebrae (posterior to the levator scapulae m.)

Innervation: Dorsal rami of cervical spinal nerves

Palpation: Deep to the upper trapezius and levator
scapulae and not palpable.

MUSCLE ACTION: SPLENIUS CAPITIS AND CERVICIS
UNILATERAL ACTIVITY

Action

Evidence

Extension of the head
and cervical spine

Supporting

Lateral flexion of the
head and cervical spine

Supporting

Ipsilateral rotation

Supporting

MUSCLE ACTION: SPLENIUS CAPITIS AND CERVICIS
BILATERAL ACTIVITY

Action

Evidence

Extension of the head
and cervical spine

Supporting

Accentuation of cervical
lordosis

Supporting

According to Basmajian [1], the splenius capitis is extremely
active in ipsilateral neck rotation and may be as important as
the sternocleidomastoid in this function. The splenius group
is intermediate in depth in this region and thus has an excel¬
lent moment arm for extension and rotation of the head and
neck. EMG studies show that the splenius group is very active
during extension of the head and cervical spine [1] but rela¬
tively silent during normal standing posture without head
movement [27].

Clinical Relevance

IMPAIRMENTS OF THE SPLENIUS CERVICIS: Little
information is found concerning clinical syndromes and the
splenius group. Kendall names it as one of the muscles

affected by the posture of forward head with slumped ,
round upper back and hyperextension of the cervical spine
(Fig. 27.2) [11]. In this condition, the splenius capitis is theo¬
retically shortened', which contributes to an overall increase
in compression on the posterior elements of the articular
processes and vertebral bodies. This evidence is largely
anecdotal and further research is necessary to verify these
relationships.

Calliet suggests that during car accidents in which the
vehicle is stopped abruptly (front end collision), the neck is
forcefully flexed, which quickly stretches posterior tissues [3].
Posterior extensor muscles are "overwhelmed" and are torn
before the neuromuscular system can prevent it. Pain after¬
ward is felt in the neck locally and referred in the distribu¬
tion of the myotomes and dermatomes. The splenius group
is likely involved in this type of injury.

LEVATOR SCAPULAE
Actions

Functionally this muscle is usually considered with the mus¬
cles that rotate or fix the scapula (see Chapter 9). Its proxi¬
mal attachments, however, are from the transverse processes
of the upper four cervical vertebrae and it can move the cer¬
vical spine when the scapula is fixed through synergistic
muscle action (Muscle Attachment Box 27.9).

MUSCLE ATTACHMENT BOX 27.9

ATTACHMENTS AND INNERVATION
OF THE LEVATOR SCAPULAE

Proximal attachment: Posterior tubercles of trans¬
verse processes of C1-C4 vertebrae

Distal attachment: Superior part of medial border of
scapula

Innervation: Dorsal scapular nerve (C5), ventral rami
of cervical nerves (C3 and C4)

Palpation: Deep to the upper trapezius, the levator
scapulae can be palpated between the upper
trapezius and sternocleidomastoid muscles. To pro¬
mote levator scapulae muscle action with a mini¬
mum of upper trapezius activity, ask the patient
to place the forearm in the small of the back to
downwardly rotate the scapula and then shrug
the shoulder.

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

499

MUSCLE ACTION: LEVATOR SCAPULAE UNILATERAL
ACTIVITY

Action

Evidence

Extension of the cervical spine
with scapula fixed

Supporting

Lateral flexion of the cervical
spine with scapula fixed

Supporting

Ipsilateral rotation of the cervical spine
with scapula fixed

Supporting

Scapular elevation, downward rotation
and adduction with cervical spine fixed

Supporting

MUSCLE ACTION: LEVATOR SCAPULAE-BILATERAL
ACTIVITY

Action

Evidence

Extension of the cervical spine
with scapula fixed

Supporting

Accentuation of cervical lordosis

Supporting

From the superior angle of the scapula, this muscle passes
medially and anteriorly to reach the transverse processes of the
upper four cervical vertebrae (Fig. 27.6). The location of this
relatively large neck muscle is significant both functionally and
clinically. As the muscle passes superiorly and anteriorly, it
twists from a frontal plane to a sagittal plane and separates into
distinctive fleshy slips that attach to the individual transverse
processes. Considered bilaterally, the levator scapulae appear to
be posterior “guy wires” that stabilize the cervical spine with co¬
contraction from antagonistic anterior muscles (Fig. 27.7). This
mechanism helps keep the head balanced over the cervical
spine more efficiently and helps maintain the cervical lordosis.

Figure 27.6: Levator scapulae runs superiorly and medially to
attach to transverse processes of cervical vertebrae.

Figure 27.7: The "guy wire" arrangement of the levator scapulae
and antagonistic anterior neck muscles. The levator scapulae
and anterior cervical muscles provide opposing forces that help
stabilize the cervical spine.

Clinical Relevance

NECK AND SHOULDER PAIN ASSOCIATED WITH
THE LEVATOR SCAPULAE: The levator scapulae
muscles probably play an important role , as previously
mentioned , in maintenance of optimal head and neck
alignment. In this role the muscle may be continuously
active to counterbalance the tendency for forward flexion.
Jull characterizes the levator scapulae as one of the
muscles in the neck-shoulder girdle region that becomes
overactive with poor posture such as forward-head [9J.
Over time , length-associated changes in this muscle
would occur as a result of this position , however , in the
short term , the overuse of the muscles could result in pain
and discomfort. Patients with neck pain and postural
problems often complain of pain in the region of the
upper medial scapula , and point tenderness is frequently
found at the superior-medial border of the scapula , site of
attachment of the levator scapulae. Articular structures in
this region may already be experiencing altered loads as
a result of suboptimal postural changes. Pain and spasm
of this muscle may further compromise these articular
structures , as they will not receive the adequate support
usually provided by the "guy wire" mechanism.

500

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 27.1

ATTACHMENTS AND INNERVATION
OF THE LONGISSIMUS CAPITIS

Proximal attachment: From superior thoracic trans¬
verse processes and the cervical transverse processes

Distal attachment: Mastoid process of the temporal
bone

Innervation: Dorsal rami of cervical spinal nerves

Palpation: Deep to the upper trapezius and levator
scapulae, it is not palpable.

LONGISSIMUS CAPITIS

This relatively small muscle is the most superior part of the
long, intermediately placed longissimus erector spinae
muscle (Muscle Attachment Box 27.10). It lies lateral to the
semispinalis capitis and proceeds to insert on the mastoid
process of the skull, deep to the attachment of the splenius
capitis and sternocleidomastoid (Fig. 27.8).

Actions

MUSCLE ACTION: LONGISSIMUS CAPITIS UNILATERAL
ACTIVITY

Action

Evidence

Extension of the head

Supporting

Lateral flexion of the head

Supporting

Ipsilateral rotation of the

Supporting

head and cervical spine

Figure 27.8: Longissimus capitis muscle lies lateral to the semi¬
spinalis capitis.

MUSCLE ACTION: LONGISSIMUS CAPITIS BILATERAL
ACTIVITY

Action

Evidence

Extension of the head

Supporting

This muscle is far smaller than the semispinalis capitis,
closer to the joints (reduced mechanical advantage), and
more laterally placed, so its lateral flexion moment arm
enhances the muscle s role in frontal plane stabilization as one
of the “guy wires” arranged around the skull. The longissimus
capitis appears to provide little stabilization in the sagittal
plane, probably because of its lateral position [10].

Superficial Plane

TRAPEZIUS

This muscle is immediately deep to the superficial fascia and
skin of the posterior neck region (Fig. 27.9). It is a very large
flat muscle extending from the superior nuchal line of the
skull to the spine of the twelfth thoracic vertebra (Muscle
Attachment Box 27.11). Its upper fibers course inferolaterally
to the clavicle and acromion, its middle fibers pass to the
scapular spine, and its lower fibers pass to the tubercle on the
base of the spine of the scapula. It is apparent from a posterior
view of the trapezius that this muscle anchors the shoulder
girdle to the axial skeleton. The primary function of the
trapezius is movement of the shoulder girdle and associated
movements of the upper extremity (Chapter 9); however,
when the scapulae are fixed, it may act on the head and cervical
spine. The trapezius is easily palpable and is responsible for
the contour of the lateral neck area.

Actions

MUSCLE ACTION: TRAPEZIUS UNILATERAL ACTIVITY

Action

Evidence

Lateral flexion of the cervical
spine with scapula fixed

Supporting

Contralateral rotation of the
cervical spine with scapula fixed

Supporting

Scapular elevation, depression,

Supporting

upward rotation, and adduction

MUSCLE ACTION: TRAPEZIUS BILATERAL ACTIVITY

Action

Evidence

Extension of the head

Supporting

Increase cervical lordosis

Supporting

Owing to the expanse of this muscle and the variety of
fiber directions, the trapezius needs to be separated into
three parts. The lower fibers depress the scapula. The mid¬
dle fibers adduct the scapula. The upper fibers elevate the tip
of the shoulder; acting with the lower fibers they rotate the
scapula so that the glenoid fossa faces superiorly (to facilitate

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

501

Figure 27.9: A. The three parts of the trapezius constitute a very large, flat back muscle primarily involved in upper extremity move¬
ments. B. The sternocleidomastoid forms the anterior border of the posterior triangle whose posterior border is the upper trapezius.

MUSCLE ATTACHMENT BOX 27.11

ATTACHMENTS AND INNERVATION
OF THE TRAPEZIUS

Proximal attachment: Medial one third of superior
nuchal line, external occipital protuberance, liga-
mentum nuchae, spinous processes of C7-T12

Distal attachment: Lateral one third of clavicle,
acromion, and spine of scapula

Innervation: Spinal root of accessory nerve (CN XI),
cervical nerves (C3 and C4)

Palpation: Palpate the entire trapezius by asking the
patient to abduct the shoulder and adduct the
scapula. To activate upper trapezius only, ask the
patient to elevate the scapula (shrug the shoulder)
and palpate between the spine of the scapula or
acromion and the medial one third of the superior
nuchal line.

glenohumeral motions). Acting on a fixed scapula, the upper
fibers flex the neck laterally and rotate it to the contralateral side
as a synergist to the ipsilateral sternocleidomastoid. Contracting
bilaterally and acting on a fixed scapula, these fibers reportedly
extend the head and increase the cervical lordosis.

Clinical Relevance

WEAKNESS AND STRAINS OF THE TRAPEZIUS
MUSCLE: The trapezius is commonly evaluated during a
neurological examination because it is innervated by a cra¬
nial nerve. Paralysis of the trapezius results in an inferiorly
sagging shoulder tip. In addition , the inferior angle of the
scapula protrudes dorsally and creates a ridge in the skin of
the back that disappears with flexion of the upper extremity
and becomes more pronounced during glenohumeral
abduction [26] (Fig. 27.10). This is in contrast to the "winging"
observed with paralysis of the serratus anterior ; which is

( continued )

502

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 27.10: With a weak trapezius there is a dorsal protrusion
of the inferior angle of the scapula while attempting to abduct
the glenohumeral joint.

(Continued)

made worse during glenohumeral flexion and helps in the
differential diagnosis between injuries to the respective
nerves (Chapter 9).

Usually, the cardinal sign of trapezius paralysis (and pos¬
sible injury to the accessory nerve) is drooping and inability
to elevate the tip of the ipsilateral shoulder. For the superior
fibers of the trapezius to elevate the shoulder ; however ;
antagonistic muscles must stabilize the cervical spine and
skull. Injury to these antagonistic muscles would result in an
inability of the trapezius to elevate the scapula, appearing
to be a problem with the trapezius or accessory nerve.
Porterfield and DeRosa have identified these supporting
muscles as the longus colli and longus capitis; which stabi¬
lize the head and neck and prevent extension moments [23].
These authors also describe the pathomechanics of shoulder
limitations after acceleration injuries such as whiplash. With
this type of injury, there is an uncontrolled extension move¬
ment that can injure the anterior muscles (longus colli and
capitis). These muscles are no longer able to stabilize the
head and neck and provide a stable base for the trapezius
to act [23].

The mechanics of acceleration injuries such as whiplash
are complex and beyond the scope of this chapter. The

above description of the injury indicates that there is an
uncontrolled extension motion that can damage the anteri¬
or musculature. Usually, following this extension move¬
ment, there is rapid forward translation of the head and,
subsequently, flexion movement that can damage the
posterior elements including musculature such as the supe¬
rior fibers of the trapezius. Calliet finds that following this
type of injury the upper trapezius becomes tender and
nodular [3j.

FLEXORS OF THE HEAD AND NECK

This group includes muscles that are located antero-
| laterally around the neck and defy anything global
Xy organization. The muscles that are discussed in this
region include the sternocleidomastoid, longus colli, longus
capitis, scalenes, and the two anterior rectus muscles.

STERNOCLEIDOMASTOID

This large muscle passes from the medial clavicle and
manubrium of the sternum to the mastoid process and lateral
half of the superior nuchal line (Muscle Attachment Box 27.12).
It is superficial, is easily palpated, and, because of its exten¬
sive course, has many functions (Fig. 27.9). Anatomically,
it forms the anterior border of the posterior triangle
an d the lateral border of the anterior triangle.
Asymmetries in these triangles may be observable in
clinical conditions such as forward head posture, primarily
because of the prominence of the sternocleidomastoid.

MUSCLE ATTACHMENT BOX 27.12

ATTACHMENTS AND INNERVATION
OF THE STERNOCLEIDOMASTOID

Proximal attachment: Superior attachment—lateral
surface of the mastoid process of the temporal bone
and the lateral one half of the superior nuchal line
of the occipital bone

Distal attachment: Sternal head—anterior surface of
the manubrium of the sternum, lateral to the jugu¬
lar notch; clavicular head—superior surface of the
medial one third of the clavicle

Innervation: Spinal root of the accessory nerve (CN
XI) and branches of the 2nd and 3rd cervical nerves
(C2 and C3)

Palpation: With the subject seated, palpate along a
line between the mastoid process and the sternoclav¬
icular joint. Ask the subject to turn the his or her head
to the opposite side you are palpating.

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

503

Actions

MUSCLE ACTION: STERNOCLEIDOMASTOID
UNILATERAL ACTIVITY

Action

Evidence

Extension of the head

Supporting

Lateral flexion

Supporting

Contralateral rotation of
the head and neck

Supporting

MUSCLE ACTION: STERNOCLEIDOMASTOID

BILATERAL ACTIVITY

Action

Evidence

Extension of the head

Supporting

Flexion of the cervical spine

Supporting

EMG analysis of the superficially located sternocleidomas¬
toid has been provided by two investigators [5,31]. They
report that the sternocleidomastoid is quiet in relaxed sitting,
breathing, deep expiration, and swallowing. As expected,
there is marked activity during resisted neck flexion, side¬
bending, and rotation to the opposite side. Inspiration, cough¬
ing, and resisted backward extension elicit variable activity.
This evidence clearly indicates the varied and frequent activ¬
ity of this important muscle.

Clinical Relevance

TORTICOLLIS: Probably the most common clinical condi¬
tion involving the sternocleidomastoid is torticollis
(Fig. 27.11). There are two general forms of torticollis:
congenital and spasmodic. The most common congenital
form is the prenatal development of a fibrous tissue tumor
in the sternocleidomastoid ' turning the infant's head to one
side in utero [25]. This may result in a breech delivery and
subsequent tearing of sternocleidomastoid fibers or damage
to the accessory nerve. Fibrosis and shortening of the fibers
may lead to the torticollis.

Spasmodic torticollis is a condition in which there is
involuntary contraction of the sternocleidomastoid ' resulting
in repeated or sustained lateral flexion, rotation , and exten¬
sion of the head and neck [6[. It usually occurs in individu¬
als between the ages of 20 and 60 and may involve more
than one muscle. It is usually accompanied by neck pain.

The previous discussion of the trapezius noted that accel¬
eration injuries often damage anterior structures including
muscles that are active in resisting extension moments [23].
In support of this; McNab finds that the sternocleidomastoid
is the most commonly damaged muscle during an accelera¬
tion injury in which the impact comes from behind [16]. This
follows from the knowledge that the sternocleidomastoid is
a strong flexor of the cervical spine and may be stretched or
injured during such an impact.

Figure 27.11: Torticollis is the deformity resulting from tightness
or spasm of the sternocleidomastoid muscle. It consists of ipsilat-
eral side-bending and contralateral rotation of the head.

LONGUS CAPITIS AND LONGUS COLLI

These prevertebral muscles are located deep in the anterior
neck and cover the cervical vertebrae (Fig. 27.12). The longus
capitis extends superomedially from cervical transverse
processes to the basilar part of the occipital bone (Muscle
Attachment Box 27.13). The longus colli has a much more
complicated arrangement (Muscle Attachment Box 27.14).
Inferior fibers pass superolaterally, superior fibers pass super¬
omedially, and intermediate fibers travel straight up from
lower cervical levels to upper cervical segments. The shape of
the muscle is triangular.

Actions

These muscles function to stabilize the head and neck as well
as flex them.

MUSCLE ACTION: LONGUS CAPITIS UNILATERAL
ACTIVITY

Action

Evidence

Ipsilateral rotation of the head

Insufficient

MUSCLE ACTION: LONGUS CAPITIS BILATERAL
ACTIVITY

Action

Evidence

Flexion of the head

Insufficient

504

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Rectus capitis:
Anterior

Figure 27.12: Deep flexor muscles include the longus colli and longus
capitis, the rectus capitis anterior and lateralis, and the scalenes.

MUSCLE ATTACHMENT BOX 27.13

ATTACHMENTS AND INNERVATION
OF THE LONGUS CAPITIS

Proximal attachment: Superior attachment—basilar
part of occipital bone

Distal attachment: Inferior attachment—anterior
tubercles of C3-C6 transverse processes

Innervation: Ventral rami of C1-C3
Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.14

ATTACHMENTS AND INNERVATION
OF THE LONGUS COLLI

Proximal attachment: Inferior attachment—bodies
of C5-T3 vertebrae, transverse processes of C3-C5
vertebrae

Distal attachment: Lowest fibers insert on transverse
processes of C3-C5, superior fibers insert on bodies
of C1-C3 and anterior tubercle of the atlas

Innervation: Ventral rami of C2-C6
Palpation: Not palpable.

MUSCLE ACTION: LONGUS COLLI UNILATERAL
ACTIVITY

Action

Evidence

Lateral flexion of the cervical spine

Supporting

Ipsilateral rotation of the cervical spine

Insufficient

MUSCLE ACTION: LONGUS COLLI BILATERAL ACTIVITY

Action

Evidence

Cervical flexion

Supporting

These two muscles are especially active in protection of
anterior structures during forceful extension motions. EMG
recordings of the longus colli show activity patterns similar to
those of the sternocleidomastoid muscle: quiet in relaxed sit¬
ting and breathing, with marked activity during resisted flex¬
ion and side-bending. No known EMG studies investigate the
longus capitis.

Clinical Relevance

WHIPLASH INJURIES TO THE LONGUS COLLI AND
CAPITIS: Injury to these muscles is discussed in the trapezius
muscle section. Forceful hyperextension movements of the neck
(auto accident) may stretch and tear the longus colli and capi¬
tis , thereby reducing the ability of these muscles to provide a
stable base on which the trapezius muscle can act Palpation in
the region of the longus colli and capitis muscles and/or resis¬
ted neck flexion (such as raising the head in the supine posi¬
tion) would be painful. In the short term , the patient may not
even be able to lift the head while lying down , although the
sternocleidomastoid and scalene muscles may substitute and
provide an adequate flexion moment to flex the neck [23].

RECTUS CAPITIS LATERALIS AND RECTUS CAPITIS
ANTERIOR

These two muscles arise from the anterior part of the atlas
and insert on the base of the skull (Muscle Attachment Boxes
27.15 and 27.16). They are, therefore, very short, have a lim¬
ited moment arm, and probably do not produce significant
force (Fig. 27.12). Their lines of action suggest that they flex
the head on the atlas when contracting bilaterally and perhaps
laterally flex when acting alone. No EMG data are available,
as these muscles are quite deep and in a dangerous location
for fine wire EMG.

MUSCLE ACTION: RECTUS CAPITIS ANTERIOR
AND LATERALIS UNILATERAL ACTIVITY

Action

Evidence

Lateral flexion of the cervical spine

Insufficient

MUSCLE ACTION: RECTUS CAPITIS ANTERIOR

AND LATERALIS BILATERAL ACTIVITY

Action

Evidence

Flexion of the head

Insufficient

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

505

MUSCLE ATTACHMENT BOX 27.15

ATTACHMENTS AND INNERVATION
OF THE RECTUS CAPITIS ANTERIOR

Proximal attachment: Base of the skull just anterior
to the occipital condyle

Distal attachment: Anterior surface of the lateral
mass of the atlas

Innervation: Branches from loop between Cl and C2
spinal nerves

Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.19

ATTACHMENTS AND INNERVATION
OF THE POSTERIOR SCALENE

Proximal attachment: Posterior tubercles of trans¬
verse processes of C4-C6 vertebrae

Distal attachment: External border of second rib

Innervation: Ventral rami of C7 and C8

Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.16

ATTACHMENTS AND INNERVATION
OF THE RECTUS CAPITIS LATERALIS

Proximal attachment: Jugular process of the occipi¬
tal bone

Distal attachment: Transverse process of the atlas

Innervation: Branches from loop between Cl and C2
spinal nerves

Palpation: Not palpable.

MUSCLE ATTACHMENT BOX 27.17

ATTACHMENTS AND INNERVATION
OF THE ANTERIOR SCALENE

Proximal attachment: Posterior tubercles of
transverse processes of C3-C6

Distal attachment: Superior surface of first rib,
anterior to groove for subclavian artery

Innervation: Ventral rami of C4-C6

Palpation: Palpated posterior to the inferior
portion of the sternocleidomastoid muscle.

MUSCLE ATTACHMENT BOX 27.18

ATTACHMENTS AND INNERVATION
OF THE MIDDLE SCALENE

Proximal attachment: Posterior tubercles of
transverse processes of C4-C6 vertebrae

Distal attachment: Superior surface of first rib,
posterior groove for subclavius artery

Innervation: Ventral rami of cervical spinal nerves
Palpation: Not palpable.

SCALENE MUSCLES

This is a group of three deeply placed muscles in the lateral
region of the neck that arise from the transverse processes of
cervical vertebrae (Muscle Attachment Boxes 27.17, 27.18,
and 27.19). The anterior and middle scalenes attach to the
first rib on either side of the neurovascular bundle leaving
the root of the neck (Fig. 27.13). This bundle includes the
subclavian artery and brachial plexus, which create a shallow
groove on the first rib. The posterior scalene passes to the
second rib, running just anterior to the levator scapulae.

Actions

This group generally functions together to laterally flex the
neck, stabilize the neck with the actions of other cervical mus¬
cles, and elevate the ribs as accessory muscles of respiration.

Figure 27.13: The scalenes are intimately related to the brachial
plexus and subclavian artery, which pass between the anterior
and middle scalenes.

506

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ACTION: SCALENES UNILATERAL ACTIVITY

Action

Evidence

Lateral flexion of the cervical spine

Supporting

Contralateral rotation

Supporting

Elevation of ribs

Supporting

MUSCLE ACTION: SCALENES BILATERAL ACTIVITY

Action

Evidence

Flexion of cervical spine

Supporting

EMG recordings from the anterior scalene confirm its
action in neck flexion and rotation [1]. The three muscles
appear to be perfectly positioned to help stabilize the verte¬
bral column in synergy with the larger muscles around the
head and neck.

Clinical Relevance

SCALENUS ANTICUS SYNDROME: The narrow triangu¬
lar opening between the anterior scalenethe middle sca¬
lene, and their attachments on the first rib transmits the
subclavian artery and brachial plexus and is a potential
problem site (Fig. 27.13). Compression of these structures in

this space can lead to symptoms such as diminished
sensation, weakness; "pins and needles" paresthesia, and
pain. Patients with this anterior scalene syndrome (scalenus
anticus syndrome) complain of numbness and tingling in
the arm and fingers [3]. The exact cause of this condition is
unknown; however; hypotheses include muscle spasm as a
result of exercise, anxiety, tension, trauma, or tightness
resulting from postural problems.

MUSCLE FUNCTION IN
THE CERVICAL SPINE

Based on previous descriptions and as a review, the muscles
of the their respective functions can be found in Table 27.1.
With the actions of these muscles described and their clinical
relevance discussed, their interaction can be explored from
both a mechanical and a motor control sense.

As noted in Chapter 26, the cervical spine is composed of
seven cervical vertebrae. These are designed to support the
head in space while at the same time allowing movement of
the head for interaction with the surrounding environment.
Two major functions of the neck musculature are (a) to stabi¬
lize the head during external perturbations or body movements
and (b) to provide orienting or voluntary head movements [22].
Stability implies support and is related to the stiffness of the

TABLE 27.1: Cervical Muscles Grouped According to Their Actions

Action

Group

Muscle Name

Extension

Flexion

Lateral

Flexion

Rotation

Extensor

Rectus capitis posterior major

Bilateral

Ipsilateral

Rectus capitis posterior minor

Bilateral

Ipsilateral

Superior oblique

Bilateral

Ipsilateral

Inferior oblique

Ipsilateral

Semispinalis capitis

Bi-, unilateral

Ipsilateral

Semispinalis cervicis

Bi-, unilateral

Ipsilateral

Splenius capitis and cervicis

Bi-, unilateral

Ipsilateral

Levator scapulae

Bi-, unilateral

Ipsilateral

Longissimus capitis

Bi-, unilateral

Ipsilateral

Trapezius

Bilateral

Ipsilateral

Contralateral

Flexor

Sternocleidomastoid

Bilateral

Ipsilateral

Contralateral

Longus colli

Bilateral

Ipsilateral

Longus capitis

Bilateral

Ipsilateral

Rectus capitis lateralis

Bilateral

Ipsilateral

Rectus capitis anterior

Bilateral

Ipsilateral

Scalene muscles

Bilateral

Ipsilateral

Contralateral

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

507

supporting structure. In the vertebral column, muscular and
ligamentous connections provide this stiffness. Cervical
mobility is provided, in part, by the discs that separate each
vertebral component. Facets that articulate with the facets of
adjacent vertebrae guide this movement. Facet orientation
promotes motion in certain directions while limiting motion
in others. The cervical vertebrae also act to protect neural and
vascular structures associated with the region and serve as
outriggers for the attachment of muscles and ligaments.

Motion of the cervical spine is described in detail in
Chapter 26. A typical cervical motion segment (two adjacent
vertebrae and the intervening disc) has six degrees of free¬
dom (DOF), translations in each plane and rotations in each
axis (Fig. 27.14). These motions are often coupled such that
motions around one axis are consistently associated with
motions around another axis. The coupling relationships
depend on the spinal posture, orientation of the articulating
facets, thickness of the intervertebral disc, and extensibility of
the muscles surrounding the joint.

In normal posture, the cervical vertebrae form a lordotic
curve. This curve is supported and accentuated by the semi-
spinalis capitis, splenius capitis and cervicis, trapezius, and
levator scapulae muscles. In a cervical lordotic position, ante¬
rior and posterior stresses on the cervical vertebral bodies are
nearly uniform and minimal compared with those in other
postures [7]. With a kyphotic cervical posture, compression
forces on the anterior margins of the vertebrae can be 6 to 10
times larger [7].

The cervical lordosis improves the ability of the spine to
absorb axial loads. When a cervical spine is straightened and
axially loaded, the time to failure and total displacement at
failure are significantly lower than their lordotic counterparts
[19]. This implies that a nonlordotic cervical spine has a

decreased ability to absorb axial force. The presence of the
cervical lordosis provides shock absorption for the head from
forces that are transferred from the body and lower extremi¬
ties. Similar curves in the thoracic and lumbar spine also con¬
tribute to shock absorption. Loss of the cervical lordosis can
result in a decrease in shock absorption capability of the spine
[32]. Cervical lordosis results from the shape of the discs and
vertebrae in this region.

For the entire spine, intervertebral discs make up 20-30%
of the column length. The discs increase in size from the cer¬
vical to lumbar region. The ratio of disc thickness to vertebral
body thickness is greatest in the cervical and lumbar regions
and least in the thoracic; the higher the ratio, the greater the
mobility due to a greater range of motion (ROM) at the sym¬
physis joints between the vertebral bodies.

Muscle Interactions and Activation
Patterns

Redundancies in the musculature of the cervical spine allow
multiple muscles to perform similar actions. In essence, there
are more muscles than there are motions. As a result, there
can be a variety of muscle activation patterns that produce or
contribute to a single movement. A single muscle can poten¬
tially contribute to head movements in multiple directions
[12]. For any given cervical muscle, the muscle moment arm,
line of action, and muscle force production determine the
resulting action of the head.

Activation of multiple muscles to produce a movement is
called a muscle synergy. This method of control is not exclu¬
sive to the cervical region and occurs throughout the human
body. An example of a synergy in the cervical region is the
activation of the trapezius muscle and sternocleidomastoid

Figure 27.14: A typical cervical vertebral motion segment is able to translate along three axes and rotate about those three axes.
Consequently, the motion segment has six degrees of freedom (DOF), three translations and three rotations: A. side-to-side translation
in the frontal plane, B. superior-inferior translation, C. anterior-posterior translation in the sagittal plane, D. side-to-side rotation in
the frontal plane, E. rotation in the transverse plane around a superior-inferior axis, F. anterior-posterior rotation in the sagittal plane.

508

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 27.15: Sagittal plane view of the head and neck illustrates
a flexion moment (M) around the point of rotation, or axis, (O)
produced by the weight of the head (W). The weight of the head is
applied at the head's center of gravity (CG). The extensor muscula¬
ture must produce an extension moment (E) to balance the head.

muscle to create contralateral cervical rotation. These mus¬
cles produce opposing force vectors. The interaction of these
force vectors produces rotation and is an example of an
anatomical force couple. Force couples can theoretically
produce pure rotational movements by canceling opposing
translatory components. They also can produce motion in
directions not available from a single muscles line of action.

When a muscle is active, it produces a moment or torque
around the joint on which it is acting. Muscle activation results
during the initiation of voluntary activities and in response to
direct and indirect forces imposed on the system. External
forces produce moments as well. In a normal sitting posture,
there is an external flexion moment on the cervical spine due
to the weight of the head, which must be countered by an
internal extension moment if the head is to remain upright
(Fig. 27.15). The extension moment in this case is provided by

the neck extensor muscles bilaterally. In this situation, one of
three things can happen, (a) If the combination of the forces
results in no movement, the muscle contraction is defined as
isometric. The flexion moment created by gravity and the
extension moment created by the muscle activation are equal.
(b) If the combination of the forces results in acceleration of
the head in a flexion direction, the flexion moment created by
gravity is greater than the extension moment provided by the
muscles, and the muscle contraction is an eccentric, or
lengthening, contraction, (c) If the combination of forces results
in acceleration of the head in an extension direction, then the
extension moment provided by the muscles is greater than the
flexion moment created by gravity. The muscle contraction is a
concentric, or shortening, contraction.

By viewing the movement of the head as resulting from
an imbalance in moments, the reader can visualize the contri¬
butions of various muscle groups and types of contractions.
The moment produced by the weight of the head is a func¬
tion of body position. As a result, muscle activity differs with
body position. Some of these differences are highlighted in
Table 27.2. The descriptions of active muscles are, for the
most part, kept in an extensor/flexor format. The read-
er can refer to Table 27.1 for the specific muscles in
each of these groups.

Effects of Posture on Cervical Muscles

Most neck muscles maintain at least 80% of their peak force¬
generating capacity throughout full cervical ROM (29). The
analysis of neck musculature is complex, however, since
length-tension relationships affect the force-generation capa¬
bility of a given muscle. The length-tension relationship,
combined with moment arm changes throughout the ROM,
alters a muscles moment or torque-generating capability
(Chapter 4). On the basis of their muscle lengths, moment
arms, and EMG activation patterns, the posterior neck mus¬
cles appear to be most efficient when the head is in a neutral
position [14]. Muscle length, which is a function of head posi¬
tion, is probably the main influencing factor in this relation¬
ship, suggesting that maintaining a neutral head position is
important in reducing the load on the cervical extensor mus¬
cles. Cervical muscles with the largest moment arms include
the sternocleidomastoid muscles (flexion and lateral flexion),

TABLE 27.2: Types of Contractions of Cervical Muscles: How Position Alters the Muscle Group and Type
of Contraction Used during Specific Motions

Movement

Position

Active Muscle

Flexion

Sitting

Eccentric extensors bilaterally

Supine

Concentric flexors bilaterally

Extension

Sitting

Concentric extensors bilaterally followed by eccentric flexors (once cervical extension reaches the
point where gravity produces an extension moment)

Prone

Concentric extensors bilaterally

Lateral flexion

Sitting

Eccentric contralateral flexors and extensors

Side-lying

Concentric ipsilateral flexors and extensors

Rotation

Sitting

Ipsilateral splenius capitis, longissimus capitis, and levator scapula (if scapula fixed); contralateral
sternocleidomastoid and upper trapezius

Chapter 27 I MECHANICS AND PATHOMECHANICS OF THE CERVICAL MUSCULATURE

509

semispinalis capitis and splenius capitis muscles (extension),
and trapezius muscles (rotation) [29]. These muscles are
expected to be the most efficient at producing their respec¬
tive movements, but the magnitude of the moment produced
is not only a function of moment arm, but also a function of
muscle force production and the muscle s physiological cross-
sectional area. A positive correlation between muscle strength
and physiological cross-sectional area is found for muscles in
the cervical region just as in muscles throughout the appen¬
dicular skeleton [15].

Changes in posture alter the moment produced by the
weight of the head by changing the location of the heads
center of gravity with respect to the point of rotation in
the cervical spine. This relationship influences the way people
interact with their environment. The posture assumed while
working on a computer, for instance, can affect the muscles
used to perform that task. Data show that increased cervical
flexion produces increased EMG activation of the trapezius
muscles bilaterally in some subjects [30]. Backward leaning
(reclining the trunk) decreases the activation of the trapezius
muscles bilaterally in some subjects. Higher computer screen
heights result in subjects assuming a more erect cervical spine
posture and a more backward-leaning position. This example
demonstrates that there are a variety of changes in muscle
activity that may occur with relatively small modifications to a
work environment. The muscle forces required to compen¬
sate for different head positions can be modeled biomechan-
ically. Fig. 27.16 demonstrates that a forward head position
can result in a fourfold increase in the requirements of the
extensor musculature.

The interaction of cervical musculature has not yet been
fully explored, in part, because of the complex nature of the
instrumentation required to perform the measurements.
Cadaver studies and EMG studies combine to provide rough
estimates or ranking of muscle moment production capabili¬
ties [5,13,15,21,27]. Biomechanical models provide suggested

Figure 27.16: Biomechanical model of the force couples required
to balance the head in two different head positions. A. Neutral
head position requires 25 N of muscle force to balance the sys¬
tem. B. Forward head position requires 100 N of muscle force to
balance the system.

ergonomic solutions for posturally related problems
[7,14,18,29,33]. This section demonstrates that an understand¬
ing of the role of the cervical muscles in functional activities
requires attention to postural attitudes, since head alignment
affects the external moments on the cervical spine. The follow¬
ing chapter provides additional examples of the loads sustained
by the cervical spine.

SUMMARY

This chapter examines the discrete actions of each muscle of
the cervical spine as well as the combined actions of muscle
groups. Because the muscles of the cervical spine are usually
paired, the effects of unilateral and bilateral contractions are
also discussed. The extensors of the head and neck are
arranged in four planes from deep to superficial. Besides
extending the head and neck, many of these muscles con¬
tribute to lateral flexion and to either ipsilateral or contralat¬
eral rotation of the head and neck. Similarly, the flexors
provide side-bending and rotation. Evidence suggests that
many muscles contract together as synergists to move the
head and neck while stabilizing the region. Available studies
of the effects of muscle impairments are reviewed, and
clinical examples provided. Finally, this chapter demonstrates
that gravity and posture play important roles in determining
muscle activation in the cervical spine.

References

1. Basmajian JV, DeLucca C: Muscles Alive: Their Functions
Revealed by Electromyography. 5th ed. Baltimore: Williams &
Wilkins, 1985.

2. Buxton DF, Peck D: Neuromuscular spindles relative to joint
movement complexities. Clin Anat 1989; 2: 211-220.

3. Calliet R: Neck and Arm Pain. 3rd ed. Philadelphia: FA Davis,
1991.

4. Cromwell RF, Aadland-Monahan TK, Nelson AT, et al.: Sagittal
plane analysis of the head, neck, and trunk kinematics and elec¬
tromyographic activity during locomotion. 1 Orthop Sports Phys
Ther 2001; 31: 255-262.

5. deSousa T, Furlani J, Vitti M: Etude electromyographique du m.
sternocleidomastoideus. Electromyogr Clin Neurophysiol 1973;
13: 93-106.

6. Fahn S, Bressman SB, Brin MF: Dystonia. In: Rowland LP, ed.
Merritt’s Textbook of Neurology. Baltimore: Williams & Wilkins,
1995.

7. Harrison DE, Harrison DD, Janik TJ, et al.: Comparison of axial
and flexural stresses in lordosis and three buckled configurations
of the cervical spine. Clin Biomech 2001; 16: 276-278.

8. Jordan A, Mehlsen J, Bulow PM, et al.: Maximal isometric
strength of the cervical musculature in 100 healthy volunteers.
Spine 1999; 24: 1343-1348.

9. Jull GA: Headaches of cervical origin. Phys Ther Cerv Thorac
Spine 2000: 261-285.

10. Kapandji IA: Physiology of the Joints: Trunk and the Vertebral
Column. New York: Churchill Livingstone, 1974.

11. Kendall HO, Kendall FP, Boynton DA: Posture and Pain.
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12. Keshner EA, Campbell D, Katz RT, Peterson BW: Neck muscle
activation patterns in humans during isometric head stabiliza¬
tion. Exp Brain Res 1989; 75: 335-344.

13. Macintosh JE, Valencia F, Bogduk N, Munro RR: The morphology
of the lumbar multifidus muscles. Clin Biomech 1986; 1: 196-204.

14. Mayoux-Behamou MA, Revel M: Influence of head position on
dorsal neck muscle efficiency. Electromyogr Clin Neurophysiol
1993; 33: 161-166.

15. Mayoux-Behamou MA, Wybier M, Revel M: Strength and
cross-sectional area of the dorsal neck muscles. Ergonomics
1989; 32: 513-518.

16. McNab I: Acceleration injuries of the cervical spine. J Bone
Joint Surg [Am] 1964; 46: 1797-1799.

17. Moore KL, Dailey AF: Clinically Oriented Anatomy. 4th ed.
Baltimore: Lippincott Williams & Wilkins, 1999.

18. Nolan JP, Sherk HH: Biomechanical evaluation of the extensor
musculature of the cervical spine. Spine 1988; 13: 9-11.

19. Oktenoglu T, Ozer AF, Ferrara LA, et al.: Effects of cervical
spine posture on axial load bearing ability: a biomechanical
study. J Neurosurg 2001; 94(1 suppl): 108-114.

20. Palastanga N, Field D, Soames R: Anatomy of Human
Movement: Structure and Function. 3rd ed. Oxford:
Butterworth-Heinemann, 1998.

21. Pauley JE: An electromyographic analysis of certain movements
and exercises. Part I. Some deep muscles of the back. Anat Rec
1966; 155: 223-234.

22. Peterson BW, Keshner EA, Banovitz J: Comparison of neck
muscle activation patterns during head stabilization and volun¬
tary movements. Prog Brain Res 1989; 80: 363-371.

23. Porterfield JA, DeRosa C: Musculature of the Cervical Spine.
In: Porterfield JA, DeRosa C, eds. Mechanical Neck Pain:

Perspectives in Functional Anatomy. Philadelphia: WB
Saunders, 1995; 47-81.

24. Queisser F, Bluthner R, Brauer D, Seidel H: The relationship
between electromyogram amplitude and isometric extension
torques of the neck muscles at different positions of the cervical
spine. Eur J Appl Physiol Occup Physiol 1994; 68: 92-101.

25. Raffensperger JG: Congenital cysts and sinuses of the neck. In:
Raffensperger JG, ed. Swensons Pediatric Surgery. Norwalk,
CT: Appleton & Lange, 1990.

26. Stern JT: Essentials of Gross Anatomy. 1st ed. Philadelphia: FA
Davis, 1988.

27. Takebe K, Vitti M, Basmajian JV: The functions of semispinalis
capitis and splenius capitis muscles: an electromyographic study.
Anat Rec 1974; 179: 477-480.

28. Travell JG, Simmons DG: Myofascial Pain and Dysfunction.
Baltimore: Williams & Wilkins, 1983.

29. Vasavada AN, Li S, Delp SL: Influence of muscle morphometry
and moment arms on the moment-generating capacity of
human neck muscles. Spine 1998; 23: 412-422.

30. Villanueva MB, Jonai H, Sotoyama M, et al.: Sitting posture and
neck and shoulder muscle activities at different screen height
settings of the visual display terminal. Ind Health 1997; 35:
330-336.

31. Vitti M, Fujiwara M, Iida M, Basmajian JV: The integrated roles
of longus colli and sternocleidomastoid muscles: an electromyo¬
graphic study. Anat Rec 1973; 177: 471^84.

32. White AA, Panjabi MM: Clinical Biomechanics of the Spine.
2nd ed. Baltimore: Lippincott Williams & Wilkins, 1990.

33. Williams P, Bannister L, Berry M, et al.: Grays Anatomy: The
Anatomical Basis of Medicine and Surgery, Br. ed. London:
Churchill Livingstone, 1995.

CHAPTER

Analysis of the Forces on the
Cervical Spine during Activity

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE LOADS ON THE CERVICAL SPINE.511

LOADS ON THE CERVICAL SPINE .514

Static Loading of the Cervical Spine.514

Dynamic Loading of the Cervical Spine .515

SUMMARY.518

T he cervical spine is a common site for complaints of pain and stiffness, and cervical spine pathology also often
produces symptoms in the upper extremity. Although the cause of such complaints often is unclear, mechani¬
cal stresses on the spine are frequently implicated [23,24]. Similarly, an appreciation of the nature of the
mechanical loads applied to the cervical spine in whiplash injuries is important for understanding the mechanisms of
injury [28,29,47]. Impact loads on the cervical spine also can produce catastrophic results including tetraplegia [6]. Thus
an appreciation of the loads sustained by the cervical spine may help health care providers optimize treatment and
develop more effective prevention strategies.

The purpose of this chapter is to examine the ability of the cervical spine to withstand loads. Specifically, the objectives
of this chapter are to

■ Use two-dimensional examples to estimate the loads to which the cervical spine is subjected

■ Examine the maximum static loads the cervical spine can sustain before it fails

■ Discuss the mechanisms of injury in dynamic loading that occur in impact and whiplash injuries

TWO-DIMENSIONAL ANALYSIS OF
THE LOADS ON THE CERVICAL SPINE

Normal upright posture is characterized by a lordotic curve
in the cervical spine so that the atlanto-occipital (AO) junc¬
tion lies anterior to the cervicothoracic junction (C7-T1)
(Fig. 28.1). Because the center of mass of the head lies anterior
to the AO joint, the head creates a flexion moment at both the
AO and C7-T1 junctions [48]. Many readers know this intu¬
itively since they have inadvertently fallen asleep while sitting
upright, only to have the head fall forward as the extensor
muscles relax. Examining the Forces Box 28.1 details the

two-dimensional analysis to determine the joint reaction force
on the occiput during upright posture. The extension moment
needed to keep the head upright is produced by the extensor
muscles, represented as a single extension force, E. The free-
body diagram reveals that the moment arm of the weight of
the head is approximately one half of the moment arm of the
extensor muscles, putting the muscles at a mechanical advan¬
tage [42]. The extensor muscle force needed to support the
head is approximately 19 N (4.3 lb), or about one half the
weight of the head. The joint reaction force on the occiput
maintaining the head in an upright position is approximately
46 N (10.3 lb), or 1.2 times the weight of the head.

511

512

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 28.1: The cervical spine is normally aligned in a lordosis
in which the middle cervical spine tends to extend and the lower
cervical spine tends to flex.

The analysis in Examining the Forces Box 28.2 deter¬
mines the extensor muscle force at the C7-T1 joint to
maintain the head’s upright position. The analysis reveals
that the extensor muscle force needed is approximately
75 N (17 lb) and the joint reaction force on C7 is 112 N
(25 lb). The greater loads in the extensor muscles and on the
vertebra are consistent with the C7-T1 joints position with
respect to the center of mass of the head. In contrast to the
AO junction, the moment arm of the weight of the head
with respect to the point of rotation at C7-T1 is twice the
moment arm of the extensor muscles at C7-T1, putting the
muscles at a mechanical disadvantage. Therefore, the mus¬
cles must exert more force to maintain the position of the
head and, consequently, the joint reaction force increases.
The results presented in Examining the Forces Boxes 28.1
and 28.2 are, at best, approximations of the real loads sus¬
tained by the structures of the cervical spine in upright pos¬
ture. The analyses examine loads in only two dimensions
and use simplifying assumptions such as activity in only one
muscle and the location of the axis of rotation at a single point.
Although the results of the calculations are only estimates,
they provide a perspective on the loads sustained by
the cervical spine and the means to examine the con¬
sequence of altered posture on these loads.

Chapter 28 I ANALYSIS OF THE FORCES ON THE CERVICAL SPINE DURING ACTIVITY

513

Clinical Relevance

CERVICAL DISC DEGENERATION: Cervical disc degen¬
eration is common; one study reports finding degenera¬
tion in over 80% of the cervical discs examined in
asymptomatic individuals over 60 years of age [ 18]. Disc
degeneration is considerably more common in the lower
cervical region than in the upper cervical region.

Although several factors contribute to the disparity in inci¬
dence between the upper and lower cervical regions, one
factor may be the magnitude of the loads to which each
region is subjected on a daily basis [12,23,45]. Many
activities require flexion of the head and neck and may
lead to increased cervical spine loads. Redesigning work
sites to decrease the amount of head and neck flexion
may help prevent disc degeneration in the cervical spine.
The loads on the lower cervical region also are likely to
increase in abnormal head alignments such as in for¬
ward head posture, in which the head is positioned
even farther anterior to the C7-T1 junction, increasing its
flexion moment on the tower cervical spine (Fig. 28.2).
Interventions to reduce forward head posture may be
important in preventing disc degeneration, as well as in
treating the symptoms associated with cervical disc
degeneration [7,19].

Figure 28.2: Forward head alignment produces an increased flex¬
ion moment at the C7-T1 junction because the moment arm of
the weight of the head increases as the head translates anteriorly.

514

Part III I KINESIOLOGY OF THE HEAD AND SPINE

LOADS ON THE CERVICAL SPINE

Studies of the loads on the cervical spine examine the static
forces attributable to external loads or muscle contraction, as
well as the loads applied dynamically during whiplash and
impact injuries. Both types of loading provide clinically relevant
information. Degenerative changes in the spine are likely
affected by both static and dynamic loads, while catastrophic
damage to the cervical spine and spinal cord typically results
from dynamically applied loads [10,12,48].

Static Loading of the Cervical Spine

Although the analyses described in Examining the Forces
Boxes 28.1 and 28.2 provide rough estimates of the loads sus¬
tained by the cervical spine in upright posture, the examples
represent oversimplifications of the real situation. As in most
anatomical regions, the cervical spine is supported by several
ligaments and by simultaneous contractions of numerous
muscles. Considerably more sophisticated analytical tools are
found in the literature, providing more realistic biomechani¬
cal models of the cervical spine.

The cervical spine is quite mobile, allowing an individual to
position the head precisely and easily, thereby optimizing the
function of the special senses of vision, hearing, and smell.
Moving the head slightly from the neutral position requires
minimal muscle force. The neutral zone of the cervical spine
describes the arc of motion that is available around the neutral
position without passive resistance to the motion (Fig. 28.3).
The neutral zone is the region in which the stiffness of the
cervical spine complex produced by the bones, discs, and soft
tissue is minimal [20,32,46]. The neutral zone for flexion and
extension is approximately 10°, for side-bending less than 10°,
and approximately 35° for rotation [1,46]. The midcervical
region (C2-C5) is stiffer and thus less mobile than the upper
and lower cervical regions [8,37], and the cervical spine is less
stiff than the thoracic and lumbar spines and uses less muscle
force to produce motion [9,22,33].

While little muscle force is required to move the head
through small arcs of motion in the upright position, as the
head moves farther from the neutral zone, the muscle force
needed to move the head increases as the resistance to
movement from the joints and ligaments increases. A model
including the trapezius, sternocleidomastoid, and rectus
capitis muscles calculates very small forces in the right and
left trapezius muscles (13 N or 3 lb) and in the right and left
sternocleidomastoid muscles (34 N or 8 lb) to maintain the
head in the upright position [38]. Like the examples in
Examining the Forces Boxes 28.1 and 28.2, the model
demonstrates a larger joint reaction force at the C7-T1 joint
(130 N or 29 lb) than at the AO joint (70 N or 16 lb).
Forward-bending from the upright position increases the
muscle and joint reaction forces at both locations. At 30° of
cervical flexion the model calculates joint reaction forces of
approximately 75 N (17 lb) and 250 N (56 lb) at the AO and
C7-T1 joints, respectively.

Figure 28.3: The neutral zone is the region through which the
head and neck can move with little passive resistance from
ligaments, joints, and muscles.

Biomechanical models simulating cervical rotations also
report that unresisted axial rotation within the neutral zone
generates minimal loading of the vertebrae and requires little
muscular force. However, axial rotation to approximately 35°
produces compressive forces on the cervical spine of approx¬
imately 100 N (22.5 lb) while developing muscle moments of
about 2 Nm [1,38]. The muscle forces increase as passive
resistance to rotation increases. The relatively large compres¬
sive loads result from the increased muscle forces and the
co-contractions of muscles on all sides of the neck necessary to
maintain an erect position of the head during the movement.
Loads on the cervical spine during more-forceful contractions
are also reported [23]. Loads on the C4-C5 junction during
maximal isometric contraction are reported in a model using
14 pairs of muscles. This model yields average compressive
loads of 1160 N (261 lb) during isometric extension
and more than 750 N (169 lb) in side-bending and
rotation.

Internal pressures within the cervical intervertebral disc
also are useful indicators of the loads on the cervical spine.
Average intradiscal pressures at C3-C4 and at C5-C6 meas¬
ured in seven human cadaveric spines range from 0.16 MPa
(megapascals) in axial rotation to 0.32 MPa in flexion/
extension [35]. These measurements are based on bending
moments of no more than 0.5 Nm with a compressive load of
10 N (2.25 lb, less than the weight of the head). Similar
intradiscal pressures are reported based on a mathematical
model of the cervical spine [8]. Simulated co-contractions of
three pairs of muscles produce varied increases in intradiscal

Chapter 28 I ANALYSIS OF THE FORCES ON THE CERVICAL SPINE DURING ACTIVITY

515

pressure, ranging from approximately 10 to 400% increases in
pressure [35]. The largest increases are reported during flexion/
extension and side-bending. The reported increases in pressure
are consistent with the increased compressive loads reported
during simulated co-contractions. The intradiscal pressures in
the cervical spine can be compared to pressures of 4.0-6.0 MPa
found at the hip joint during weight bearing [14] and 2.3-3.6
MPa at the elbow during vigorous elbow extension [21] and
reveal substantial loading of the cervical spine, even though it
supports only the weight of the head.

STRENGTH OF THE CERVICAL SPINE TO RESIST
STATIC LOADS

Failure strength of a tissue is the maximum load the tissue
can sustain and still fulfill its function (Chapter 2). Bones fail
by fracturing, ligaments and muscles fail by tearing. Although
the loads in the cervical spine reported so far are well below
the failure strengths of the cervical spine, they may be impor¬
tant in the degenerative changes in cervical discs and in arthritic
changes at the facet joints as the result of repetitive or pro¬
longed loading. In the upright posture, most of the load is
borne through the intervertebral disc, and flexion of the cervi¬
cal spine increases the load on the discs [8,15]. Extension of the
cervical spine reduces the load on the intervertebral discs while
increasing the loads on the facet joints [8].

During small movements of the head from the neutral
position only slight loads are generated in the cervical spine.
Larger movements of the head produce larger loads on the
vertebrae and discs. To put the loads sustained by the cervical
spine on a regular basis in perspective, it is useful to compare
these values with the reported loads at which the cervical
spine fails. Most of the data describing the failure strength of
the cervical spine are based on mechanical testing of cadaver
specimens and thus do not adequately represent the physio¬
logical response of an intact cervical spine. Yet these data are
useful in understanding how the loads sustained by the cervi¬
cal spine during everyday activities compare with the theo¬
retical failure points of the spine.

The failure strength of the cervical spine in static loading
typically reflects the ability of the cervical spine to withstand
bending in either flexion or extension, with and without the
addition of a compressive load [22,34,37,47]. Failure of the
cervical spine occurs by vertebral fracture, disc disruption, or
tears of the ligaments or muscles so that the cervical spine is
no longer able to support or move the head. Available studies
are hard to compare because the modes of loading and the
specific regions of the cervical spine tested vary. Failure of the
cervical spine is reported when it is subjected to flexion
moments of approximately 7 Nm in the midcervical region,
but a 12-Nm flexion moment with an additional 2000-N com¬
pression load (450 lb) is reported before failure occurs in the
lower cervical region [37]. Nightingale et al. report an average
moment at failure of 24 Nm in the upper cervical region
(occiput to C2) during flexion and 43 Nm for extension [26].
Another study also reports failure of the entire head-neck
complex with approximately 2000 N when loading with the

cervical spine flexed [34]. Testing the cervical spine when it is
held in combined axial rotation and flexion reveals increases in
the stiffness of the spine, and the moment at failure is greater
than when the cervical spine is flexed alone. However, the
damage to the tissue is greater at failure when the spine fails
in combined flexion and rotation [13,37,47]. Failure moments
reported for the cervical spine are lower than those reported
for the lumbar spine [22]. These data demonstrate that the
loads sustained by the cervical spine during active motion are
well below the static loading limits of the cervical spine. To put
these moments in perspective, it is useful to recall that the
elbow reportedly sustains moments of approximately 12 Nm
during propulsion of a wheelchair [36].

Dynamic Loading of the Cervical Spine

Typically, failures of the cervical spine in healthy individuals
result from high-velocity dynamic loading. Loading rate affects
the mechanical properties of bones and connective tissues
(Chapters 3 and 6). Like the studies examining static loads on
the cervical spine, the studies that examine the response of the
cervical spine to dynamic loads vary in the rates and modes of
loading studied and in the part of the spine analyzed, as well as
its position [16,25,28,34,44,47]. Despite these differences, data
suggest that the stiffness of the cervical spine and the load that
it can sustain before failure increase with increasing loading
rates [34,44]. The increased stiffness and load to failure with
increased loading rates demonstrate the viscoelastic behavior
of the cervical spine. Studies of impact loading are useful in
understanding the incidents that most often lead to spinal cord
injuries. Acceleration, or whiplash, injuries to the cervical spine
also occur, frequently as the result of automobile accidents.
Although whiplash injuries rarely produce the catastrophic
results that occur in impact injuries, they are extremely
common and can be very costly, both physically and financially
[28,47].

IMPACT LOADING OF THE CERVICAL SPINE

Most catastrophic injuries to the cervical spine result from
high-velocity collisions between the head and relatively fixed
objects, such as when a football player uses his head to tackle
another player or when a swimmers head hits a rock or pool
bottom [2,5].

Several factors contribute to the severity of the injuries
that result from impact loading of the head and neck. The
force of impact is quite large because the head comes to an
abrupt stop after traveling at a high speed. Recalling the rela¬
tionship between force and acceleration (2F = ma) discussed
in Chapter 1, it is clear that the deceleration from a high
velocity to zero velocity requires large deceleration forces.
Burstein provides an example of a football player moving at a
velocity of 5 m per second whose average deceleration at the
time of impact with another player is about 415 m/sec 2 , com¬
pared with the acceleration of gravity, 9.8 m/sec 2 [5]. The
average force of impact for the football player is approxi¬
mately 2000 N (450 lb).

516

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

AMERICAN FOOTBALL: American professional football
players may be exposed to compression forces on the
cervical spine of more than 5,000 N (approximately
1,125 lb) when involved in helmet-to-helmet impacts in
which the striking player lowers his head and hits the
other player with the top of his helmet [43]. Such tackling
is known as "spearing." While exceptional strength of the
cervical muscles appears to help protect most professional
players from catastrophic cervical damage, players with
less strength are unlikely to sustain such impacts safely.
These data emphasize the need for parents, coaches, and
officials to enforce the no-spearing (tackling a player by
hitting with a lowered head) rules that currently exist
in football.

An impact force on the head may produce axial loading
and compressive loads on the cervical spine or a flexion or
extension moment on the head and neck, depending on the
location of the force with respect to the joints of the cervical
spine (Fig. 28.4). Studies of the force of impact on the head
when it collides with a more rigid structure while traveling at
similar, or even slower, speeds than the football player reveal
impacts ranging from 2000 to 11,000 N (450-2472 lb) and
bending moments on the neck from 40 to 150 Nm [25].
Loads and moments of such magnitudes produce diffuse and
catastrophic damage to the bones, ligaments, and discs of the
cervical spine and ultimately to the spinal cord housed with¬
in it [25].

Another important factor in the morbidity of impact acci¬
dents is the continued movement of the body after the head
has impacted the object. At the time of impact, the body is
traveling at approximately the same speed as the head.

However, after the head comes to a stop, the rest of the
body continues to move toward the head, so the entire mass
of the body exerts a force on the cervical spine, producing
additional deformation of the cervical spine [5,25] (Fig.
28.5). Studies show that if the head is capable of moving out
of the way of the oncoming trunk, the cervical spine may
survive the impact and avoid significant damage [25].
Studies of the mechanics of such collisions and the resultant
injuries have lead to national standards for swimming pool
designs and changes in athletic equipment, including foot¬
ball helmets. Clinicians who understand the mechanisms
leading to the failure of the cervical spine are better able to
participate in public education and equipment design to
prevent such accidents [6].

ACCELERATION INJURIES TO THE CERVICAL SPINE

Although impact injuries of the cervical spine are the primary
cause of catastrophic injuries of the cervical spine, the most
common trauma to the cervical spine is whiplash injuries.
Rarely catastrophic, these injuries nevertheless cause sub¬
stantial cost in human suffering, lost wages, and medical
expenditures [28,47]. As in impact injuries, major contribu¬
tors to the cervical spine injuries in whiplash incidents are the
accelerations imparted to the head and neck when the body
is suddenly slowed, as well as the continued movement of the
trunk toward the head and neck. In an automobile accident in
which a car at rest is hit from behind, the car and its contents
are accelerated forward. If the driver is secured in the seat by
a lap and shoulder belt, the driver accelerates forward with
the car [3]. However, the flexible cervical spine allows the
head to lag behind the trunk, producing cervical hyperexten¬
sion, stretching the anterior structures of the neck, and caus¬
ing compression loading of the posterior structures [3,33]
(Fig. 28.6). Almost simultaneously, the occupants trunk rises
toward the head, applying a compressive load to the lower
cervical region [3]. Conversely, the car that impacts an

Figure 28.4: Impact loads on the head may produce extension moments (A) or flexion moments (B) on the cervical spine.

Chapter 28 I ANALYSIS OF THE FORCES ON THE CERVICAL SPINE DURING ACTIVITY

517

Figure 28.5: The trunk's effect on the
cervical spine in impact injuries. After
the head comes to a stop during an
impact injury, the body continues to
move toward the head, contributing to
additional deformation of the spine.

immovable object from the front comes to an abrupt halt, but
the relatively mobile head and neck accelerate forward. High-
velocity impacts may cause a rebound movement of the head
and neck before the head finally comes to rest, producing dif¬
fuse injury to the cervical spine [25].

Studies of rear impacts at relatively low velocities, 8 kph
(approximately 5 mph), suggest that the head and neck com¬
plex are subjected to accelerations as high as 13 times the

acceleration due to gravity and extension moments of approx¬
imately 30 Nm [16,29]. Investigations into the effects of front
or rear impacts on the structures of the cervical spine present
conflicting conclusions [17,27]. However, the weight of the
evidence suggests that even impacts at low velocities can pro¬
duce increased deformation in ligaments and muscles,
increased pressures within the intervertebral discs, and
increased loads on the facet joints [3,11,16]. Experiments

Figure 28.6: When a car is rear-ended,
the vehicle and driver accelerate for¬
ward, but the head lags behind in
hyperextension, putting strain on the
anterior structures of the cervical spine.

518

Part III I KINESIOLOGY OF THE HEAD AND SPINE

using cadaver specimens yield evidence of strain (percentage
change in length) beyond physiological tolerance within the
intervertebral discs and excessive narrowing of the interverte¬
bral foramena following impact [30,31]. Individuals who sus¬
tained whiplash injuries with their heads rotated also exhibit
evidence of torn alar, apical, and transverse ligaments [13].
These anatomical changes are consistent with the frequent
complaints of headaches, neck pain, and radicular pain (pain
radiating into the arm) that many individuals report following
motor vehicle accidents [4,31]. The combined use of shoulder
and lap belt restraint systems and properly positioned head
rests in automobiles decreases collisions between the head
and rigid objects within the car and limits the excursion of the
head and neck on the more stationary body, thereby reducing
the soft tissue injuries sustained in whiplash injuries [16].

Clinical Relevance

MOTOR VEHICLE ACCIDENTS: Health care providers
frequently treat the pain and impairments resulting from
motor vehicle accidents. The most common of these is
whiplash injury. Because not all individuals involved in
front or rear-impact collisions report neck pain, there is a
tendency among some health professionals to dismiss the
complaints. However , the biomechanical data suggest
that if the conditions are right a small "fender bender"
can indeed produce a significant injury. Abnormal head
posture appears to increase the risk of neck injury in such
motor vehicle accidents [39]. Women demonstrate more
cervical motion and higher accelerations of the head
following low-velocity impacts, which may help to
explain why women have an increased incidence of
whiplash injuries [40,41].

Another common cause of acceleration injuries to the
cervical region is deployment of the front seat airbag. The
injury occurs when the bag deploys with an explosive
force into the face and head of the passenger, forcing the
neck into hyperextension. Children and small adults are
particularly susceptible because of the position of the
airbag in relationship to the passenger's head. Unlike
whiplash injuries, these injuries typically involve the upper
cervical region despite the fact that the upper cervical
region appears to be stronger in extension than the lower
and middle regions of the cervical spine [26]. These data
help reinforce the need to keep children riding in the back
seat of the car until they attain the required height.

SUMMARY

This chapter provides simple two-dimensional analyses of
the forces on the upper and lower cervical spine that are
generated while holding the head upright. The analyses
demonstrate the differences in mechanical advantage of the
cervical extensor muscles between the upper and lower

regions. These differences lead to differences in the joint
reaction forces sustained by the two regions. In upright
postures, calculations determine loads of approximately 1.2
times the weight of the head at the AO joints and loads of
approximately 3 times head weight at the C7-T1 juncture.
Loads on the cervical spine increase as the head and neck
move beyond the neutral zone, and loads of over 50 lb on
the C7-T1 joint are reported during forward-bending.
However, typical loads in the cervical region to move the
head and neck are well below those that produce failure.

This chapter also discusses the mechanics of injury to the
cervical region produced by dynamic loading. Injuries to the
cervical spine typically involve large accelerations that pro¬
duce very large forces and moments on the cervical spine,
resulting in significant soft tissue and bony tissue trauma. An
understanding of the mechanics of such injuries allows a clini¬
cian to contribute to the development of prevention strategies
and may lead to more effective treatment regimens.

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CHAPTER

Structure and Function of the Bones
and Joints of the Thoracic Spine

CHAPTER CONTENTS

STRUCTURE OF THE THORACIC VERTEBRAE .521

Bodies of Thoracic Vertebrae.521

Vertebral Arch of a Thoracic Vertebra .522

BONES OF THE THORACIC CAGE .524

Ribs.524

Sternum.524

JOINTS OF THE THORACIC REGION .525

Joints between Adjacent Vertebrae .525

Articulations Joining the Ribs to the Vertebrae and Sternum .526

MOVEMENTS OF THE THORACIC SPINE AND THORAX.529

Thoracic Spine Motion .529

Motion of the Rib Cage .531

MECHANICS OF RESPIRATION.535

SUMMARY .536

T horacic vertebrae exhibit the articular attachments typical of much of the vertebral column, including the inter¬
body and facet (zygapophyseal) joints. The thoracic spine differs from the cervical and lumbar spinal segments by
virtue of its participation in the thoracic cage that encloses the thoracic viscera (Fig. 29.1). Along with interverte¬
bral articulations, the thoracic vertebrae provide attachments for the ribs, and these additional articulations influence the
structure of the individual thoracic vertebrae and the mobility and stability of the thoracic spine. The articulations between
thoracic vertebrae and the rest of the thorax also affect the mechanics of respiration.

The three chapters on the thoracic spine review its structure and how its structure affects the mechanics and patho-
mechanics of the region. The first chapter presents the skeletal composition of the thorax and the mobility allowed by
the articulations. The second chapter discusses the function of the muscles of the thoracic spine, and the third chapter
discusses the forces sustained by the thoracic spine and how those forces contribute to common pathologies.

The current chapter discusses the structure of the thoracic vertebrae and ribs and how their architecture influences the
mobility, stability, and function of the thoracic cage. The objectives of this chapter are to

■ Describe the unique structural features of the thoracic vertebrae and the functionally relevant features of the ribs
■ Discuss the joints of the thorax and their supporting structures
■ Compare the mobility and stability of the thoracic, cervical, and lumbar spines
■ Discuss the mobility of the costal articulations
■ Review the basic mechanics of ventilation

520

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

521

STRUCTURE OF THE THORACIC
VERTEBRAE

The thoracic region of the spine, the longest segment of the
vertebral column, consists of 12 separate vertebrae and acts as
a transition between the cervical and lumbar spines [47]. The
thoracic vertebrae have several features in common with each
other and contain the typical elements of a vertebra, the body
and the vertebral arch with its sites for muscular attachments,
the transverse and spinous processes. However, some individ¬
ual variations have lead to the identification of three distinct
regions within the thoracic spine [34]. The upper thoracic
spine consists of the first through fourth thoracic vertebrae

(T1-T4), which have many features similar to those of the
lower cervical vertebrae. The middle region extends from the
fourth through the ninth or tenth thoracic vertebrae and
exhibits the classic characteristics of thoracic vertebrae. The
lower region consists of the lowest two or three thoracic
vertebrae, which have features similar to those of the upper
lumbar vertebrae [35].

Bodies of Thoracic Vertebrae

The bodies of the vertebrae increase in size from the second
cervical vertebra through the lumbar vertebrae. Consequently,
the body of the twelfth thoracic vertebra is larger than the
body of the first thoracic vertebra [37,54] (Fig. 29.2). This
progressive increase in size is consistent with the increasing
load that is borne by underlying vertebrae. The superior and

Figure 29.2: The bodies of the thoracic vertebrae increase in size
from superior to inferior vertebrae and within each vertebra
from superior to inferior surface.

522

Part III I KINESIOLOGY OF THE HEAD AND SPINE

inferior surfaces of the vertebral bodies, known as endplates,
exhibit similar size progressions, the inferior surface of each
vertebra being larger than its superior surface. The diameters
of the bodies are slightly larger in an anterior-posterior direc¬
tion than in a medial-lateral direction [34,37]. The ratio
between the anterior-posterior and medial-lateral diameters
varies only slightly throughout the thoracic spine.

The bodies of the thoracic vertebrae are wedge shaped,
thicker posteriorly than anteriorly (Fig. 29.3). Wedging of the
vertebral bodies is the primary cause for the normal kyphotic
curve of the thoracic spine that is characterized by a posterior
convexity [26,34]. The normal kyphosis of the thoracic spine
and wedging of its vertebrae result in large loads applied to the
thoracic vertebral bodies and may help explain why compres¬
sion fractures of the vertebral bodies in individuals with osteo¬
porosis are more common in the thoracic spine [2,49].

Figure 29.3: The wedge-shaped bodies of the thoracic vertebrae
are the primary source of the normal thoracic kyphosis.

Clinical Relevance

COMPRESSION FRACTURES OF THE THORACIC
VERTEBRAE: Osteoporosis, common in postmenopausat
women , decreases the ioad-bearing capacity of bone ;
predisposing an individual to fragility fractures and pro¬
gressive kyphosis , the so-called widow's hump deformity.
Approximately 25% of postmenopausal women and more
than 50% of women 85 years or older are affected by verte¬
bral fractures [20,27]. Fragility fractures are most commonly
seen in the lower thoracic spine [27].

The bodies of the thoracic vertebrae contain facets for articu¬
lation with the heads of the ribs. With the exception of the
first, tenth, eleventh, and twelfth vertebral bodies, the bodies
possess half, or demi, facets at their posterolateral aspects on
both the superior and inferior borders [34,39,54] (Fig. 29.4).
The half facet on the superior aspect of one vertebral body
pairs with the inferior half facet of the body above to form the
socket for the head of a rib. The first, tenth, eleventh, and
twelfth vertebral bodies have full facets and provide the
entire attachment for the head of a rib.

Vertebral Arch of a Thoracic Vertebra

The vertebral arch in the thoracic region, as in the rest of the
vertebral column, is formed by the posteriorly projecting
pedicles and medially projecting laminae (Fig. 29.5). As the
laminae converge and join together, the vertebral arch is
formed. In the thoracic region, the pedicles project less

Demifacets

Figure 29.4: Demifacets on the thoracic vertebrae are located
at the posterolateral aspect of the superior and inferior sur¬
faces of the vertebral bodies and provide attachment for the
heads of articulating ribs. The inferior vertebra attaches to
the rib of the same number.

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

523

Figure 29.5: The spinal canal within the thoracic region is narrow
because the pedicles of the vertebrae project posteriorly with lit¬
tle lateral angulation.

laterally than they do in either the cervical or lumbar regions,
contributing to a narrower spinalcanal.

The spinal canal contains the spinal cord, and its size and
shape are important factors in avoiding impingement of the
spinal cord. In general, the spinal canal is smaller in the tho¬
racic region than in the cervical or lumbar regions where the
spinal cord enlarges, providing the large spinal nerves that
form the brachial and lumbosacral plexuses of the upper and
lower extremities, respectively. The spinal cord occupies
approximately 40% of the spinal canal in the thoracic region
but only about a quarter of the canal in the cervical region
[34,47]. Within the thoracic region, the spinal canal is largest
at the level of the first thoracic vertebra and is smallest in the
midthoracic region. The area increases again in the lower tho¬
racic region.

Clinical Relevance

SPINAL CORD IMPINGEMENT: Because the spinal canal
is relatively small in the thoracic region , space-occupying
lesions such as tumors or disc herniations put the spinal cord
at risk [47]. Careful screening for signs of spinal cord com¬
pression is an important component of the assessment of an
individual with thoracic spine dysfunction. Signs of spinal
cord compression include motor or sensory changes in the
lower extremities as well as hyperreflexia. Loss of bowel or
bladder control also may suggest spinal cord involvement.

ARTICULAR PROCESSES OF A THORACIC VERTEBRA

The articular processes extend superiorly and inferiorly from
the junction of the pedicles and laminae. Each process con¬
tains an articular facet that provides articular surfaces for a
facet, or zygapophyseal, joint (Fig. 29.6). The orientation of
the articular processes and facets directly affects the available

Superior articular

Figure 29.6: The articular processes of the thoracic region are
almost vertically aligned. The superior articulating facets in the
thoracic region face posteriorly and slightly laterally and superi¬
orly; the inferior articulating facets face anteriorly and slightly
medially and inferiorly.

spinal motion. In the thorax, the articular processes are more
vertically aligned than in the cervical region [33]. The angle
between the facets and the transverse plane increases steadily
through the cervical and upper thoracic vertebrae. The facets
throughout most of the thoracic region lie approximately
70-80° from the transverse plane, with the facets of the lower
thoracic vertebrae slightly more vertical than those of the
upper thoracic vertebrae [25,33]. In addition, the facets on
the superior articular processes face posteriorly and slightly
laterally, while the inferior facets face anteriorly and slightly
medially. These facets lie at approximately 10° from the
frontal plane, which is similar to those of the cervical verte¬
brae [25,30,33]. In contrast, the lumbar facets lie closer to the
sagittal plane [33,37,46].

The alignment of the articular facets helps explain the
regional differences in mobility. The vertical alignment of the
thoracic facets has a limiting effect on flexion, since the infe¬
rior facet of the vertebra above can glide only slightly anteri¬
orly on the superior facet of the vertebra below. Because the
thoracic facets lie close to the frontal plane, the articular sur¬
faces provide little limitation to axial rotation. Side-bending
also is relatively unobstructed by bony contact at the facet
joints. However, as the facets become more medially and lat¬
erally aligned in the lower thoracic vertebrae, rotation is more
limited, as it is throughout the lumbar region.

MUSCULAR PROCESSES OF A THORACIC VERTEBRA

The transverse and spinous processes of the thoracic verte¬
brae exhibit unique characteristics. The transverse processes
of the thoracic vertebrae vary in length. They are longest in
the upper thoracic region and shortest in the lower thoracic
region [34]. The lateral aspect of the anterior surface of each
transverse process contains a facet for articulation with the
tubercle of a rib. The spinous processes are longer in the tho¬
racic region than anywhere else in the vertebral column and
project inferiorly, so that throughout most of the thorax the
palpable tip of a thoracic spinous process is in line with the

524

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 29.7: The spinous processes of the thoracic region are long
and project interiorly so that the palpable tip of the spinous
process is in line with the body of the vertebra below.

body of the inferior thoracic vertebra (Fig. 29.7). This rela¬
tionship is found from approximately the second or third tho¬
racic vertebra to the ninth or tenth vertebra. The upper tho¬
racic vertebrae have more horizontally aligned spinous
processes, similar to the cervical vertebrae. The spinous
processes of the eleventh and twelfth thoracic vertebrae are
somewhat shorter than those of the rest of the thoracic verte¬
brae and project more posteriorly becoming more like the
spinous processes of the lumbar vertebrae.

Clinical Relevance

PALPATION OF THE THORACIC SPINE: Clinicians fre¬
quently palpate the spinous processes of the vertebral col¬
umn. A dear understanding of the relationship between a
spinous process and neighboring vertebrae allows accurate
location of a patient's complaints or the site of impairment

BONES OF THE THORACIC CAGE

Ribs

Twelve pairs of ribs and the sternum, along with the thoracic
vertebrae, form the thoracic cage. Ribs are long, curved strips
of bone composed of head, neck, and body (Fig. 29.8). The
head and neck form the most dorsal portion of each rib and
articulate with the vertebral column. The body, or shaft,
constitutes most of each rib and provides attachment to the
costal cartilages for all but the last two pairs of ribs [39,54].

Neck

The second through ninth pairs of ribs consist of typical
ribs. The head of a typical rib has a superior and an inferior
facet that articulate with the demifacets on the vertebral bodies.
The neck of the rib extends laterally and slightly posteriorly
from the head, ending with a tubercle on the posterior surface
of the rib for articulation with a transverse process. The body of
the rib extends laterally from the neck and then curves anteri¬
orly from the angle of the rib. The bodies of the ribs provide
attachments for several muscles, including the intercostal mus¬
cles, the erector spinae muscles, and the abdominal muscles.
The anterior tip of the rib is slightly concave for articulation with
a costal cartilage.

The first rib is the shortest and most curved and
has a single facet on its head to articulate with the first
thoracic vertebra. The tenth, eleventh, and twelfth
ribs also typically have only one facet on the heads of the ribs
to articulate with their respective vertebrae [39,54].

Sternum

The sternum as a whole is convex anteriorly and concave pos¬
teriorly, contributing to the normal contour of the anterior
chest wall. It is a flat bone that is composed of three seg¬
ments, the manubrium, the body, and the most inferior and
smallest segment, known as the xiphoid process (Fig. 29.9).
The manubrium is the proximal and widest segment of the
sternum. Its superior border lies at approximately the level of
the third thoracic vertebra and contains the palpable sternal,
or jugular, notch, which is bordered laterally by the facets for
the sternoclavicular joints. The sternal notch is a useful land¬
mark to identify the sternoclavicular joints [39,54].

The body is the longest segment of the sternum, spanning
the fifth through ninth thoracic vertebrae [54]. It is notched
laterally by facets for the costal cartilages of the second
through seventh pairs of ribs. The manubrium and body are
joined by a cartilaginous joint that may ossify with age. This
joint, the sternomanubrial junction, is readily palpable, since
the manubrium and sternal body join at an angle of approxi¬
mately 160°, known as the sternal angle, or angle of Louis.
While mobile, the joint bends a few degrees in the sagittal

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

525

Sternal notch Clavicular notch

Figure 29.9: The sternum consists of the manubrium, body, and
xiphoid process. A lateral view reveals the angle of the sternum,
or sternomanubrial junction, formed by the cartilaginous joint
between the manubrium and body.

plane during respiration, especially during forced respiration
[15,40,54]. The xiphoid process is the smallest portion of the
sternum and attaches to the inferior aspect of the sternal
body Its shape is more variable than that of the other portions
of the sternum, but typically ends in a point inferiorly. The
xiphoid process projects inferiorly or inferiorly and posteriorly
and may or may not be palpable. The xiphisternal junction is
cartilaginous and typically ossifies by 40 years of age.

JOINTS OF THE THORACIC REGION

The thoracic vertebrae are joined at the vertebral bodies by the
intervertebral discs and at the spinal arches by the facet joints
(Fig. 29.10). In addition, the ribs articulate with the vertebrae
and with the sternum by way of the costal cartilages.

Joints between Adjacent Vertebrae

The joints tethering the thoracic vertebrae together consist of
the interbody joints, or symphyses, and the synovial facet joints.

INTERBODY JOINTS

The joints between adjacent vertebral bodies are formed by the
binding of the intervertebral disc to the adjacent vertebrae.
The disc consists of the gelatinous nucleus pulposus and the
anulus fibrosus composed of concentric cartilaginous rings that
bind the adjacent vertebrae together at their endplates [36].

Figure 29.10: The joints of the thoracic region participate in artic¬
ulations between adjacent vertebral bodies ( 1 ), between articulat¬
ing facets ( 2 ), between the ribs and vertebrae at the bodies ( 3 )
and transverse processes ( 4 ), and indirectly with the sternum ( 5 ).

The structure and mechanical properties of a typical lum¬
bar intervertebral disc are discussed thoroughly in Chapter 32.
Thoracic discs differ from discs in the lumbar region in size
and shape. Although the discs generally increase in size from
superior to inferior, the thinnest discs of the vertebral column
are found in the upper thoracic region. The ratio between the
disc height and the height of the vertebral body is smaller in
the thoracic region than in the cervical and lumbar regions
[15,54] (Fig. 29.11). Because deformation of the intervertebral
discs contributes to motion of the spine, the decreased ratio

Figure 29.11: The ratio of intervertebral disc height to vertebral
body height is smallest in the thoracic region.

526

Part III I KINESIOLOGY OF THE HEAD AND SPINE

between disc and vertebral body height in the thoracic region
contributes to lower mobility in the thoracic spine than in the
cervical and lumbar regions. The thoracic intervertebral discs
are almost equal in height from anterior to posterior and con¬
tribute little to the thoracic kyphosis [26,34].

Clinical Relevance

HERNIATION OF THORACIC DISCS: Herniations of the
intervertebral disc occur in the thoracic region , although
most apparently remain asymptomatic. Symptomatic disc
herniations are less common in the thoracic region than in
the cervical or lumbar regions [47]. A herniation may pro¬
duce nerve root compression with dermatomal or myotomal
signs at the level of impingement or ; because the spinal
canal is relatively small in the thoracic region , spinal cord
compression with signs and symptoms into the lower
extremities.

FACET JOINTS

The facet joints of the thoracic region are gliding synovial
joints supported by joint capsules like facet joints throughout
the vertebral column. The capsules in the thoracic and lumbar
regions are tauter than those in the cervical region and help
limit flexion and anterior translation of a superior vertebra on
an inferior vertebra [32,36].

SUPPORTING STRUCTURES

Besides the capsular ligaments, the thoracic spine is supported
by several sets of ligaments common to the rest of the vertebral
column: the anterior and posterior longitudinal ligaments, the
inter- and supraspinous ligaments, the ligamentum flavum, and
the intertransverse ligaments (Fig. 29.12). Serial transection of
these ligaments in the thoracic region using cadaver specimens
reveals that anterior instability of the thoracic spine with intact
vertebrae occurs only after transection of all of the posterior
ligaments and the posterior portion of the disc [31]. Similarly,
posterior instability of the thoracic spine occurs only with tran¬
section of the anterior longitudinal ligament and the anterior
portion of the disc. Removal of small portions of the disc alone
for treatment of disc herniations does not appear to impair
spinal stability in the thoracic region [3].

Articulations Joining the Ribs
to the Vertebrae and Sternum

The ribs, thoracic vertebrae, and sternum form the thoracic
cage, which must be rigid enough to protect the heart and
lungs but flexible enough to participate in respiration and to
allow spinal motion. The rib cage plays an important role in
supporting the entire thoracic spine [8,31,32,35]. With an
intact thoracic cage, the thoracic spine can sustain four times

Figure 29.12: Ligaments supporting the thoracic spine consist
of the capsular ligaments of the facet joints, the anterior and
posterior longitudinal ligaments (ALL and PLL), the supra- and
interspinous ligaments (SSL and ISL), the ligamentum flavum
(iLF), and the intertransverse ligaments (ITL).

the compressive load it can support without the thoracic cage
[1,37]. In fact, without the rib cage, the thoracic spine would
barely be able to support the weight of the head [35].

ARTICULATIONS BETWEEN THE RIBS
AND THE VERTEBRAE

The posterior ends of the ribs articulate with the vertebrae at
the bodies and transverse processes, forming the joints of the
costal heads (also known as the costovertebral joints) and
the costotransverse joints, respectively. Although each artic¬
ulation is described as a gliding, or plane, joint, together they
allow rotational movement of a rib.

The joints of the costal heads typically consist of the junction
between the head of a rib and the demifacets of two vertebral
bodies [39,54] (Fig. 29.13). With the exception of the first,
tenth, eleventh, and twelfth pairs of ribs, the head of each rib
attaches to the bodies of two adjacent vertebrae, to the verte¬
bra at the same thoracic level as the rib, and to the vertebra
above. The rib also attaches to the intervening intervertebral
disc. The first pair of ribs attaches only to the lateral surfaces of
the first thoracic vertebra. Typically, the tenth through twelfth
pairs of ribs articulate with single vertebrae, T10 through T12.

The joints of the costal heads are supported by synovial
capsules and by intraarticular ligaments that extend medially
from the tips of the heads of the ribs to the intervertebral
discs. Each joint capsule is reinforced on its superficial sur¬
face by a radiate ligament, so named because it radiates from

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

527

Joint of

Figure 29.13: Joints of the costal heads consist of the head of
a rib, the posterolateral surface of the inferior surface of the
superior vertebral body, and the superior surface of the
vertebral body of the adjacent vertebra.

the rib to the bodies of the superior and inferior vertebrae
and to the intervening disc [38,52] (Fig. 29.14).

The costotransverse joints are the junctions between the
facet on the tubercle of the neck of each rib and the facet on
the anterior surface of each transverse process. Each rib artic¬
ulates with the transverse process of the vertebra of the same
number. The costotransverse joint is supported by a joint cap¬
sule that is reinforced by a costotransverse ligament, and by
posterior and lateral costotransverse ligaments (Fig. 29.15).
Ligamentous support affords considerable stability to the
joints of the costal heads and the costotransverse joints.

Consequently, blows to the chest more frequently produce
rib fractures than dislocations of these joints [56].

Clinical Relevance

RIB FRACTURES: Individuals often sustain rib fractures
as a result of motor vehicle accidents or from a chest-high
tackle in football. In both cases the individual receives a
blow to the chest usually from the front. The attachments
of the rib are generally very secure so the rib bends , particularly
in its lateral aspect. If the bending exceeds the ultimate
strain (percent length change) of the bone ’ the rib fractures.
As a result the fracture occurs some distance from the
site of the blow [41]. (Refer to Chapter 2 for more details
about strain.)

ARTICULATIONS BETWEEN THE RIBS AND STERNUM

All but the last two pairs of ribs join the sternum by costal
cartilages that are lengths of hyaline cartilage (Fig. 29.16).
The first pair of ribs attaches to the lateral facets on the
manubrium just inferior to the sternoclavicular joints. The
second pair articulates with the sternum at the junction of
the manubrium and body. The second rib is readily palpated
at the sternomanubrial junction (Fig. 29.17). The third
through seventh pairs attach to the lateral sides of the sternal
body. Because of their direct attachment with the sternum, the
first seven pairs of ribs are known as vertebrosternal ribs.
Rib pairs eight through ten join the costal cartilages of the rib
above and eventually join the costal cartilages of the seventh
ribs. These ribs are referred to as vertebrochondral ribs.

Posterior

costotransverse

ligament

Intertransverse

ligament

Figure 29.14: A joint of the costal head is supported by a capsule that is reinforced by the radiate ligament (B) and by an intraarticular
ligament between the head of the rib and the adjacent intervertebral disc (superior view) (A).

528

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 29.15: A costotransverse joint is supported by a capsule,
a costotransverse ligament, and posterior and lateral costotrans¬
verse ligaments.

Figure 29.16: The seven superior pairs of ribs articulate with the
sternum by individual costal cartilages. Each rib from eight
through ten joins the costal cartilage of the rib immediately
superior to it. Ribs eleven and twelve have only small cartilagi¬
nous tips and do not articulate with the sternum.

Figure 29.17: The second rib is easily palpated at the
sternomanubrial junction.

The last two pairs of ribs, known as vertebral ribs, have
no costal cartilage attachments, so their ventral tips, covered
by a thin layer of cartilage, have no bony attachment anteri¬
orly and are often palpable. Although often referred to as the
floating ribs, these last two rib pairs are secured by muscles
and ligaments and are not free to “float” in the thorax.

The costal cartilages adhere to the ribs by a blending of
the periosteum and perichondrium, as well as by a continu¬
ation of the collagen matrix within the bones and cartilages.
Consequently, no motion is available at the junctions
between ribs and costal cartilages. In contrast, most of the
costal cartilages join the sternum by synovial joints with
small joint capsules and supporting ligaments that permit
motion [38,54]. The junction of the first rib to the sternum
is cartilaginous.

Clinical Relevance

PAIN AT THE COSTOSTERNAL JUNCTIONS: Because
the junctions between most of the costal cartilages and
the sternum are synovial , they are relatively mobile and
capable of subluxation and joint inflammation. An indi¬
vidual with bronchitis or pneumonia may develop inflam¬
mation at a costosternal joint as the result of repeated
vigorous coughing that generates repetitive loading on
the joints by muscles that attach to the ribs and produce
the cough.

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

529

Combined
fiexion/extension
(± x-axis rotation)

One side
lateral bending
(z-axis rotation)

One side
axial rotation
(y-axis rotation)

I_I_I_I_I_I I_i_I_I I_I_I_J_//-Ji_1

5° 10° 15° 20° 25° 5° 10° 15° 5° 10° 15° 35° 40°

Figure 29.18: Segmental motion varies throughout the vertebral column. (Reprinted with permission from White AA III, Panjabi MM:
Kinematics of the spine. In: White AA III, Panjabi MM, eds. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia: JB Lippincott, 1990.)

MOVEMENTS OF THE THORACIC SPINE
AND THORAX

Thoracic Spine Motion

Motion of the thoracic spine, like that of the cervical and lum¬
bar regions (Chapters 26 and 32), depends on the orientation
of the facet joints and on the thickness of the intervertebral
discs. In addition, the motion of the thoracic spine is influ¬
enced greatly by the presence of the ribs [32,37]. To under¬
stand the motion in the thoracic spine, the clinician must
appreciate the segmental mobility that is available at an
individual thoracic motion segment (two adjacent thoracic
vertebrae with the intervening disc) as well as the total
motion from all of the thoracic vertebrae. Thoracic spine
motion is less thoroughly studied than the motions in other
regions of the spine.

SEGMENTAL MOTION

In general, the thoracic spine exhibits less segmental mobility
than the cervical or lumbar regions [24,36,52] (Fig. 29.18). The
segments in the upper and middle regions of the thoracic spine
display approximately 2-6° of combined flexion and extension,
with approximately equal flexion and extension excursions
[24,32,36,43,52]. The thoracic spine is slightly less stiff in flex¬
ion than in extension, requiring less force to flex than to extend
[30]. Flexion and extension mobility increases in the lower
thoracic spine in the presence of the vertebral ribs.

Segmental side-bending is less in most of the thoracic
region than in the cervical region and similar to that available
in the lumbar region. The ribs limit side-bending in the tho¬
racic region. Consequently, side-bending increases in the
lower thoracic region, where the ribs have no sternal attach¬
ment and provide little barrier to side-bending motion.
Segmental rotation in the upper and middle thoracic regions

530

Part III I KINESIOLOGY OF THE HEAD AND SPINE

is greater than segmental rotation in the lumbar region.
Rotation in the lower thoracic region is similar to lumbar seg¬
mental rotation.

As in the cervical and lumbar regions, the motions of the
thoracic spine are coupled. A coupled motion consists of a
primary motion that occurs in one plane and is accompanied
automatically by motion in at least one other plane. Although
coupling appears to occur in all motions of the vertebral col¬
umn, it is greatest in side-bending and rotation [52]. In the
upper thoracic region, side-bending is coupled with ipsilateral
rotation, similar to the coupled motion in the middle and
lower cervical regions (Chapter 26). Throughout the rest of
the thoracic region, the coupling is less extensive and more

variable. Side-bending in the middle and lower thoracic
regions can be accompanied by either ipsilateral or contra¬
lateral rotation.

Differences in segmental mobility among the spinal
regions can be attributed in large degree to the differences
in facet joint orientation. The facets of a typical thoracic ver¬
tebra are progressively more vertically aligned, with the
superior facets facing posteriorly and slightly laterally and
the inferior facets facing anteriorly and slightly medially [30]
(Fig. 29.19). In contrast, the facets of a typical cervical ver¬
tebra are more horizontally aligned, the superior facets fac¬
ing posteriorly and superiorly and the inferior facets facing
anteriorly and inferiorly. The similarity in coupled motions

Superior View

Lateral View

Figure 29.19: Superior and lateral views reveal that the thoracic and lumbar vertebral facets are more vertically aligned than the cervi¬
cal facets. The cervical and thoracic facets face more anteriorly and posteriorly; the lumbar facets face more medially and laterally.

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

531

Figure 29.20: Flexion of the thoracic spine requires anterior
translation of the inferior facet, limited by the vertical orienta¬
tion of the facet and by the facet joint capsule.

between the cervical and upper thoracic regions results
from the gradual transition from cervical to thoracic facet
orientations [30]. Typical lumbar facets are almost vertical
but face more medially or laterally than either the cervical
or thoracic vertebrae [36].

The facet alignment in the thoracic region allows easy
mobility in axial rotation, limited by the posterior ligaments
[29,52]. The vertical orientation of the thoracic facets and
their position close to the frontal plane require that flexion of
one vertebra on another be accompanied by superior transla¬
tion of the inferior facets on the superior facets (Fig. 29.20).
The facet alignment and joint capsules help limit the transla¬
tion [29,47]. Similarly, the facet orientation produces com¬
pressive forces between articulating facets during extension,
limiting extension range of motion (ROM). The anterior
longitudinal ligament and intervertebral disc limit extension
excursion that may also be limited by the inferiorly projecting
spinous processes of the thoracic region [52]. The
ribs contribute to a reduction in segmental mobility
in all directions [1,32,35,37].

TOTAL MOBILITY OF THE THORACIC SPINE

Few reports exist describing the total excursion available in
the thoracic spine, and only one known report describes the

Clinical Relevance

IMPAIRMENT IN THORACIC RANGE OF MOTION:

The variability in reported ranges of thoracic motion provides
little clinically useful information on the normal mobility that
is expected in a person with a healthy thoracic spine. The
clinician must be careful to base clinical decisions on more
than measurement of impairments in ROM. Additional infor¬
mation that may prove useful to a clinician trying to under¬
stand the basis of a patient's complaints is symmetry of
motions to the left and right and the pattern of pain reported
by the patient during motion. Assessment of segmental mobil¬
ity may also provide insight into the pathomechanics con¬
tributing to the clinical problem.

source of the data [9,10]. Table 29.1 reveals wide variations in
reported total motion in the thoracic spine and demonstrates
the lack of accepted norms for the excursions available in the
thoracic region in individuals without pathology. Despite the
lack of normative data for total excursions, evidence demon¬
strates that left-right symmetry in rotation and side-bending
excursions is a normal finding in individuals without impair¬
ments [18]. Research is required to determine the relative
flexibility in rotation, side-bending, flexion, and extension as
well as to determine the range of mobility available in indi¬
viduals with no spinal impairments. Establishing normative
data describing total excursions of the thoracic spine
will assist clinicians in identifying joint impairments
in patients with thoracic spine dysfunction.

Motion of the Rib Cage

The ribs attach to the vertebrae by gliding synovial joints, and
all but the last two pairs are attached indirectly to the corre¬
sponding rib on the opposite side by the costal cartilages and
sternum. Thus the rib pair forms a closed loop, or closed
kinetic chain, fixed at both ends to the thoracic vertebrae.
The primary motion of the ribs is the elevation and depres¬
sion that is a part of respiration. Because the ribs are attached
to the thoracic vertebrae, they also move in response to
thoracic motion. Motions of the ribs apply forces on the costal
cartilages, and the resulting alterations in the shapes of the
costal cartilages also play a role in respiration.

TABLE 29.1: ROM of the Thoracic Spine in Individuals without Spinal Impairment

Little information is available on the total motion exhibited by the thoracic spine in individuals with

no thoracic impairments.

Flexion/Extension

Side-Bending to One Side

Rotation to One Side

American Academy of

Orthopaedic Surgeons [10]

63°

68°

62°

Gerhardt and Rippstein [9]

85° flexion;

30° extension

30°

45°

532

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 29.21: A. Pump handle motion occurs in the sagittal plane. B. Bucket handle motion occurs in the frontal plane.

ELEVATION AND DEPRESSION OF THE RIBS

Although the ribs exhibit complex three-dimensional motion
[39,54], the motions of the ribs in elevation and depression
can be described mechanically as hingelike. Rib movements
classically are compared to the hinged movements of a pump
handle and a bucket handle [12,23,55] (Fig. 29.21). The han¬
dle represents the closed kinetic chain that consists of a pair
of ribs attached by costal cartilage and the sternum. The axis
of motion passes approximately through the length of the rib
neck, and the costotransverse joint and joint of the head of the
rib together constitute a hinge-like unit on each side of the

vertebral column. Pump-handle motion of the ribs refers to
their motion in the sagittal plane, and bucket-handle
motion represents frontal plane excursion.

Although each rib moves in both the sagittal and frontal
planes, sagittal plane, or pump-handle, movement predom¬
inates in the upper thoracic region, where the neck of the
ribs, and hence the putative axis of motion, lies closer to the
frontal plane. [7,17,22]. The lower thoracic region exhibits
more equally distributed motion in both the sagittal and
frontal planes, and it is unclear if one motion predominates
[7,17,22,55]. Angular measurements of elevation and

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

533

depression reveal larger total excursions of the upper ribs
than of the middle and lower ribs [21,55].

Total chest expansion measurements are more clinically
feasible than measurements of discrete rib motion and are
standard components of the assessment of pulmonary func¬
tion [13]. Average chest expansion during forced inspiration
and expiration in individuals with normal pulmonary function
is presented in Table 29.2. Young adults show average chest
expansions of approximately 7.0 cm or more; women exhibit
slightly less excursion than men [28]. Chest expansion
appears to increase from adolescence to adulthood and then
begins to decrease in elders [4,28]. Measurements are influ¬
enced by the location of the measurement within the chest
and by the position of the subject during the measurement.
Chest excursion is a common clinical assessment in individuals
with suspected pulmonary dysfunction, and clinicians
must choose consistent measurement techniques to
ensure valid assessments.

Clinical Relevance

CHEST EXPANSION IN PATIENTS WITH ANKYLOSING
SPONDYLITIS: Many disorders restrict the motion of the
ribs and affect pulmonary function negatively One such dis¬
order is ankylosing spondylitis, which is an inflammatory
disease affecting the joints of the spine and thorax. Joints of
the lower extremities such as the hips and knees may also be
affected [16]. Inflammation and subsequent ankylosis , or
fusion , of the joints of the spine and ribs lead to decreased

chest expansion. Measurements of chest expansion are useful
outcome variables to assess a patient's progress or the effec¬
tiveness of an intervention [48].

MOTIONS OF THE RIBS WITH THORACIC MOTION

Because ribs are attached to all of the thoracic vertebrae, move¬
ment of these vertebrae also produces rib motion [40, 42].
Flexion and extension of the thoracic spine are accompanied
by depression and elevation of the ribs, respectively. Flexion of
the thoracic spine causes approximation of the ribs, which con¬
tributes to the limitation of total thoracic flexion ROM.
Similarly, extension of the thoracic spine causes the ribs to sep¬
arate and consequently tends to expand the chest. Side-bend¬
ing of the thorax produces approximation of the ribs on the
side of the concavity and separation on the side of the convex¬
ity, contributing to the limitation in side-bending excursion.

Rotation of a thoracic vertebra in the transverse plane affects
the paired ribs attached to it asymmetrically. Rotation of a ver¬
tebra is named according to the side to which the vertebral body
turns; hence right rotation indicates that the body of the verte¬
bra turns to the right. Because the center of rotation (COR) for
a thoracic vertebra rotating in the transverse plane lies some¬
where in the vertebral body, rotation to the right is accompanied
by anterior movement of the left transverse process and poste¬
rior movement of the right transverse process, producing asym¬
metric movement of the left and right ribs [15,51] (Fig. 29.22).
Rotation to the right tends to push the left rib anteriorly and to
pull the right rib posteriorly. Thus rotation of the thoracic spine
alters the contour of the thorax [15,22].

TABLE 29.2: Circumferential Chest Excursions in Subjects with Normal Pulmonary Function

Circumferential chest expansion measurements are similar in men and women but appear to vary with age, subject position,
and measurement site.

Axillary Site (cm)

Xiphoid Process Site (cm)

Carlson [5] a

8.48 ± 0.64

Harris et al. [12]

7.6 ± 1 2 b

7.4 ± 1.7

7 A ± 1.3°

6.9 ± 1.6

6.8 ± 1.6^

8.2 ± 1.4

6.8 ± 1.3 e

7.6 ± 1.5

LaPier et al. [19]

4.75'

4.75

Moll and Wright [28]

6.0 ± 2.149

4.82 ± '\.29 h

Burgos-Vargas et al. [4]

5.6 ±1.76'

a 13 females and 6 males, aged 20 to 30 years, in supine; mean ± standard error of the mean.

b 30 males, aged 19 to 34 years, in supine; mean ± standard deviation.

c 30 females, aged 19 to 34 years, in supine; mean ± standard deviation.

d 30 males, aged 19 to 34 years, in standing position; mean ± standard deviation.

e 30 females, aged 19 to 34 years, in standing position; mean ± standard deviation.

f 20 male and female subjects, aged 20 to 69 years, in standing position; data reported graphically without standard deviations.
g/ \6 males, aged 45-54 years, in standing position; mean ± standard deviation.
h 26 females, aged 45-54 years, in standing position; mean ± standard deviation.

'57 adolescents (112 boys and 45 girls, mean age of 13 years ± 1.1), in standing position; mean ± standard deviation.

534

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 29.22: Right rotation of
the thoracic spine in the trans¬
verse plane rotates the right
transverse process posteriorly
and the left transverse process
anteriorly. This rotation tends to
pull the right rib posteriorly and
to push the left rib anteriorly.

Clinical Relevance

COUPLED MOTIONS AND IDIOPATHIC SCOLIOSIS IN
THE THORACIC SPINE: Although coupled motions in the
thoracic spine are variable in normal motion , side-bending
and contralateral rotation appear more systematically
coupled in individuals with idiopathic scoliosis. Idiopathic
scoliosis is the most common form of scoliotic deformity, typ¬
ically arising in adolescent girls with apparently normal mus¬
culoskeletal systems [50]. The deformity is characterized by a
frontal plane curve that is accompanied by transverse plane
rotation of the involved thoracic vertebrae to the side of the
convexity [44,45,51,53]. Rotation applies posteriorly directed
stresses to the rib on the convex side and anteriorly directed
stresses to the rib on the concave side. The angle of the rib
on the convex side becomes more prominent and produces
the characteristic rib hump (Fig. 29.23). The rib hump ,

always found on the convex side of the curve, is the
401 result of the asymmetrical stress on the attached ribs ,
K&y contributing to asymmetrical growth of the ribs.

Figure 29.23: An individual with idiopathic scoliosis in the tho¬
racic region exhibits a rib hump.

MOVEMENTS OF THE COSTAL CARTILAGES
AND STERNUM

Because most ribs are attached to the vertebral column and
to each other by means of the costal cartilages and sternum,
movement of the ribs is accompanied by movement of these
other structures as well. During elevation of the ribs the ster¬
num rises and moves slightly anteriorly [14,23] (Fig. 29.24).
The difference in lengths of the superior and inferior ribs pro¬
duces an unequal anterior movement that induces a slight
bending or flexion of the sternum at the sternomanubrial
junction [15,40].

Although the sternum rises during rib elevation, its move¬
ment is less than the movement of the ribs. The difference in
movement between the ribs and sternum twists the costal
cartilages [11]. The passive torsion applied to the costal carti¬
lages allows the cartilages to store elastic energy that is

Figure 29.24: Elevation of the ribs causes a slight anterior and
superior movement of the sternum and torsion of the costal
cartilages.

Chapter 29 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE THORACIC SPINE

535

released as the cartilages recoil [38]. Their passive recoil
helps lower the ribs and reduce thoracic volume during exha¬
lation, without the need for muscle contraction.

Clinical Relevance

INCREASED STIFFNESS OF THE COSTAL CARTILAGES
WITH AGING: Aging produces increased stiffness in the
costal cartilages as well as in the lung tissue itself Increased
stiffness increases ventilatory resistance, intensifying the work
of respiration in elders [6,15].

MECHANICS OF RESPIRATION

Respiration, the exchange of oxygen and carbon dioxide
within the lungs, is largely a mechanical process that pumps
air in and out of the lungs. This process, known as ventilation,

is the function of rib motion and operates on the simple
inverse relationship between volume and pressure in a
closed space. Boyle’s law relating pressure and volume of a
gas states:

PjVj = PV = constant

where P is pressure and V is volume. The respiratory pump
moves air by altering the volume of the thoracic cage, thus
altering the internal pressure within the thoracic cavity (Fig.
29.25). When pressure within the cavity is high, air is
expelled, and when pressure is low, air flows in. Specifically,
elevation of the ribs or contraction of the diaphragm muscle
increases the volume of the thorax, decreasing the internal
pressure, and inspiration begins. As the lungs fill with air,
internal pressure rises, and the muscles that have increased
the thoracic volume gradually relax. The elastic recoil of the
chest wall and of the lungs themselves reduces the volume of
the thorax, elevating the internal pressure, and air flows out of
the lungs [11]. Active contraction of the abdominal muscles
can facilitate the descent of the ribs, contributing to a rapid,
more forceful retraction of the thorax and volume reduction
during forced expiration.

Figure 29.25: An increase in thoracic volume decreases thoracic pressure and air rushes in {left); a decrease in thoracic volume increases
thoracic pressure and forces air out of the thoracic cage {right).

536

Part III I KINESIOLOGY OF THE HEAD AND SPINE

SUMMARY

This chapter reviews the structure and motion of the thoracic
spine and rib cage. As in other regions of the vertebral column,
the mobility of the thoracic spine depends on the alignment of
the articular facets and the ligamentous supports. The superior
facets of the thoracic spine face posteriorly and slightly laterally,
and the inferior facets face in the opposite direction. The
alignment of the facets in the thoracic spine contributes to lim¬
its in flexion and extension while providing little restraint to
axial rotation. The mobility and stability of the thoracic spine
also are influenced by the rib cage, which limits excursion of
the thoracic spine while helping to stabilize it. Segmental flex¬
ion and extension in the thoracic spine are smaller than that
available in the cervical and lumbar regions. Thoracic seg¬
mental rotation is greater than lumbar rotation except in the
lower thoracic region. Segmental side-bending in the thoracic
and lumbar regions is less than that in the cervical regions.

Motions of the ribs are less well studied, but data suggest
that normal chest expansion is approximately 7.0 cm (approx¬
imately 3 in) in young adults without impairments and
decreases in elders. The motion of the ribs and thoracic spine
contributes significantly to the mechanics of respiration.
Respiration includes the mechanical pumping action of the
ribs, known as ventilation, which is governed by Boyles law.
Elevation of the ribs increases thoracic volume and decreases
thoracic pressure, allowing air to flow into the thoracic cavity.
Depression of the ribs reverses the volume and pressure so
that air is expelled.

Motions of the thoracic spine also influence the position of
the ribs. In particular, rotation of the thoracic spine distorts
the thorax by pushing one rib anteriorly and pulling the oppo¬
site rib posteriorly. This normal rib and costal cartilage defor¬
mation helps explain the characteristic rib hump in structural
scoliosis deformities. Thus the structure and function of the
thoracic portion of the vertebral column and the rib cage
attached to it are intimately related to each other. The fol¬
lowing chapter reviews the muscles that support and move
the thoracic region and discusses their role in respiration.

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Eng 1995; 117: 48-52.

CHAPTER

Mechanics and Pathomechanics
of the Muscles of the Thoracic Spine

CHAPTER CONTENTS

MUSCLES OF THE POSTERIOR THORAX .538

Superficial Layer .539

Deep Layer of the Posterior Thoracic Region.541

INTRINSIC MUSCLES OF THE THORAX .545

Serratus Posterior Superior and Inferior.546

Intercostal Muscles .546

Transversus Thoracis, Subcostales, and Levator Costarum.549

Diaphragm.550

MUSCLE ACTIVITY DURING RESPIRATION .552

Muscles of Inspiration.552

Muscles of Expiration .553

SUMMARY .553

T he preceding chapter describes the bony architecture of the thoracic region, its supporting structures, and

available motion. The current chapter presents the muscles of the thoracic region. These muscles support and
move the thoracic spine and also participate in respiration.

The goals of this chapter are to

■ Present the reported actions of the muscles found in the thoracic region
■ Discuss the muscles' functional significance

■ Consider the functional consequences of impairments in these muscles
■ Examine the role these muscles play in respiration

Muscles affecting the thoracic region include the muscles of the shoulder, posterior thoracic spine, and abdomen, and
those intrinsic to the thoracic cage, which attach the ribs to the sternum, to vertebrae, or to each other (Fig. 30.1).
Muscles of the shoulder are discussed in Chapter 9 and are discussed only briefly in the current chapter. Abdominal
muscles produce flexion of both the thoracic and lumbar spines and are presented in Chapter 33.

MUSCLES OF THE POSTERIOR THORAX

Muscles of the posterior thorax are grouped in a variety of
ways. The current chapter describes two layers, a superficial
and deep layer. The superficial layer consists of shoulder

muscles, specifically the trapezius, the rhomboids major and
minor, and the latissimus dorsi. The deep layer contains the
erector spinae, the transversospinales muscles, and the inter-
spinous and intertransverse muscles.

538

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

539

Figure 30.1: Muscles affecting the thorax include shoulder muscles, muscles of the posterior spine, abdominal muscles, and muscles
intrinsic to the thorax.

Superficial Layer

MUSCLE ACTION: TRAPEZIUS AND RHOMBOIDS

Action

Evidence

Thoracic spine extension
(bilateral contraction)

Inadequate

Thoracic spine side bending
(unilateral contraction)

Inadequate

Contralateral rotation of thoracic spine
(unilateral contraction)

Inadequate

MUSCLE ACTION: LATISSIMUS DORSI

Action

Evidence

Thoracic spine extension
(bilateral contraction)

Conflicting

MUSCLE ACTION: LATISSIMUS DORSI (Continued)

Action

Evidence

Thoracic spine side bending
(unilateral contraction)

Supporting

Ipsilateral rotation of thoracic spine
(unilateral contraction)

Supporting

The muscles of the superficial layer of the posterior tho¬
rax assist in supporting the thorax (Fig. 30 . 2 ). They attach to
spinous processes of thoracic vertebrae. With their distal
attachments on the scapula and humerus fixed, these
muscles may move the thoracic spine. Just as bilateral con¬
traction of the upper trapezius produces extension of the
cervical spine, bilateral contraction of the trapezius and/or

540

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 30.2: Superficial muscles of the posterior thorax are
shoulder muscles and include the trapezius, the rhomboids major
and minor, and the latissimus dorsi.

rhomboids major and minor may help support the thoracic
spine in extension, although there are minimal data verify¬
ing such a role [43].

The latissimus dorsi exhibits an extension moment arm in
the lumbar region and participates in extension of the lumbar
spine [35,38]. Its role in flexion and extension in the thoracic
spine is controversial, because as the latissimus dorsi projects
to its distal attachment on the humerus, it appears to pass
from the posterior to the anterior surface of the thorax
(Fig. 30 . 3 ). Measurement of its flexion and extension moment
arms using magnetic resonance imaging (MRI) suggests that
it continues to have an extension moment arm to at least the
fifth thoracic vertebra [35]. Others suggest that it develops a
flexion moment arm on the thoracic spine at its humeral end

Figure 30.3: The latissimus dorsi lies diagonally on the thorax from
the posterior to the anterior surface. It has an extension moment
arm on the lumbar and thoracic spines but may exert a flexion
moment on the upper thoracic spine.

[12,42]. Tightness of the latissimus dorsi reportedly con¬
tributes to excessive thoracic kyphosis, although there are no
known studies verifying this association [25].

Unilateral contraction of the trapezius, rhomboids, and
latissimus dorsi reportedly produces side-bending and rota¬
tion of the thoracic spine, although only the latissimus dorsi
has data directly supporting its contributions to these actions
[35,38]. In individuals with idiopathic scoliosis, the trapezius
on the convex side exhibits a relative increase in type I mus¬
cle fibers with a slight atrophy of type II fibers, and the
trapezius and rhomboid muscles may contribute to the spinal
deformity [41,60,61].

Electromyography (EMG) data reveal activity of the latis¬
simus dorsi during ipsilateral rotation with its inferior attach¬
ment fixed [39] (Fig. 30 . 4 ). With their distal attachments fixed,
contraction of the trapezius and rhomboids major and minor
could potentially produce contralateral rotation by pulling the
spinous processes toward the distal attachment. No known
data are available to identify if, or under what conditions, the

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

541

Figure 30.4: The latissimus dorsi pulls the shoulder girdle posteri¬
orly and participates in ipsilateral rotation of the thorax. The
trapezius and rhomboids major and minor pull on the spinous
processes of the thoracic spines and, consequently, produce con¬
tralateral rotation.

trapezius and rhomboid muscles contribute to movement of
the thoracic spine, and further research is needed to clarify
their participation in thoracic spine motion. [25]

Deep Layer of the Posterior
Thoracic Region

The deep muscular layer of the back is separated from the
superficial layer by the thoracolumbar fascia. The deep layer
can be divided into additional muscular subsets: the erector
spinae, the transversospinales, and the deepest layer consist¬
ing of the interspinales and intertransversarii.

Figure 30.5: The three groups of the erector spinae are, from lat¬
eral to medial, the iliocostalis, longissimus, and spinales muscle
groups.

ERECTOR SPINAE

The erector spinae extends from the pelvis to the occiput and
consists of three main groups of muscles: the iliocostalis,
longissimus, and spinalis muscles (Fig. 30.5) (Muscle
Attachment Boxes 30.1-30.3). The spinales are found in the
thoracic, cervical, and occipital regions only. The longissimus
consists of longissimus thoracis, cervicis, and capitis. The
longissimus thoracis contains both a thoracic portion
(the longissimus thoracis pars thoracis) and a lumbar portion
(the longissimus thoracis pars lumborum) [28,29]. The ilio¬
costalis consists of cervical, thoracic, and lumbar segments
but, like the longissimus thoracis, the iliocostalis lumborum

542

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 3

ATTACHMENTS AND INNERVATION OF
THE ILIOCOSTALIS MUSCLES AFFECTING
THE THORACIC AND LUMBAR SPINES

lliocostalis thoracis:

Inferior attachment: Upper borders of the lower
six rib angles medial to the insertion of the ilio-
costalis lumborum

Superior attachment: Superior borders of the
upper six ribs at their angles and the posterior
aspect of the transverse processes of the seventh
cervical vertebra. The iliocostalis thoracis lies
between the iliocostalis lumborum pars thoracis
and the longissimus thoracis pars thoracis.

Iliocostalis lumborum pars thoracis:

Inferior attachment: Crest of the ilium from the
posterior superior iliac spine laterally approxi¬
mately 5 cm

Superior attachment: Angles of all 12 ribs

Iliocostalis lumborum pars lumborum:

Inferior attachment: Iliac crest

Superior attachment: Transverse processes of the
first four lumbar vertebrae and thoracolumbar
fascia

Innervation: Dorsal rami of the thoracic and lum¬
bar spinal nerves.

Palpation: The erector spinae can be palpated as a
group through the thoracolumbar fascia as the
muscles parallel the vertebral column in the lum¬
bar and thoracic regions. The iliocostalis can not
be distinguished from the rest of the erector
spinae.

contains a thoracic component (the iliocostalis lumborum pars
thoracis) and a lumbar portion (the iliocostalis pars lumborum).

Action

MUSCLE ACTION: ERECTOR SPINAE

Action

Evidence

Trunk extension (bilateral contraction)

Supporting

Trunk side bending (unilateral contraction)

Supporting

Ipsilateral rotation of trunk (unilateral

Supporting

contraction)

The spinalis thoracis and iliocostalis thoracis span only the
thoracic region and act only to extend the thoracic spine. In

MUSCLE ATTACHMENT BOX 30.2

ATTACHMENTS AND INNERVATION OF
THE LONGISSIMUS MUSCLES AFFECTING
THE THORACIC AND LUMBAR SPINES

Longissimus thoracis pars thoracis:

Inferior attachment: Fibers contribute to the
erector spinae aponeurosis and the spinous
processes of the lumbar and sacral vertebrae
and onto the ilium.

Superior attachment: Transverse processes of all
thoracic vertebrae and the lower eight or nine ribs

Longissimus thoracis pars lumborum:

Inferior attachment: Posterior superior iliac spine

Superior attachment: Transverse and accessory
processes of the lumbar vertebrae

Innervation: Dorsal rami of the lumbar and tho¬
racic spinal nerves

Palpation: Cannot be differentiated from the rest of
the erector spinae.

contrast, the longissimus thoracis and iliocostalis lumborum
with their pars thoracis and pars lumborum components cross
the thoracic and lumbar regions, producing combined tho¬
racic and lumbar extension [28,29]. A detailed description of
their effects on the lumbar spine is found in Chapter 33.

EMG data verify the role of the erector spinae as a whole
in extension of the trunk during bilateral contraction [1,2]. The

MUSCLE ATTACHMENT BOX 30.3

ATTACHMENTS AND INNERVATION
OF THE SPINALIS MUSCLES OF THE
THORACIC SPINE

Spinalis thoracis:

Inferior attachment: Arises by three to four ten¬
dons from the eleventh thoracic to the second
lumbar vertebral spines running medial to, and
blending with, iliocostalis thoracis

Superior attachment: Spines of the upper four to
eight thoracic vertebrae by separate tendons and
blending with semispinalis thoracis

Innervation: Dorsal rami of thoracic spinal nerves

Palpation: Cannot be differentiated from the rest
of the erector spinae.

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

543

Figure 30.6: EMG studies demonstrate
that the erector spinae cease electrical
activity about halfway through a full for¬
ward bend and remain silent until the
individual returns about halfway toward
the upright position.

Active EMG EMG Activity Ceases

muscles contract eccentrically during forward-bending from a
standing position and concentrically as the spine returns to an
erect position. The erector spinae becomes electrically silent
during forward flexion when the trunk reaches approximately
two thirds of its maximal available excursion, and remains
silent as the trunk initiates the return to the erect posture
[1,2,10,19,23,26] (Fig. 30.6). Only after the trunk reaches
approximately 45° do the muscles resume activity. The poste¬
rior ligaments of the spine and intervertebral discs provide the
primary support to the spine in the maximally flexed position,
and the passive recoil of these tissues, combined with the
action of the superficial back muscles and hip extensors, helps
initiate the return to upright posture [2,19,40].

Unilateral contraction of the iliocostalis and longissimus
muscles is associated with side-bending and with ipsilateral
rotation of the spine [1,46,58]. The erector spinae as a whole
exhibits a side-bending moment arm of 25 to 35 mm in the
thoracic region, compared with moment arms of approximately
50 mm for extension [35,38]. The moment arms of muscles
within the erector spinae are quite varied, so that substantial
moment arms in both extension and side-bending are exhibit¬
ed by at least some segments of the erector spinae [29,30].

It is important to recognize that from the erect posture, the
erector spinae contract eccentrically to control forward-bending
of the trunk and contract concentrically briefly to initiate either
extension or side-bending (Fig. 30.7). Continuation of hyperex¬
tension or side-bending is facilitated by the moments exerted by
the weight of the head and trunk. The abdominal muscles con¬
tract eccentrically to control the extension moment of the head
and trunk [2]. Similarly, side-bending of the trunk from
the upright posture is controlled by the abdominal mus¬
cles and contralateral erector spinae [1,2,36].

The erector spinae in the thoracic region are composed
primarily of type I fibers, or slow-twitch fibers (approximately

75%), but the erector spinae in the lumbar region exhibit
a more even distribution of type I and type II fibers
(approximately 57% type I) [50]. The preponderance of
fatigue-resistant muscle fibers in the thoracic spine suggests
that the erector spinae in the thoracic region plays a primary
role in postural support and in stabilizing the costovertebral
joints [47].

Figure 30.7: The erector spinae muscles typically contract to con¬
trol forward-bending or side-bending, requiring an eccentric con¬
traction of the whole group during forward-bending and the
contralateral group during side-bending.

544

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 30.8: Transversospinales muscles include the semispinalis,
multifidus, and rotatores muscles.

TRANSVERSOSPINALES

The transversospinalis muscle group consists of the semi-
spinales, multifidus, and rotatores muscles (Fig. 30.8) (Muscle
Attachment Boxes 30.4-30.6). The group is so named because
of its attachments, which consist of an inferior attachment on a
transverse process and a superior attachment on a spinous
process [46,58]. Like the erector spinae, these muscles are
more thoroughly studied in the lumbar region.

Action

MUSCLE ACTION: TRANSVERSOSPINALES

Action

Evidence

Trunk extension (bilateral contraction)

Supporting

Trunk side bending (unilateral contraction)

Supporting

Contralateral rotation of trunk (unilateral
contraction)

Supporting

MUSCLE ATTACHMENT BOX 30.4

ATTACHMENT AND INNERVATION
OF THE SEMISPINALIS MUSCLES

The transversospinales muscle group lies deep to the
erector spinae with fibers running superiorly and me¬
dially from transverse processes to spinous processes.

Semispinalis thoracis:

Inferior attachment: Transverse processes of the sixth
to the tenth thoracic vertebrae by tendinous slips

Superior attachment: Spinous processes of the lower
two cervical and upper four thoracic vertebrae

Innervation: Dorsal rami of the cervical and tho¬
racic spinal nerves

Palpation: Cannot be palpated directly.

MUSCLE ATTACHMENT BOX 30.5

ATTACHMENT AND INNERVATION
OF THE MULTIFIDUS

Inferior attachment: Arises from the transverse
processes of thoracic vertebrae

Superior attachment: Fibers run superiorly and medi¬
ally in the space between the transverse processes to
their spinous process insertions along the length of
the spine. Fasciculi vary in length, with the most
superficial running from one vertebra to the third or
fourth above, the next deepest running from one
vertebra to the second or third above and the deep¬
est connecting adjacent vertebrae.

Innervation: Dorsal rami of the thoracic spinal nerves
Palpation: Cannot be palpated.

MUSCLE ATTACHMENT BOX 30.6

ATTACHMENT AND INNERVATION
OF THE ROTATORES

Rotatores thoracis:

Superior attachment: Eleven pairs of muscles that
take origin from the lower border and lateral
surfaces of the lamina from the first to the
eleventh vertebrae

Inferior attachment: Upper and posterior portions
of the transverse processes of the vertebra below

Innervation: Dorsal rami of the thoracic spinal
nerves

Palpation: Cannot be palpated.

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

545

Figure 30.9: The transversospinales muscles produce contralateral
rotation by pulling the spinous process of the superior vertebrae
toward the transverse process of the inferior vertebrae.

Like the erector spinae, these muscles contract eccentri¬
cally to control flexion and contralateral side-bending from
the upright position. They produce contralateral rotation by
pulling the spinous processes of the superior vertebrae
toward the transverse processes of the inferior vertebrae
(Fig. 30.9). The thoracic multifidi are longer and thinner, with
longer and more obliquely aligned tendons than those in the
lumbar region [4]. Thus while the multifidi in the lumbar
region may contribute significantly to the compressive loads
on the lumbar vertebrae, the multifidi in the thoracic region
may contribute more to side-bending and rotation moments.
The multifidi in the thoracic region are, like the erector
spinae, composed mostly of type I fibers (approximately 75%)
and exhibit a slightly smaller concentration of type I fibers
(63%) in the lumbar region [50].

The rotatores are more fully developed in the thoracic
region than anywhere else, yet they are still very small and
hence not very powerful. They may act more as position sen¬
sors than as torque producers, but additional research is
needed to verify this hypothesis.

INTERSPINALES AND INTERTRANSVERSARII

The interspinales and intertransversarii muscles are found at
only a few of the thoracic vertebral levels, primarily at the
superior and inferior aspects of the thorax, and may even be
absent completely in the thoracic region [46,58] (Muscle
Attachment Boxes 30.7 and 30.8). Their functional signifi¬
cance is unclear.

MUSCLE ATTACHMENT BOX 30.7

ATTACHMENT AND INNERVATION
OF THE INTERSPINALES

Attachments: The interspinales are most distinct in
the cervical and lumbar regions running along
either side of the interspinous ligaments to connect
the apices of contiguous spinous processes.
Occasionally a pair may be found between the last
thoracic and first lumbar vertebrae and between
the fifth lumbar and the sacrum, and they may be
absent in the thoracic region.

Innervation: Dorsal rami of the spinal nerves

Palpation: Cannot be palpated.

IMPAIRMENT OF THE MUSCLES
OF THE POSTERIOR THORAX

Discrete impairments of the deep posterior thoracic muscles
are difficult to identify. The erector spinae muscles in the tho¬
racic region contribute to the extension moment of the thoracic
and lumbar regions. Consequently, weakness of the thoracic
erector spinae contributes to a decrease in total trunk extension
strength. Individuals with idiopathic scoliosis exhibit atrophy of
the muscles of the posterior thorax, particularly on the concave
side, and a higher than normal percentage of type I muscle
fibers on the convex side of the deformity, but the clinical
significance of these differences is unclear [15,60,62].

INTRINSIC MUSCLES OF THE THORAX

The intrinsic muscles of the thorax consist of those muscles
that attach the ribs to the vertebral column, to the sternum,
or to other ribs. This group includes the posterior serratus

MUSCLE ATTACHMENT BOX 30.8

ATTACHMENT AND INNERVATION
OF THE INTERTRANSVERSARII

Attachments: The intertransversarii are sets of mus¬
cles running between vertebrae and are most dis¬
tinct in the cervical spine. In the thoracic region,
down to the first lumbar vertebra, intertransversarii
connect transverse processes as a single muscle slip.

Innervation: Dorsal rami of thoracic spinal nerves
with ventral rami contribution

Palpation: Cannot be palpated.

546

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 30.10: The superior and inferior serratus posterior muscles
lie deep to the superficial muscles of the posterior thorax and
superficial to the erector spinae.

superior and inferior; the external, internal, and innermost
intercostals; the transversus thoracis, subcostales, levator
costarum, and the diaphragm.

Serratus Posterior Superior and Inferior

MUSCLE ACTION: SERRATUS POSTERIOR SUPERIOR
AND INFERIOR

Action

Evidence

Elevate the ribs (superior)

Inadequate

Depress the ribs (inferior)

Inadequate

MUSCLE ATTACHMENT BOX 30.9

ATTACHMENTS AND INNERVATION OF
THE SERRATUS POSTERIOR SUPERIOR

Superior attachment: Arises from the inferior part
of the ligamentum nuchae and the spines of the
upper three or four thoracic vertebrae and seventh
cervical vertebrae

Inferior attachment: Four digitations descend inferi-
orly and laterally to insert just lateral to the angles
of the second, third, fourth, and fifth ribs on their
superior and superficial surfaces.

Innervation: Second to the fourth intercostal nerves.
The fifth intercostal nerve may also contribute.

Palpation: Cannot be palpated.

The serratus posterior superior and inferior muscles attach
the ribs to the thoracic vertebrae (Fig. 30.10) (Muscle
Attachment Boxes 30.9 and 30.10). Neither muscle is well
studied, and the posterior serratus inferior is absent in some
individuals [58]. The serratus posterior superior is aligned to
elevate the ribs and thus may be active in inspiration; the ser¬
ratus posterior inferior may lower the ribs and perhaps par¬
ticipates in exhalation.

Intercostal Muscles

The intercostal muscles include the external, internal, and
innermost intercostal muscles (Fig. 30.11) (Muscle Attachment
Boxes 30.11-30.13).

MUSCLE ATTACHMENT BOX 3

ATTACHMENTS AND INNERVATION OF
THE SERRATUS POSTERIOR INFERIOR

Superior attachment: The outer surface and inferior
borders of the lower four ribs just lateral to their
angles by four digitations. Fewer digitations may be
present, and infrequently the entire muscle may be
absent.

Inferior attachment: The spines of the upper two or
three lumbar vertebrae and lower two thoracic ver¬
tebrae and their supraspinous ligaments. It may also
attach to the thoracolumbar fascia.

Innervation: Ninth through eleventh or twelfth
intercostal nerves.

Palpation: Cannot be palpated.

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

547

Figure 30.11: The intercostal muscles include the external, inter¬
nal, and innermost intercostal muscles.

MUSCLE ATTACHMENT BOX 3

ATTACHMENT AND INNERVATION
OF THE INTERNAL INTERCOSTALS

Superior attachment: Eleven pairs of muscles arise
anteriorly at the costal cartilages of the first seven
ribs and the cartilaginous ends of the remaining
ribs, with continuing attachment along the floor of
the costal grooves back to the rib angles. From the
rib angles, they continue as an aponeurotic layer
called the internal intercostal membrane and blend
with the superior costotransverse ligaments.

Inferior attachment: Fibers descend obliquely from
the superior rib to the upper border of the rib below
in a direction nearly perpendicular to fibers of the
external intercostals. Fibers of the lower two rib
spaces may blend with the internal oblique muscle.

Innervation: The corresponding intercostal nerves

Palpation: In the spaces between the upper ribs, just
lateral to the sternum where the external inter¬
costal muscles are membranous.

MUSCLE ATTACHMENT BOX 30.11

ATTACHMENT AND INNERVATION
OF THE EXTERNAL INTERCOSTALS

Superior attachment: Tubercles of the ribs, blend¬
ing with the posterior fibers of the costotrans¬
verse ligaments, continuing along the lower rib
borders almost to the costal cartilages, where it
then proceeds anteriorly toward the sternum as
an aponeurotic layer called the external inter¬
costal membrane.

Inferior attachment: Eleven pairs of muscles run
from their superior attachment to the upper bor¬
der of the rib below. Fibers travel obliquely down¬
ward and laterally at the back of the thorax and
downward, medially and forward on the anterior
thorax. In the lower two rib spaces, fibers attach
to the ends of the costal cartilages; in the upper
two or three rib spaces, they do not quite reach
the rib ends. Fibers from the lower intercostal
spaces may blend with the external oblique
muscle.

Innervation: The corresponding adjacent intercostal
nerves

Palpation: The intercostals are palpated together in
the intercostal spaces.

ACTION

MUSCLE ACTION: INTERCOSTALS EXTERNAL
AND INTERNAL

Action

Evidence

Contralateral trunk rotation (external)
Ipsilateral trunk rotation (internal)

Supporting

Inspiration (external)

Conflicting

Exhalation (internal)

Conflicting

MUSCLE ATTACHMENT BOX 30.13

ATTACHMENT AND INNERVATION OF
THE INNERMOST INTERCOSTAL MUSCLES
(INTERCOSTALES INTIMI)

Attachments: Pairs of muscles attach to the internal
aspect of two adjacent ribs. They lie deep to the inter¬
nal intercostals, with fibers running in the same direc¬
tion. The innermost intercostal muscles become more
substantial posteriorly and in the middle two quarters
of the lower intercostal spaces. They are smaller and
may be absent at the higher thoracic levels.

Innervation: The corresponding intercostal nerves

Palpation: Cannot be palpated directly.

548

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Several studies investigated the role of the external and
internal intercostal muscles but few studies have investigated
the function of the innermost intercostal muscles that lie par¬
allel to the internal intercostals and are smaller, more vari¬
able, and sometimes absent. Their actions are inferred from
studies of the internal intercostal muscles [58]. Despite
efforts to define the functional role of the external and inter¬
nal intercostal muscles, controversy remains. Intercostal mus¬
cles are difficult to study because they are thin and overlie
each other through most of their lengths [57]. The external
intercostal is the only intercostal muscle belly in an intercostal
space between the angle of the rib and tubercle of the rib.
Similarly, the only location where the internal intercostal
muscles are not covered by external intercostal muscles is in
the spaces between the costal cartilages. In this region they
are known as the parasternal intercostal muscles. Most of
the studies of the internal intercostal muscles are investiga¬
tions of the parasternal muscles [28,48].

EMG data reveal activity of the external intercostal mus¬
cles during contralateral rotation and activity in the internal
intercostal muscles during ipsilateral rotations [45,56]. These
actions parallel the actions of the oblique abdominal muscles,
which have similar lines of pull. The intercostal muscles
appear to play an important role in trunk rotation; however,
their short fiber length suggests that additional muscles are
necessary to produce an excursion through the full available
range of motion.

Some EMG studies and biomechanical models support the
traditional view that the external intercostal muscles partici¬
pate in inspiration and the internal intercostal muscles in expi¬
ration [28,31,44,47,59], although most studies agree that at
least the parasternal portion of the internal intercostal muscles
are active in inhalation [6,8,11,20,28,55]. Other studies identify
activity in both muscle groups during both inspiration and
exhalation [11,58]. Simultaneous activity of the intercostal
muscles suggests that the muscles may work together to stabi¬
lize the rib cage against the changing pressures of the thoracic
cavity and the displacement of the diaphragm [6,11,47,58].

The mechanics of ventilation are based on the relationship
between pressure and volume of a gas. This relationship dic¬
tates that as volume increases, pressure decreases, and as
volume decreases, pressure increases. Decreased pressure
within the thoracic cavity causes air to enter the lungs. The
internal pressure of the thorax tends to have the same effect
on the flexible walls of the thorax (Fig. 30.12). To prevent the
collapse of the rib cage during inhalation and its expansion
during exhalation, the intercostal muscles contract simultane¬
ously to support the thorax. The fibers of the internal and
external intercostal muscles are approximately perpendicular
to each other and thus are well aligned to function together to
stiffen the thoracic walls and stabilize the ribs.

IMPAIRMENT OF THE INTERCOSTAL MUSCLES

Weakness of the intercostal muscles may occur in individuals
following cervical or thoracic spinal cord injury as well as in
other neuromuscular and musculoskeletal disorders [3,7,

Figure 30.12: During inspiration, the pressure within the thorax is
low and tends to collapse the rib cage. During expiration, the
pressure within the thorax is high and tends to expand the thorax.

18,51]. Isolated evaluation of these muscles is impossible
because of their location and because they function as a group
with the diaphragm during respiration. Consequently, assess¬
ment is based on clinical observations and pulmonary func¬
tion tests, including maximum inspiratory and expiratory
pressures and vital capacity [3,7,18]. Studies show that mus¬
cles of respiration are amenable to exercise and demonstrate
improved function following exercise [34,44].

Clinical Relevance

PARADOXICAL BREATHING: Paradoxical breathing is a

breathing pattern in which the change in circumference of
either the thorax or abdomen is opposite that expected from
the relationship of volume and pressure in normal respiration.
A patient with flaccid paralysis of the intercostal muscles fol¬
lowing a cervical or high thoracic spinal cord injury demon¬
strates paradoxical breathing with abnormal movement of the
rib cage during respiration. Inspiration occurs by means of
contraction of the diaphragm , which increases the volume of
the thoracic cage and decreases the pressure. In the absence
of intercostal muscle activity to stabilize the ribs , the decrease
in thoracic pressure also causes the thorax to collapse toward

(< continued )

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

549

Figure 30.13: Paradoxical breathing is movement of the thorax or
abdomen in a direction opposite to the expected direction for
the phase of respiration. Pictured is paradoxical breathing in
which the circumference of the abdomen decreases during inspi¬
ration, when it is expected to expand.

(Continued)

the lungs. Consequently, the circumference of the thorax
decreases during inspiration instead of increasing.

Paradoxical breathing also occurs if the intercostal muscles
contract in the absence of diaphragmatic activity; inspiration
occurs with chest expansion , but the diameter of the abdomen
decreases as the diaphragm is pulled into the thoracic cavity
(Fig. 30.13). Paradoxical breathing patterns , identifiable by
careful observation , provide valuable clinical information to
the clinician regarding the patient's respiratory function.

Tightness of the intrinsic muscles of the thorax may occur fol¬
lowing thoracic surgery or be present on the concave side of
a thoracic scoliosis. Tightness of the intrinsic muscles restricts
the mobility of the rib cage and, in particular, limits chest
expansion. Individuals with scoliosis demonstrate decreased
pulmonary function attributed to the diminished chest expan¬
sion resulting from the deformity [27,53]. Stretching exercises
may improve pulmonary function in these patients [49,54].

Transversus Thoracis, Subcostales,
and Levator Costarum

MUSCLE ACTION: TRANSVERSUS THORACIS,
SUBCOSTALES, LEVATOR COSTARUM

Action

Evidence

Depress the ribs (transversus thoracis)

Supporting

Depress the ribs (subcostales)

Inadequate

Elevate the ribs (levator costarum)

Supporting

MUSCLE ATTACHMENT BOX 3

ATTACHMENT AND INNERVATION
OF THE SUBCOSTALES

Superior attachment: Internal surface of the ribs
near the angle

Inferior attachment: Internal surface of the second or
third rib below, traveling with the innermost inter¬
costal muscles between the intercostal vessels and
nerves and the pleura. They usually are most devel¬
oped in the lower thorax, with the fibers running
parallel to those of the internal intercostal muscles.

Innervation: The corresponding intercostal nerves

Palpation: Cannot be palpated.

The transversus thoracis and subcostales lie on the deep sur¬
face of the thoracic cage, and the levator costarum muscles
lie on the posterior aspect of the thorax. These muscles are
not well studied (Fig. 30.14) (Muscle Attachment Boxes
30.14-30.16). The transversus thoracis (also known as the
triangularis sterni) depresses the lower ribs and participates
in expiration [13,28,46,58]. The subcostales muscles lie par¬
allel to the internal intercostal muscles and are found pri¬
marily in the lower thoracic region [46,58]. They appear to
depress the ribs, although no known studies verify that func¬
tion. The levator costarum, as its name suggests, appears to
elevate the ribs [28,46,58].

MUSCLE ATTACHMENT BOX 3

ATTACHMENT AND INNERVATION
OF TRANSVERSUS THORACIS

Superior attachment: Fibers diverge into slips pass¬
ing from their inferior attachment to insert onto
the lower borders and inner surfaces of the costal
cartilages of the second to the sixth ribs.

Inferior attachment: Lower third of the posterior
surface of the sternum, the xiphoid process, and the
costal cartilages of the lower three or four true ribs
near their sternal ends. Lower slips run horizontally
and are contiguous with superior fibers of transver¬
sus abdominis; the upper slips run obliquely upward
and laterally.

Innervation: The corresponding intercostal nerves
Palpation: Cannot be palpated.

550

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 30.14: The deep layer of intrinsic muscles of the thorax includes the subcostales and transversus thoracis, lying on the deep sur¬
face of the thoracic wall, and the levator costarum on the posterior surface of the thorax.

MUSCLE ATTACHMENT BOX 30.16

ATTACHMENT AND INNERVATION
OF THE LEVATORES COSTARUM

Superior attachment: Twelve pairs of triangular
muscle bundles arise from the tips of the transverse
processes of the seventh cervical and the first to the
eleventh thoracic vertebrae.

Inferior attachment: Superior and outer edge of the
rib immediately below the vertebra of origin
between the tubercle and angle. Fibers run laterally
and inferiorly, paralleling the posterior borders of
the external intercostals. The lower four muscle
pairs may have an additional attachment to the sec¬
ond rib below their origin.

Innervation: Lateral branches of the dorsal rami of
the corresponding thoracic spinal nerves

Palpation: Cannot be palpated.

Diaphragm

The diaphragm is an unusual muscle because it is a somewhat
circular sheet of muscle with a central tendon and bony
attachment only along its circumference. During contraction
it pulls from its periphery to its central tendon (Fig. 30.15)
(.Muscle Attachment Box 30.17).

ACTION

MUSCLE ACTION: DIAPHRAGM

Action

Evidence

Lower floor of thoracic cavity

Supporting

Elevate the lower ribs

Supporting

Because the diaphragms actions increase thoracic volume,
the diaphragm is unquestionably a muscle of inspiration
[8,9,11,17,46,58]. The diaphragm contracts from its peripher¬
al attachments on the lower ribs and vertebrae and pulls on
the central tendon, thereby pulling the central tendon inferi¬
orly and increasing the vertical length of the thoracic cavity.

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

551

Central

tendon

Esophageal

hiatus

Aortic hiatus

Figure 30.15: The diaphragm forms a movable floor of the thoracic cavity, attached peripherally to the sternum, lower ribs, lumbar ver¬
tebrae, and the fibrous arches surrounding the aorta and esophagus.

As the thoracic floor lowers, the volume of the abdominal cav¬
ity decreases, and abdominal pressure increases. If the
abdominal wall remains relaxed, the abdominal viscera are
pushed anteriorly, and the anterior-posterior diameter of the
abdominal cavity increases [16]. Although the diaphragm lies

MUSCLE ATTACHMENT BOX 3

ATTACHMENT AND INNERVATION
OF THE DIAPHRAGM

Attachments: The diaphragm attaches peripherally
in three parts: sternal, costal, and lumbar or crural.
The sternal portion attaches to the posterior surface
of the xiphoid process. This attachment may be
absent. The costal portion attaches to the deep sur¬
faces of the lower six costal cartilages and ribs. The
lumbar portion arises from the lumbar vertebrae
and from two aponeurotic arches and the medial
and lateral arcuate ligaments. The peripheral
attachments converge to attach on a central tendon
that has no bony attachments.

Innervation: Phrenic nerve (C3-5)

Palpation: Cannot be palpated.

deep to the lower ribs and cannot be palpated, its contraction
is readily inferred by observing the movements of the abdom¬
inal contents.

The abdominal viscera limit the full descent of
the diaphragm allowable by the contractile length of the
diaphragms muscle fibers. Continued contraction of
the diaphragm after it reaches its maximum descent onto the
viscera elevates the lower ribs, continuing to increase thoracic
volume [46,58] (Fig. 30.16).

Clinical Relevance

VALSALVA MANEUVER: The Valsalva maneuver is the sus¬
tained simultaneous elevation of thoracic and abdominal pres¬
sure and is a natural response during vigorous muscle con¬
tractions such as lifting a heavy weight or defecating. It is per¬
formed at the end of inspiration by holding the breath and
contracting the abdominal muscles. At the end of inspiration,
the diaphragm is contracted', which increases abdominal pres¬
sure. Simultaneous contraction of the muscles of the abdomi¬
nal wall increases abdominal pressure further. At the same
time, thoracic pressure is high because air fills the thoracic
cavity, and the airway is closed. High thoracic pressure inhibits
venous return to the heart and elevates blood pressure. It also

( continued )

552

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 30.16: Action of the diaphragm. A. Contraction of the diaphragm lowers the thoracic floor. B. When the descent of the
diaphragm is stopped by the viscera, continued contraction elevates the lower ribs.

(Continued)

increases the resistance to blood flow to and from the lungs.
These changes are dangerous in an individual with hyperten¬
sion or other forms of cardiopulmonary disorders. Individuals
at risk of cardiopulmonary dysfunction must be instructed to
avoid the maneuver.

IMPAIRMENT OF THE DIAPHRAGM

Weakness and or paralysis of the diaphragm can occur in indi¬
viduals with high cervical spinal cord injuries (C3) who also
exhibit weakness or loss of the intercostal muscles and
scalenes. Such patients have profound impairment of inspira¬
tion and require at least intermittent mechanical ventilation.
Weakness of the diaphragm, even with intact rib cage muscles,
produces substantial impairment of the inspiratory apparatus.
Isolated weakness of the diaphragm produces a paradoxical
breathing pattern in which the circumference of the abdomi¬
nal cavity decreases during inspiration.

MUSCLE ACTIVITY DURING
RESPIRATION

The primary role of the muscles of respiration is to regulate
volume of the thoracic cavity and hence to control the pres¬
sure within the cavity. Muscles of inspiration are those that

increase thoracic volume, and muscles of expiration decrease
thoracic volume.

Muscles of Inspiration

Although individuals exhibit variability in respiratory muscle
activation, the diaphragm is the primary muscle of inspiration
[14,58]. It is responsible for approximately 60% of vital capac¬
ity (the amount of inspired and expired air during maximal
inspiration and expiration) and 70% of tidal volume (the vol¬
ume of inspired and expired air during relaxed breathing)
[14,58]. The diaphragm is not, however, the sole muscle of
inspiration [8,11,16]. The intercostal muscles participate
either directly to elevate or depress the ribs or indirectly to
stabilize the thorax. The parasternal intercostal muscles and
the scalene muscles in the cervical region also participate in
relaxed inspiration, elevating the ribs in the sagittal plane as
the diaphragm increases the vertical and lateral dimensions of
the thorax [6,11,14]. Quiet breathing in standing relies more
on rib cage movement than on abdominal excursion [11,53].
There are many additional muscles, including the sternoclei¬
domastoid, the suprahyoids, and the pectoralis major and
minor, that may be recruited with more-vigorous inspiratory
efforts [11,21,22,24,32,52]. Increased respiratory challenge
induces greater recruitment of the muscles that
IRI attach to the upper rib cage than of the diaphragm

KSX [5,11,37].

Chapter 30 I MECHANICS AND PATHOMECHANICS OF THE MUSCLES OF THE THORACIC SPINE

553

Clinical Relevance

RESPIRATORY MUSCLE TRAINING IN HEALTH
AND DISEASE: Muscles of respiration are voluntary mus¬
cles that are under both voluntary and reflex control. But
like all other voluntary muscles , they are amenable to train¬
ing. Research demonstrates that respiratory muscle training
can increase muscular endurance and enhance physical
performance [33]. Exercises include inspiration against
increased airflow resistance. Patients with chronic obstruc¬
tive pulmonary disease (COPD) experience a chronic
increase in respiratory resistance and frequently recruit
accessory muscles of inspiration , including the sternocleido¬
mastoid and scalenes. Exercises to facilitate the use of the
diaphragm can help the efficiency of the patient's ventilatory
pump and may be beneficial in improving respiratory
function in individuals with mild or moderate COPD.

Figure 30.17: In forced expiration, the muscles of the abdominal
wall compress the abdominal cavity and depress the ribs,
decreasing the volume of the thoracic cavity and forcing air out.

Muscles of Expiration

The passive recoil of the rib cage and lungs provides most of
the volume reduction of the thorax necessary for quiet breath¬
ing. Activity of the diaphragm, parasternal intercostal muscles,
and the scalenes continues into early expiration, contracting
eccentrically to control the recoil of the rib cage [8,24]. As res¬
piratory effort increases, however, as during a cough or sneeze,
active muscle contraction facilitates the volume reduc-
tion in the thorax. Contraction of the muscles of the
abdominal wall compresses the abdominal contents
and depresses the ribs, pushing the diaphragm superiorly
and decreasing the thoracic cavity (Fig. 30.17). The quadratus
lumborum and the erector spinae also can depress the ribs.
Shoulder muscles appear to function as either inspiratory or
expiratory accessory muscles, depending upon the position
and stabilization of the shoulder. Both the trapezius and latis-
simus dorsi are reportedly active during respiration [58].

SUMMARY

This chapter discusses the muscles of the thorax, including
the muscles that support and extend the spine and those that
move the ribs. The extensors of the thoracic spine include
shoulder muscles as well as muscles that span the entire ver¬
tebral column. The thoracic erector spinae extend the tho¬
racic spine but also contain segments that extend both the
thoracic and lumbar regions together. The extensor muscles
of the spine function primarily to control forward-bending
through eccentric contractions. The superficial and deep
extensor muscles also contribute to side-bending and rotation
of the trunk. The deep extensor muscles of the thoracic spine
are characterized by an unusually high type I muscle fiber
content, consistent with a primary role in postural support.

The intrinsic muscles of the thorax are responsible for ven¬
tilation by elevating and depressing the ribs and, in the case of
the diaphragm, by lengthening and shortening the vertical
dimension of the thoracic cavity. Action of these muscles alters
the volume of the thorax and induces inspiration or expiration.
The diaphragm is the primary muscle of inspiration, but
increased resistance to inspiration yields increased recruitment
of intercostals, cervical, and shoulder muscles. Exhalation
occurs by the passive recoil of the costal cartilages, although
contraction of the abdominal muscles contributes to forced
exhalation. Assessment of the muscles of the thorax must
include careful observations of a patients breathing pattern as
well as an assessment of the patients respiratory function.

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CHAPTER

Loads Sustained by
the Thoracic Spine

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE FORCES ON THE THORACIC SPINE .556

LOADS ON THE THORACIC SPINE.560

SUMMARY.561

T he preceding two chapters discuss the structure and function of the bones, joints, and muscles of the thoracic
spine. It is well supported by ligaments, muscles, and the rib cage. Yet the thoracic spine undergoes mechani¬
cal failure when it sustains excessive loads or when the spine is weakened so that it is unable to withstand
normal loads. Fractures are a common form of mechanical failure in the thoracic spine and can result from trauma that
exerts excessive loads on the vertebrae. More commonly, fractures of the thoracic vertebrae are fragility fractures pro¬
duced by normal loads applied to bones weakened by osteoporosis [12]. The current chapter describes the mechanical
factors that play a role in compression fractures as well as in progressive kyphotic deformities of the thoracic spine.
The specific objectives of this chapter are to

■ Use two-dimensional examples to calculate the in vivo forces on the thoracic spine
■ Discuss the mechanical factors that lead to compression fractures or excessive kyphotic deformities
in the thoracic spine

■ Examine the strength of thoracic vertebrae

TWO-DIMENSIONAL ANALYSIS OF THE
FORCES ON THE THORACIC SPINE

The normal thoracic kyphosis subjects the vertebral bodies to
compressive loads. The two-dimensional examples presented
in Examining the Forces Boxes 31.1 and 31.2 demonstrate the
relationship between the kyphosis and compressive loads on
the vertebral bodies. In the upright posture, the superincum¬
bent weight of the head and neck is distributed between the
vertebral bodies and the columns formed by the articular
processes, with more load on the bodies than on the articular
processes [19] (Fig. 31.1). The anterior concavity of the tho¬
racic spine places the center of gravity of the head and cervical

spine anterior to much of the thoracic spine, thereby produc¬
ing a flexion moment on the thoracic spine [24] (Examining the
Forces Box 31.1) [14, 23]. The farther a thoracic vertebra is
from the line of force of the head and neck weight, the greater
the flexion moment on the thoracic vertebra. An increase in
thoracic flexion or in the thoracic kyphosis lengthens the
moment arm of the head and neck weight and increases the
flexion moment on the thoracic spine.

In static equilibrium, external moments produced by the
weight of a body segment or external load must be balanced by
internal moments produced by muscles and ligaments. An
increase in the external flexion moment on the thoracic spine
resulting from an increased thoracic kyphosis is balanced by an

556

Chapter 31 I LOADS SUSTAINED BY THE THORACIC SPINE

557

EXAMINING THE FORCES BOX 31.1

FLEXION MOMENTS ON THE THORACIC SPINE

External flexion moments (M E xt) are exerted on the
thoracic vertebrae by the superincumbent weight
of the head, neck, and superior vertebrae (W s ).

In normal upright posture, the flexion moment
on the first thoracic vertebra (T1) is smaller than the
flexion moment on the fourth thoracic vertebra (T4)
because the moment arm (x) of the superincumbent
weight is shorter for T1 than for T4 and because the
superincumbent weight at T1 includes the head and
cervical vertebrae, while the force producing a flexion
moment on T4 includes the weight of the head,
cervical spine, and the first three thoracic vertebrae.

m ext = W s X X

As the thoracic kyphosis increases, the external
flexion moment on the thoracic spine increases
because the moment arm (x) of the superincumbent
weight increases, even though the weight (W s )
remains the same.

EXAMINING THE FORCES BOX 31.2

COMPRESSIVE LOADS ON A THORACIC
VERTEBRA

To determine the reaction force on the fifth thoracic
vertebra (T5), the extension force in the muscles and
posterior ligaments (E) needed to balance the flexion
moment of the weight of the head, neck, and upper
thoracic vertebra (W) must first be determined (Figure, A).
The following anthropometric data are based on data
from the literature [2,14,21,24].

Weight of head, neck and
superior vertebrae (W):

Moment arm of W:

Moment arm of the extensor
muscles and ligaments:

11 % of body
weight (BW)

5 cm

2 cm

2M = 0

M ext + M int - 0

(W x 5.0 cm) - (Ex 2.0 cm) = 0

0.11 BW X 5.0 cm = E X 2.0 cm

E = 0.275 BW, or 27.5% of body weight

Calculate the compressive and shear forces (J c and J s )
on T5. The coordinate system is placed within the
vertebra so that the compressive force (J c ) is along
the y axis and the shear force (J s ) is along the x axis.

(continued)

558

Partlll I KINESIOLOGY OF THE HEAD AND SPINE

J S + W x - E x = 0

J s + (W x sin 15°) - (Ex sin 5°) = 0
J s = (E x sin 5°) - (W x sin 15°)

J s = (E x sin 5°) - (0.11 BW X sin 15°)

J s = — 0.004 BW, or 0.4% of body weight

EXAMINING THE FORCES BOX 31.2 (Continued)

(W X 9.5 cm) - (EX 2.0 cm) = 0

0.11 BW X 9.5 cm = E X 2.0 cm

E = 0.5225 BW or 52.25% of body weight

SF y :

XF y :

j c - w Y - e y = 0

J c - (W X cos 15°) - (EX cos 5°) = 0

J c = (W X cos 15°) + (E X cos 5°)

J c = 0.106 BW + 0.274 BW

J c = 0.38 BW, or 38% body weight

Increased thoracic kyphosis increases the moment arm
of the superincumbent weight to 9.5 cm, producing
changes in the compression and shear forces on the
vertebra (Figure, B).

2M = 0

M EXT i M int - 0

Js - E x + w x = 0

J s - (E x sin 5°) + (W x sin 30°) = 0
J s = (E x sin 5°) - (W x sin 30°)

J s = 0.046 BW - 0.055 BW

J s = -0.009 BW, or almost 1.0% body weight

SF V

J c - W Y - E y = 0

J c - (W X cos 30°) - (EX cos 5°) = 0
J c = (W X cos 30°) + (E X cos 5°)

J c = 0.095 BW + 0.52 BW

J c = 0.61 BW, or 61 % body weight

Chapter 31 I LOADS SUSTAINED BY THE THORACIC SPINE

facet joints, with more load on the vertebral bodies.

increased internal extension moment to keep the thoracic
spine from flexing more (Examining the Forces Box 31.2).
The extension moment can be exerted by extensor muscles
and also by the posterior ligaments [1,7] (Fig. 31.2). The
reaction force on the vertebral body is determined from the
static equilibrium relationship, 2F = 0. As the kyphosis
increases, the external flexion moment and resulting internal

559

Figure 31.2: The extensor muscles and posterior ligaments apply an
internal extension moment (M INT ) to balance the external flexion
moment (M EXT ) applied by the superincumbent weight (W s ).

extension moment increase, producing an increased reaction
force on the vertebral bodies, including the compressive
component. Increased compressive forces on the vertebral
bodies can lead to compressive failures. A compression fail¬
ure in the thoracic region commonly occurs in the anterior
portion of a vertebral body, creating a wedge fracture.
The collapse of the anterior portion of the body of a

560

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 31.3: Compression fractures in the thoracic region typically
occur in the anterior portion of the vertebral body and contribute
to an increase in the kyphosis. (Reprinted with permission from RB
Salter: Textbook of Disorders and Injuries of the Musculoskeletal
System. 3rd ed. Baltimore: Williams & Wilkins, 1999.)

thoracic vertebra increases the kyphotic deformity (Fig. 31.3).
A wedge fracture contributes to a downward spiral of excessive
kyphosis, compressive failure, increased kyphosis, and further
compressive failure [4] (Fig. 31.4).

Clinical Relevance

COMPRESSION FRACTURES IN THE THORACIC
SPINE, WEDGE AND BURST FRACTURES: Fractures of
the vertebral body in the thoracic spine result from compres¬
sive loading. When the loading is accompanied by signifi¬
cant flexion, the anterior portion of the body fractures; pro¬
ducing a wedge fracture. Large compressive forces applied
to a relatively straight spine produce a burst fracture in
which the endplate of the vertebral body fractures, and the
nucleus pulposus is forced into the vertebral body [11,17].
Fortunately, vertebral body fractures alone in the midtho-
racic region rarely produce spinal cord impingement [10].

Because wedge fractures contribute to an increased kyphosis
and an increased risk of additional fractures, these fractures
are sometimes treated with kyphoplasty, in which an inflat¬
able balloon is inserted into the fractured vertebral body. The
insert is designed to elevate the compressed vertebral body,
thereby reducing or controlling the kyphotic deformity [15].

Figure 31.4: Severe thoracic kyphosis secondary to osteoporosis.
Osteoporosis can produce a downwardly spiraling set of events
in which the osteoporosis leads to a compression fracture that
increases the thoracic kyphosis, producing an increased flexion
moment and further compression failures.

LOADS ON THE THORACIC SPINE

Loads on the thoracic spine are contributing factors in the
failure of the thoracic vertebrae, although they are less well
studied than in the cervical and lumbar regions. Knowledge
of the strength of healthy thoracic vertebrae may help clini¬
cians and scientists develop strategies to prevent thoracic
fractures in the future. Most studies of the mechanical prop¬
erties of the thoracic spine examine the ultimate strength of
the thoracic spine in compression, since most of the failures
of the thoracic spine in vivo occur under compressive load¬
ing. The ultimate strength of bone is the maximum load
the bone can support without fracture. Studies of cadaver
specimens from adults ranging in age from 26 to 98 years
demonstrate increasing load to failure from the superior to
inferior thoracic vertebrae [3,5,9,18]. These findings are
consistent with the increase in the size of the vertebral
bodies from the upper to lower thoracic spine described in
Chapter 29 [16,18]. As the size and load to failure increase
from the upper to the lower thoracic vertebrae, the load each
vertebra bears also increases. Anther important measure
of bone strength is ultimate stress, the maximum stress

Chapter 31 I LOADS SUSTAINED BY THE THORACIC SPINE

561

(stress = load/area) sustained without failure. Ultimate stress
remains relatively constant or actually decreases from the
upper to the lower thoracic vertebrae [5,18]. This may help
explain the increased incidence of vertebral fractures in the
middle and lower thoracic regions.

The magnitude of the load to failure in the thoracic spine
depends on many factors, including subject characteristics as
well as characteristics of the mechanical testing procedures used
in the experiments. Subject characteristics that influence the
ultimate strength of the thoracic vertebrae in compression
include gender, bone mineral density, and bone mineral content
[3,9,18]. Although there is no known study that specifically
examines the effect of age on the strength of thoracic vertebrae,
it follows that since bone mineral content and bone mineral
density decrease with age, failure strength of thoracic vertebrae
also decreases with age.

Testing procedures have a significant influence on the
determination of ultimate strength of thoracic bone. Chapter
2 describes the effect that loading rate can have on the
mechanical properties of materials, and this is borne out in
the thoracic spine, where tests reported in the literature vary
in loading rates from approximately 0.1 mm/sec to almost
1000 mm/sec [3,9,18]. Position of the spine during loading
also affects the strength of the spine [13]. Consequently, there
is no single value of bone strength for the thoracic spine (or
any other biological tissue, for that matter). The clinician can
use the literature to obtain a general concept of the range of
strength in thoracic vertebrae and to recognize the clinically
relevant factors that influence strength.

Reported loads to failure applied at slow loading rates in
the upper thoracic region are on the order of approximately
2600 N (approximately 600 lb) [3,9,18]. However, in a study
of cadavers of elderly subjects (aged 46-98 years), failure
loads as low as 613 N (138 lbs) are reported [3]. At higher
loading rates, upper thoracic vertebrae exhibit ultimate
loads of 3000-4500 N (675-1000 lb) in the same region
[3,9]. Slowly applied loads to failure in the lower thoracic
spine range from 4000 to 5000 N (approximately 900-1100
lb) [3,9]. Failure loads of approximately 8500 N (almost
2 tons) are reported in the lower thoracic spine during rap¬
idly applied loading [9]. These data demonstrate the
increased strength of the lower thoracic vertebrae. They
also reveal that bone strength, as measured by ultimate fail¬
ure loads, increases with increased loading rates, consistent
with the viscoelastic nature of bone.

Bone mineral content and bone mineral density are corre¬
lated with bone strength [3,18]. These relationships help
explain the increase in ultimate strength in the lower thoracic
spine where the vertebrae are larger. These correlations also
help explain the greater bone strength in men than in women,
since men, on average, have larger bones. In fact, research sug¬
gests that ultimate stress (force/area) in the thoracic spine is
similar in men and women [5]. Finally, these relationships are
particularly useful in explaining the increase in incidence of
vertebral fractures identified in postmenopausal women who
undergo accelerated loss of bone mass at menopause [8,10].

Clinical Relevance

SPONTANEOUS VERTEBRAL FRACTURES: Individuals
reach peak bone mass in their mid-20s. After reaching their
peak bone mass; premenopausal women begin to lose
approximately 0.3% of their bone mass per year.

Menopausal and postmenopausal women experience
accelerated bone loss during menopause and for approxi¬
mately the next 5 or 10 years, losing approximately 2% of
bone mass yearly, although later in life bone loss slows
again. Individuals with extremely low body weight and body
mass index (BMI) also experience accelerated rates of bone
loss [6,25]. The sustained increased rate of bone loss leads
to a cumulative loss that drastically alters the ultimate
strength of bone [22]. Osteoporosis is defined as bone mass
more than 2.5 standard deviations below peak bone mass
[10,22]. Postmenopausal women are most likely to be affect¬
ed by osteoporosis; however, younger women with a very
low BMI, such as highly trained athletes or women with eat¬
ing disorders such as anorexia nervosa, also are at
increased risk of osteoporosis [20,25].

Individuals with osteoporosis are at risk for vertebral frac¬
tures and may report a sudden sharp pain in the midback,
perhaps following a sneeze but often with no pre¬
cipitating event at all. Some individuals may deny
any discomfort but report an increase in a midback
hump or a loss of standing height. These clinical findings
are consistent with a spontaneous fracture of one or more
thoracic vertebrae. A precipitating event such as a sneeze
produces a large flexion moment that cannot be sustained
by the thoracic vertebrae weakened by loss of bone mass. In
the presence of severe osteoporosis, the superincumbent
weight in an individual with an excessive kyphosis may be
sufficient to produce a fracture with no precipitating event.

SUMMARY

This chapter presents two-dimensional models to demon¬
strate the mechanical factors that contribute to fractures of
the thoracic spine. The bodies of the thoracic vertebrae are
predisposed to high loads because of the normally occurring
thoracic kyphosis. Although thoracic vertebrae can sustain
compressive loads of several hundred pounds or more before
failure, fractures of thoracic vertebrae occur as the result of
excessive loads or, more commonly, from normal loads
applied to weakened thoracic vertebrae. As the thoracic
kyphosis increases, the flexion moment applied by the weight
of the head and neck increases, producing larger compressive
loads on the vertebral bodies, which may result in fracture.
The presence of osteoporosis is a primary factor that precipi¬
tates a cascade of events producing a downward spiral:
kyphotic deformity, increased load, fracture, increased defor¬
mity, and increased load.

562

Part III I KINESIOLOGY OF THE HEAD AND SPINE

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CHAPTER

Structure and Function of the Bones
and Joints of the Lumbar Spine

PAUL F. BEATTIE , P.T., PH.D.

CHAPTER CONTENTS

STRUCTURE OF THE BONES AND LIGAMENTS OF THE LUMBAR SPINE.564

General Overview of the Osteocartilaginous Lumbar Spine.564

Ligamentous Support of the Lumbar Spine .568

Thoracolumbar Fascia.570

Palpable Bony and Ligamentous Structures of the Lumbar Spine .570

STRUCTURE OF THE JOINTS OF THE LUMBAR SPINE.571

Facet Joints .571

Intervertebral Joint.572

MECHANICAL PROPERTIES OF THE IVD .575

Compression .575

Bending .575

Rotation.576

IVD Pressures during Activities of Daily Living.577

MOTION OF THE LUMBAR SPINE .578

Gross Motion of the Lumbar Spine.578

Joint Coupling in the Lumbar Spine .579

Segmental Motion of the Lumbar Spine.580

Clinical Methods of Lumbar Range of Motion Assessment.581

Normative Values for Lumbar Range of Motion .583

RELATING THE OSTEOCARTILAGINOUS LUMBAR SPINE TO FUNCTIONAL DEMANDS .583

SUMMARY .584

T he lumbar spine functions as a complex interplay of musculoskeletal and neurovascular structures creating a
mobile, yet stable, transition between the thorax and pelvis. The lumbar region repetitively sustains enor¬
mous loads throughout one's lifetime, while still providing the mobility necessary to allow a person to per¬
form myriad tasks associated with daily living. In addition, the lumbar spine provides the fibro-osseous pathway for
the inferior portion of the spinal cord, the cauda equina and lumbosacral spinal nerves traveling to and from the trunk
and lower extremities. Considering the magnitude and complexity of these functional demands, it is not surprising that
the low back is a common site of dysfunction, with low back pain syndromes representing the most frequent muscu¬
loskeletal problem encountered by health care professions [16,17,26]. The high prevalence of this condition and the
enormous variability of its clinical manifestations create a challenge when studying spinal motion or when diagnosing
sources of low back pain.

563

564

Part III I KINESIOLOGY OF THE HEAD AND SPINE

The purpose of this chapter is to describe the bony structures of the lumbar spine as well as its joint components and
to relate these to the spine's functional demands of stability, mobility, and protection of neurovascular elements.
Emphasis is placed upon the clinical significance of various structures as they relate to trauma and degeneration.

The specific objectives of this chapter are to

■ Describe and discuss the bony geometry and other unique morphology of the lumbar vertebrae

■ Describe the structure and unique biomechanical functions of the lumbar spinal ligaments, facet joints,
intervertebral discs (IVDs), and intervertebral joints

■ Identify and define spinal gross and segmental motions

■ Compare various methods of lumbar motion assessment

■ Relate the objectives above to commonly observed clinical conditions

STRUCTURE OF THE BONES AND
LIGAMENTS OF THE LUMBAR SPINE

General Overview of the
Osteocartilaginous Lumbar Spine

The human spine acts as a multisegmental, flexible rod form¬
ing the central axis of the neck and trunk. The normal bony
spine consists of 24 presacral vertebrae that combine to form
the three major curves on the sagittal plane. Lordotic curves
(apex anterior) are present in the lumbar and cervical spines,
with a kyphotic curve (apex posterior) present in the thoracic
spine. These curves help to enhance the repetitive load-bear¬
ing capacity of the spine by providing “flex,” or damping func¬
tion. The junctions between these curves are areas of great
force concentration and are called transitional zones (Fig.
32.1). These zones are frequent sites of tissue injury resulting
in dysfunction and nociception. For example, in the lumbar
spine the junction between L5 and SI (lumbosacral joint) is a
very common site of pain complaint [19]. Vertebrae near and
within transitional zones have unique characteristics and are
referred to as atypical vertebrae.

Interposed between the vertebrae are the fibrocartilagi¬
nous IVDs, the principle component of the intervertebral
joints. These symphysis-type joints create a flexible interspace
to maintain the vertical length of the lumbar spine and permit
three-dimensional displacement [23]. Posteriorly, articular
processes projecting from adjacent vertebrae join to form the
paired facet or apophyseal joints. These synovial joints act to
guide and restrict the directions of motion available at differ¬
ent segmental levels [13,19]. Conceptually, the intervertebral
joints and the paired facet joints act to form the motion seg¬
ment, or “articular tripod” [53], in which these three joints
function as a closed chain system, i.e., displacement of one
joint requires a specific displacement of the other two joints
(Fig. 32.2A). Consider the spine, therefore, to be a series of
links (segmental levels) of three joint systems helping to fill the
lumbar spines demands of mobility and stability (Fig. 32.2B).

In addition to the mechanical demands of mobility
and stability, each motion segment forms three important

Figure 32.1: Sagittal view of the entire spine. Note the anteriorly
convex lumbar and cervical lordosis and the posteriorly convex
thoracic kyphosis. A plumb line dropped through the center of
the spine transects the transitional zones.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

565

Figure 32.2: A. Lateral view of two adjacent vertebrae and the interposed intervertebral disc. This system, along with associated soft
tissues, is referred to as a lumbar motion segment. Note the intervertebral joint anteriorly and the paired facet joints posteriorly.

B. When the motion segments are joined, a complex multijoint system is formed.

fibro-osseous canals to house and protect the important neu¬
rovascular elements of the lumbar spine. The vertebral fora¬
men is formed within the center of the vertebrae, and the
paired intervertebral foramina are formed laterally between
adjacent vertebrae.

The osseous lumbar spine comprises five vertebrae (LI-5).
Occasionally, the junction between the first and second sacral
vertebrae fails to fuse, creating a condition known as lum-
barization. This results in six mobile lumbar vertebrae. In
some cases, the lumbosacral junction fuses during growth and
development, resulting in sacralization of L5. This results in
only four mobile lumbar vertebrae. While the mechanical
influence of these anatomical variations is uncertain, it is
important to note that neither lumbarization nor sacralization
appears to increase the risk of low back pain [72].

Anatomically, each vertebra consists of a large, cylindrical
vertebral body anteriorly, with a bony ring or “neural arch” pos¬
teriorly (Fig. 32.3). The vertebral bodies, along with the IVDs,
provide the vertical dimension (length) of the lumbar spine and
sustain most of the compressive loading [13,23,46]. The neural
arch forms a protective bony ring around the neural elements of
the lumbar region while providing numerous bony projections
or processes that serve to form the surfaces of the facet joints or
act as sites of attachment for spinal muscles and ligaments. The
neural arch, along with the IVDs, sustains most of the torsional
load bearing acting on the lumbar spine [13,59].

VERTEBRAL BODIES

The vertebral bodies are primarily composed of well-vascu-
larized cancellous bone. Roughly cylindrical, they narrow

slightly in their midsection to create a shape similar to an
“hourglass” [13]. This unique biconcave arrangement pro¬
vides a deep passageway for neurovascular structures along
the midportion of the vertebral body and, coupled with the
vertical and transverse arrangement of the bony trabeculae,
creates a system that is well designed to tolerate compres¬
sive loads. For example, in upright postures the vertebral
bodies of the lumbar spine assume 80-90% of the compres¬
sive load bearing [13,23]. This capacity is further enhanced
by an abundance of potential spaces within the cancellous
bone that are occupied by blood and hematopoietic tissues
helping to reinforce the bony trabeculae by occupying the
empty spaces. Interestingly, the “reinforced scaffolding”
created by the alignment of the bony trabeculae lacks a sub¬
stantial number of obliquely orientated trabeculae, which
results in a poor capacity of the lumbar vertebrae to tolerate
rotational stresses. In situ, this is compensated for by the
IVDs and posterior bony structures as well as by muscular
support.

At their superior and inferior margins, the lumbar verte¬
bral bodies widen slightly and are covered by the cartilagi¬
nous vertebral endplates. This widening corresponds to the
epiphyseal ring and forms the site of the strong peripheral
attachments of the IVD. The vertical dimension of the lum¬
bar vertebral bodies is higher anteriorly, forming a slight
wedge shape that results in adjacent vertebrae forming a nat¬
ural lordotic curve. Consistent with the rest of the spine, the
vertebral bodies become progressively larger from superior
(LI) to inferior (L5) as a function of progressively increasing
load demands from rostral to caudal.

566

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

AREAS OF THE VERTEBRAE THAT ARE PREDISPOSED
TO INJURY OR DISEASE: Fractures from compressive loads
sustained with the spine in flexion or in a midrange position
(lumbar neutral) usually occur in the vertebral bodies of the
upper lumbar or lower thoracic area. These fractures are quite
common in persons with osteoporosis and may result from
commonly performed activities such as going rapidly from
standing to sitting in a chair.

The epiphyseal ring at the superior and inferior portions
of the vertebral bodies is an important site of ossification
during growth and development. Abnormal ossification here
can occur in adolescents and leads to a painful condition
known as vertebral epiphysitis , or Scheuermann's disease .

The abundant vascularity of the vertebral bodies helps
load bearing and allows most fractures to heal quickly;
however ; it also predisposes these structures as a common
site for metastatic lesions. While metastases are most com¬
monly found in the thoracic spine in persons with breast or
lung cancers , they may also be present in the lumbar spine.

NEURAL ARCH

The neural arch is a bony ring spanning posteriorly from the
vertebral bodies. It consists of the paired pedicles that bind

the vertebral bodies and neural arch together and the paired
laminae that enclose the posterior portion of the vertebral
foramen. Arising from the neural arch are seven bony projec¬
tions: two superior and inferior articular processes, two trans¬
verse processes, and one spinous process.

The pedicles in the lumbar region are stout and roughly
cylindrical. Composed of strong cortical bone, they arise from
the upper posterior portion of the vertebral body and project
posteriorly. Functionally, the pedicles are the only bony
attachment between the vertebral body and the neural arch
and strongly anchor these structures to one another. The
pedicles are often called upon to sustain high compressive
and tensile loads that occur during spinal rotation, flexion,
and extension. Their strong, tubular shape and amount of cor¬
tical bone make them well suited to this task. Additionally, the
pedicles form the superior and inferior boundaries of the
intervertebral foramina, providing a reinforced pathway for
the spinal nerves.

The laminae are relatively flat, blade-shaped bones that
project posteriorly from lateral to medial, converging at the
posterior midline of the trunk to give rise to the spinous
process. Functionally, the laminae act primarily as a posteri¬
or bony boundary of the neural arch. While having less load-
bearing demand than the pedicles, they are responsible for
shunting forces between the spinous processes and the
articular processes, as may occur with forceful lumbar rota¬
tion [13]. Recognizing this, spinal surgeons use great care to

Figure 32.3: A typical lumbar vertebra (L3) in four views. The views identify all of the relevant landmarks of a typical lumbar vertebra.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

567

minimize removal of laminae (laminectomy) during posterior-
approach spinal surgery.

Arising from the junction of the posterior pedicles and lat¬
eral laminae are the important superior and inferior articular
processes. These processes articulate with the opposite artic¬
ular processes from adjacent vertebrae (i.e., a superior artic¬
ular process articulates with the inferior articular process of
the vertebra above it). The superior articular process is the
larger of the two articular processes. It projects upward, pro¬
viding an articular surface on its medial aspect, thus forming
the outer bony component of the facet joint. The thick bone
of the superior articular process is critical in resisting lumbar
rotation and, by doing so, protecting the IVD from excessive
torsional stress [13]. The inferior articular process projects
downward and provides an articular surface on its lateral side.
As it “nests” into the vertebra below, it forms the inner por¬
tion of the facet in a manner similar to two paper cups being
placed one into the other.

As noted in the cervical and thoracic regions, the orien¬
tation of the articular processes as they form the facet joint
is critical to understanding the directions in which a motion
segment is able to displace and thus is critical to the under¬
standing of spinal motion [13,19,50,60]. In the lumbar
spine, the facet joint planes lie roughly parallel to the sagit¬
tal plane; thus movements in this plane (flexion and exten¬
sion) have larger excursions than movements in the trans¬
verse plane (lumbar rotation) or frontal plane (lumbar side¬
bending) [50,69]. It is important to note, however, that
anatomical variation in the facet joint planes is common. For
example, the facet joint plane on one side of a vertebra may
be orientated more obliquely than the facet joint plane on
the opposite side, leading to asymmetrical side-bending or
rotation [19,77].

Clinical Relevance

SPONDYLOLYSIS: Between the superior and inferior articu¬
lar processes is a relatively flat isthmus of bone known as the
pars interarticularis. Clinically, this area is of great impor¬
tance, as it is often the site of bone failure during excessive or
repetitive lumbar extension and/or rotation. Fractures in this
area are called spondylolysis and are visible on plane film
oblique radiographs (Fig. 32.4). Occasionally , stress fractures
can occur in this area that are not easily detectable radi¬
ographically. These are extremely common in young gym¬
nasts and springboard divers. In some cases; bilateral
spondylolysis can occur. This can result in anterior slippage of
the lumbar vertebra known as spondylolisthesis [2,41].

The spinous and transverse processes have no joint sur¬
faces but serve a very important function as “outriggers” for
the attachment of muscles, ligaments, and fascia. The spin¬
ous processes in the lumbar spine are relatively thick, with a

IT/ s;jjg. *

Figure 32.4: An oblique view radiograph of the lumbar spine
reveals a normal pars interarticularis (upper arrow) and a frac¬
tured pars interarticularis (i.e., spondylolysis) (lower arrow).

quadrangular shape. Their posterior tips are easily palpable
and are on the same transverse plane as the vertebral body.
Along its superior and inferior surface, the spinous process is
the point of attachment of the interspinous ligament, and
posteriorly it provides an enhanced moment arm for the
attachment of the thoracolumbar fascia (TLF) and the mul-
tifidus muscle [74,77].

The transverse processes are long and flat. The widest
transverse processes (an important radiographic landmark) are
found on the third lumbar vertebra and the thickest are part of
the fifth lumbar vertebra. Several structures that provide sta¬
bility in the frontal plane have attachments to the transverse
processes, including the quadratus lumborum muscle, fibers
from the TLF, and the iliolumbar ligaments (L4-5).

VERTEBRAL FORAMINA

Of critical importance are the three fibro-osseous passage¬
ways within the bony lumbar spine. Located centrally is the
vertebral foramen, while the paired intervertebral foramina
are positioned laterally. The vertebral foramen is typically tri¬
angular, bordered anteriorly by the posterior aspects of the
vertebral body and IVD, laterally by the pedicle, and posteri¬
orly by the lamina and ligamentum flavum. When consider¬
ing the lumbar spine as an entire unit, the vertebral foramina
and associated soft tissues form the spinal or vertebral canal.
In the upper lumbar spine this canal is oval and contains the
conus medullaris, the lower portion of the spinal cord.
Progressing inferiorly, the canal becomes wider and flatter
while containing the cauda equina [15]. The lateral portions

568

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 32.5: Parasagittal lumbar magnetic resonance image of
the intervertebral foramen (IVF) of a lumbar motion segment.
Note the oblong shape. In situ the spinal nerve roots (seen as
small gray structures within the IVF) and their supporting tissues
travel through the superior portion of the IVF and thus are a
considerable distance from the IVD. At the L3-4 level, a disc her¬
niation encroaches upon the intervertebral foramen but does
not compress the spinal nerve (arrow).

of the vertebral foramen just medial to the intervertebral
foramen are known as the lateral recesses and are a com¬
mon location for nerve entrapment by disc herniation [9].

The intervertebral foramina are bordered anteriorly by the
posterior aspects of the vertebral bodies and anulus fibrosus.
The pedicles form the superior and inferior borders, while
posteriorly the ligamentum flavum and anterior portion of the
facet joint capsule complete the perimeter (Fig. 32.5).

In the normal lumbar spine the ratios between the size
of the vertebral foramen and the size of the nerves is
quite large, providing, in the normal state, ample room
for the neural and vascular structures within the vertebral
foramina [9,15]. Representative values are reported by
Dommisse [15], who describes the anterior-posterior (AP)
diameter at LI level as approximately 16 mm and the trans¬
verse diameter as approximately 21 mm, with the neural
contents approximately 10 mm. At the L3 level, the canal
becomes flatter and wider (AP, 15 mm; transverse, 22 mm),
and at the SI level, it narrows slightly, with an AP diameter
of approximately 13 mm and a transverse diameter of
approximately 30 mm.

Of great interest is the change in the shape and diameter
of the vertebral and intervertebral foramina during spinal
motion and how these changes influence the neural struc¬
tures. The theoretical basis behind many spinal treatments
relates to relieving nerve compression by lumbar movement
[40,41,73]. In normal subjects, there is an approximately 10%

increase in the area of the vertebral foramen during flexion
and a 10% decrease during extension [13].

Clinical Relevance

SPINAL STENOSIS: Spinal stenosis is a narrowing of the
vertebral foramina. Persons with spinal stenosis typically
find positions of lumbar flexion more comfortable than posi¬
tions of extension. This clinical finding is consistent with the
data that show an increase in the vertebral foramina with
flexion. This increased space reduces compression on the
neural structures [2,4'i]-

Panjabi et al. [49] compare the size and shape of the inter¬
vertebral foramina in normal and degenerative motion seg¬
ments (i.e., two adjacent vertebra). The authors report a 20%
decrease in the area during lumbar extension and a 30%
increase during lumbar flexion. Hasue [22] and Mayoux-
Benhamou [38] describe the intervertebral foramen as pear-
shaped in flexion and triangular in extension.

Despite the changes in shape and area, the ratio between
the intervertebral foramen and the nerve root remains quite
large, so that nerve root compression rarely occurs in the
intervertebral foramen of the lumbar spine. In a recent study,
only 4 of 408 subjects with presumed nerve compression had
evidence of compression in the intervertebral foramen [9].
However, a very large number of these individuals demon¬
strated compression in the lateral portion of the vertebral
foramen, the area known as the lateral recess.

Ligamentous Support
of the Lumbar Spine

The lumbar spine contains a very complex ligament system
that provides a critical component of its mobility and stability
characteristics. On gross inspection, the ligaments are inter-
meshed with fascia, tendinous attachments of muscle, and, in
some cases, the outer portion of the IVD [13,77]. The lumbar
ligaments may be classified as extrasegmental (anterior longi¬
tudinal, posterior longitudinal, and supraspinous), segmental
ligaments (ligamentum flavum, interspinous, and intertrans-
verse) or regional (iliolumbar) (Fig. 32.6).

A primary function of the lumbar ligaments is to provide
a restraint for motion. Biomechanically, the spinal ligaments,
with the exception of the ligamentum flavum, are relatively
inelastic and exhibit a viscoelastic response, or time-
dependent elongation, to loading [13]. (See Chapter 2 for a
more detailed discussion of viscoelasticity.) By identifying the
location of a ligament and the direction of its fibers, one may
hypothesize the motions that a given ligament resists. For
example, those ligaments posterior to the axis of rotation of a
motion segment: posterior longitudinal, interspinous, liga¬
mentum flavum, and supraspinous ligament are restraints

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

569

Ligamentum

Figure 32.6: Midsagittal view of the lumbar spine demonstrates
the spinal ligament system.

against flexion, while the anterior longitudinal ligament
restrains extension (Table 32.1) [13,77].

The anterior longitudinal ligament is a large, broad band
that spans the anterior portion of the vertebral bodies and anu-
lus fibrosus. It is strongly anchored to the anterior sacrum and
is a strong reinforcing tissue against anterior displacement of
the IVD [13]. The posterior longitudinal ligament spans the
posterior aspect of the vertebral bodies. It is characteristically
hourglass shaped, with the widest portion covering the poste¬
rior, but not posterior lateral, portions of the IVD. The inter-
spinous ligament runs between the spinous process, while the
supraspinous ligament travels from the posterior tips of the
spinous processes. These two ligaments help to provide poste¬
rior stability for the motion segment [20].

TABLE 32.1: Displacements Opposed
by Lumbar Ligaments

Ligament

Displacements Resisted

Anterior longitudinal

Vertical separation of anterior vertebral bodies
(e.g., lumbar extension, anterior bowing of the
lumbar spine)

Posterior longitudinal

Separation of posterior vertebral bodies

Supraspinous

Separation of the spinous process

Ligamentum flavum

Separation of the laminae

Interspinous

Separation of posterior vertebral bodies, i.e.
lumbar flexion, posterior translation of superior
vertebral bodies

Intertransverse

Separation of transverse processes

Iliolumbar

Flexion, extension, rotation, and lateral bending

Unique among the lumbar ligaments is the ligamentum
flavum. This ligament travels between adjacent laminae ante¬
riorly and blends with the anterior portion of the facet joint
capsule. In so doing, it forms the posterior aspect of the ver¬
tebral foramen. Characterized by its yellow color, this liga¬
ment contains large amounts of the elastic protein known as
elastin. Approximately 80% of its mass is elastin. Unlike
other ligaments, the ligamentum flavum can be passively
elongated to 40% of its resting length without tissue failure.
This elasticity allows the ligamentum flavum to tolerate the
large displacements between adjacent laminae during lumbar
flexion and yet not buckle and displace into the vertebral fora¬
men during lumbar extension [13].

The iliolumbar ligament is a series of bands that run from
the transverse processes of L5 to the ilium. Basadonna et al.
[4] describe it as having an anterior band traveling from the
anterior-inferior-lateral part of the transverse process and
widening to attach on the anterior part of the iliac tuberosity.
Additionally, a posterior band arises from the apex of the
transverse process and attaches superior to the anterior band.
Because of its central location at the lumbosacral joint it acts
to resist flexion, extension, rotation, and lateral bending.
Sprain of this ligament, especially the weak posterior band,
has been hypothesized as a common cause of low back pain.

While traditional beliefs have classified lumbar ligaments
as the primary stabilizers of the spine, the actual role of these
structures may be more complex. Work by Lucas and Bresler
[33] indicates that the spine, without muscular support, buck¬
les under only 2 kg of loading, implying that ligaments pro¬
vide only a small portion of the stability necessary for the
spine. Others confirm that the stress-strain characteristics of
the spinal ligaments provide minimal support to spinal stabil¬
ity during normal lumbar motion (see Chapters 33 and 34).

What then is the primary role of the lumbar ligament sys¬
tem? Close inspection reveals that the lumbar ligaments are
blended and interconnected with many other structures such
as deep and superficial fascia as well as tendon and muscle
fibers. Histological studies identify large densities of sensory
end-organs, including free nerve endings and mechanorecep-
tors [12,28,54,78]. This observation has led authors [62] to
postulate that the ligament system may be a major part of a
reflex arc, with the lumbar muscles providing important
information regarding the position of the motion segment,
which in turn influences lumbar muscle tension. Examining
this hypothesis on animal models and a small sample of
patients, Solomonow et al. report that a primary reflex arc
exists between the mechanoreceptors in the supraspinous lig¬
ament and the multifidus muscle [62]. When the supraspinous
ligament is loaded, the multifidus muscle contracts to increase
the stiffness in the motion segment. The magnitude of con¬
traction increases as the load increases, implying a protective
mechanism. The authors postulate that similar arcs exist from
the other spinal ligaments as well as the IVD and the facet
joint capsule. This fascinating theory supports the importance
of interplay of the fibro-osseous and neuromuscular struc¬
tures in the normal function of the spine.

570

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

MOTOR CONTROL: Clinically, the potential of a reflex arc
between the fibroosseous and neuromuscular structures
lends great support to the importance of muscle training
and proprioceptive retraining in the rehabilitation of persons
with low back pain [39,48,53]. Improved strength and
motor control will play an important role in stabilization of
the lumbar spine against injurious forces.

Thoracolumbar Fascia

The TLF is a complex array of dense connective tissue cover¬
ing the lumbar region. It interconnects with an extraordinary
number of bony and soft tissue structures while providing crit¬
ical support to the spine during lumbar flexion and lifting
activities [18,74]. Anatomically it consists of three layers (Fig.
32. 7). The anterior and middle layers arise from the transverse
processes of the lumbar vertebrae and join together laterally,
encompassing the quadratus lumborum while blending with
the fascia of the transversus abdominis and internal oblique
abdominis muscles. This creates a direct connection between
the bony spine and the deep abdominal muscles and appears
to be an important relationship for the dynamic stabilization of

the lumbar spine. The large posterior layer of the TLF arises
from the spinous processes of the thoracic, lumbar, and sacral
vertebrae and covers the erector spinae muscles. Laterally, it
blends with the latissimus dorsi muscle, and inferiorly it blends
with the gluteus maximus muscle, thus forming a direct con¬
nection between the proximal humerus (the distal attachment
of the latissimus dorsi) and proximal femur (distal attachment
of the gluteus maximus muscle).

To conceptualize one of the important functions of the
TLF, imagine being in the position of lumbar forward-bend¬
ing, with the hips and knees slightly flexed, while pulling an
object toward you. This requires activity of the gluteus max¬
imus, lumbar erector spinae, abdominal muscles, and latis¬
simus dorsi, all of which have central attachments to the TLF.
The TLF is then strongly tensed, providing stability to the
posterior aspect of the lumbar spine as it reinforces the pos¬
terior ligaments and muscular system [18,74].

Palpable Bony and Ligamentous
Structures of the Lumbar Spine

Palpation of the bony structures of the lumbar spine is a vital
component of the physical examination. Palpation can assist
in identifying the segmental level of pain [35] and may pro¬
vide general information about the mobility of a given motion
segment [67].

Posterior layer
of thoracolumbar
fascia

Internal oblique

External oblique

Quadratus laborum

Posterior layer
of thoracolumbar
fascia

Gluteus maximus

Figure 32.7: A. Posterior view of the TLF. Note how various muscles act to exert tension on this structure, thus providing dynamic
stability to the low back. B. Axial (transverse) view of the posterior lumbar spine shows the layers and attachments of the TLF.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

571

The only bony structures that can be easily palpated in the
lumbar spine are the spinous processes. Clinically a useful
technique for this is to use the radial surface of the index fin¬
ger to identify the iliac crests in a standing or prone patient.
If the examiner brings his or her thumbs directly toward the
midline, they intersect roughly at the level of the L4-5 inter¬
space. The spinous process below is that of L5, while that
above is L4. A second technique is to palpate the inferior sur¬
faces of the posterior superior iliac spines (PSISs). This cor¬
responds to the S2 level. After identifying the spinous process
of L5, the remaining lumbar vertebrae can be determined by
counting the spinous processes. The interspace
between the spinous processes is occupied by the
supraspinous and interspinous ligaments and the TLF.

STRUCTURE OF THE JOINTS
OF THE LUMBAR SPINE

The joint system of the lumbar spine comprises the large sym¬
physis joint (the intervertebral joint) anteriorly and the paired
synovial joints (the facet or apophyseal joints) posteriorly.

These three joints form an anatomically unique three-joint
complex [29], or “articular tripod” [53]. This joint system
forms the basis of dynamic stability, allowing the spine to tol¬
erate loads while traveling through an arc of motion.

Facet Joints

The paired facet joints of the lumbar spine are located
posteriorly but are intimate with the vertebral and interver¬
tebral foramina. These unique joints are formed by the infe¬
rior articular processes of the vertebrae above “nesting” into
the superior articular process of the vertebrae below (Fig.
32.8). Because of their flat appearance, the facet joints are
classed as planar joints. However, upon closer inspection,
the articular surfaces are typically J-shaped, with the lower
portion or hook of the “J” being most anterior [13]. The
facet joints have a unique joint capsule. As with all synovial
joints, the capsule is lined with synovium and covered by a
layer of dense ordinary connective tissue. The capsule
attaches just beyond the periphery of the joint surfaces.
Inferiorly and superiorly, the capsule tends to bow outward
away from the joint surface, creating a redundancy. This

Vertebral body

Figure 32.8: Posterior view of a lumbar motion segment illustrates the bony components of the lumbar facet joints. Note how the infe¬
rior articular processes of the superior segment "nest" into the superior articular processes of the inferior segment.

572

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 32.9: An axial view MRI demonstrates the attachment of
the lumbar multifidus muscle to the facet joint capsule (arrow).

appears to allow extra “joint play,” increasing the magnitude
of joint motion [67]. Overall, however, the redundancy is not
large, since the capacity for fluid within the facet joint is
only approximately 2 mL [24]. Interestingly, the ligamen-
tum flavum attaches to the anterior-superior portion of the
capsule and exerts tension during lumbar flexion. The mul¬
tifidus muscle sends fibers to attach on the superior-poste¬
rior portion of the capsule and exerts tension when active
concentrically during lumbar extension or eccentrically dur¬
ing lumbar flexion (Fig. 32.9).

As previously stated, a primary function of the facet joint is
to guide segmental motion. This is a function of the direction
of the facet planes. The general direction of the facet planes
in the lumbar spine is parallel to the sagittal plane; thus the
lumbar spine flexes and extends through a large arc of
motion, while rotation and side-bending are much less.

The facet joints also serve other important roles in the load
bearing of the lumbar spine. They act to resist anterior shear
forces and, along with the IVD, resist torsion [59].
Additionally, the facet joints play a role in resisting compres¬
sive forces. During upright posture, approximately 18-20% of
the compressive load acting upon the lumbar spine is exerted
at the facets [13]. This value, however, varies as a function of
the position of the center of gravity of the head, arms, and
trunk. With increased lordosis, the center of gravity shifts pos¬
teriorly, producing an extension moment on the lumbar spine
and increasing the load on the lumbar facets. A decreased lor¬
dosis shifts the center of gravity of the head, arms, and trunk
anteriorly and shifts the load to the vertebral bodies and inter¬
vertebral joints [30,46,58].

Clinical Relevance

THE CONTRIBUTION OF THE FACET JOINTS TO LOW
BACK PAIN: The capsules of the lumbar facet joints often
have fibroadipose, meniscoid inclusions [67]. These structures
are quite small and are typically located near the periphery
of the joint. A common clinical hypothesis is that entrapment
of these meniscoid inclusions can occur with certain sudden,
unguarded motions; resulting in painful limited range of
motion (ROMl eliciting exclamations such as, "I just threw
my back out!" Anecdotally, patients with such episodes often
have rapid relief of symptoms following spinal manipulation.
However ; the role of the facet joints in symptom production
is controversial [25,45]

The capsule of the facet joint is richly innervated and
typically has strong afferent connections to the segments
above and below. This large receptor field is one of many
reasons why precise identification of the source of low back
pain remains elusive [12,41].

Because the facet joint is a synovial-type joint, it is sub¬
jected to a variety of arthritic and synoviolytic disorders
such as osteo- and rheumatoid arthritis. Although degenera¬
tive changes of the facet joint are frequently visualized on
radiographs, the relationship of these findings to pain is
uncertain [72].

Intervertebral Joint

The intervertebral, or interbody, joint is a symphysis-type
articulation that joins two adjacent vertebral bodies. Its pri¬
mary components are the superior and inferior surfaces of the
vertebral bodies, the vertebral endplates, and the IVD (Fig.
32.10 ) [23,31]. The intervertebral joint serves the critical
function of providing a mechanism for motion and load bear¬
ing between the vertebrae.

VERTEBRAL ENDPLATE

The vertebral endplate is a flat structure composed of hyaline
and fibrocartilage that is approximately 0.6-1.0 mm thick [13].
Located within the inner margin of the epiphyseal rings on the
superior and inferior surfaces of the vertebral bodies, the end¬
plate acts as a boundary between the IVD and vertebra [23].
In certain places, the subchondral bone deep to the endplate
is thin or absent, creating a portal for tissue fluid to travel
between the bone marrow and the IVD. This is an important
consideration in understanding the nutrition of the largely
avascular IVD. The endplate is more strongly bound to the
disc than to the vertebral body; thus certain types of trauma
can tear the endplate away from the bone [1]. Additionally, the
vertebral endplate may be fractured following rapidly applied
compression loads to the spine such as those sustained when a
person slips and falls on the ischial tuberosities. Although usu¬
ally quite painful, these fractures may not be easily visible on

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

573

Figure 32.10: The lumbar intervertebral joint consists of the IVD, the vertebral endplate, and the ring apophysis.

radiographs and may require further investigation using mag¬
netic resonance imaging or bone scan procedures.

INTERVERTEBRAL DISC

The IVD is an extraordinary structure that is the central fig¬
ure in spinal mechanics and pathology [1,6,31,40,42]. It is
typically described as consisting of an outer fibrous covering,
the anulus fibrosus, and an inner gel-like region known as the
nucleus pulposus [13,23,24]. This distinction between the
nucleus pulposus and the anulus fibrosus often is used to
describe disc biomechanics; however, in vivo these are nei¬
ther independent nor isolated structures [7,23,24]. The
nuclear zone actually develops as a transition from a lesser
hydrated area in the periphery to a more highly hydrated cen¬
tral region [7,27]. This distinction becomes less apparent with
disc degeneration. For clarity, however, the current discus¬
sion describes the anulus fibrosus and the nucleus pulposus as
separate structures.

Anulus Fibrosus

The anulus fibrosus is predominately composed of rings of
fibrocartilage forming the outer portion of the IVD. Taylor
describes the typical lumbar anulus fibrosus as consisting of
between 10 and 20 layers of collagen fibers that are obliquely
oriented to one another [66]. This plywood-like system forms a

strong surrounding band of tissue to protect and isolate the
nucleus pulposus while tolerating high-magnitude tensile loads
[13,23]. Strong attachments exist between the anulus fibrosus
and the outer portion of the vertebral bodies and vertebral end-
plates as well as with the anterior longitudinal ligament.

When viewed on the transverse plane, the IVD is not cir¬
cular but rather has a noticeable concavity in its central, pos¬
terior portion (Fig. 32.11 ) [13]. This concavity increases the
amount of anulus fibrosus material posteriorly to resist the
flexion loads common in daily activities. The posterior longitu¬
dinal ligament also reinforces the posterior anulus; however,
the posterior-lateral portion of the anulus is not as well rein¬
forced. This contributes to the predominance of posterior and
posterior-lateral disc herniations [9,41].

In addition to its capacity for load bearing, recent work
demonstrates an abundance of mechanoreceptors and free
nerve endings in the outer layer of the anulus, suggesting an
important role in proprioception as well as in pain production
[29,41,62]. A recent surgical development, anuloplasty, ther¬
mally denervates the outer anulus in an attempt to control
pain. The efficacy of this procedure is not currently known.

Nucleus Pulposus

The nucleus pulposus represents the inner portion of the IVD.
Histologically, the nucleus pulposus is a mucopolysaccharide

574

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 32.11: Axial view of the lumbar IVD. Note the posterior
concavity and the close relationships of the anterior and
posterior longitudinal ligaments to the anterior and posterior
anulus fibrosus.

gel that is approximately 70-90% water, although this water
concentration typically decreases with aging. The dry weight
of the nucleus pulposus is composed of 65% proteoglycans,
approximately 20% of which is collagen, with the rest being
elastic fibers and various proteins [13,23,27]. These structures
aggregate to form a relatively soft, gel-like substance that
interfaces with the anulus fibrosus to provide a hydraulic load-
bearing system [70,76]. Because it is more hydrated than the
anulus fibrosus, the nucleus pulposus of the IVD is clearly vis¬
ible on T2-weighted magnetic resonance images (Fig. 32.12)
[3,6,9,10].

Its hydrophilic capacity, (i.e., its ability to bind water) is crit¬
ical to the function of the IVD [13,23,70,71]. Consider that
with the exception of the very periphery of the anulus fibrosus,
the IVD is avascular [13]. Such avascularity is necessary
because if this structure relied upon arterial flow, sustained
compression such as occurs during upright activities would
impede blood flow and lead to ischemia. Thus the disc main¬
tains its hydration by diffusion of tissue fluid, mediated by
mechanical forces and osmotic gradients. To clarify this,
Urban et al. [70,71] describe the fluid content of the disc as a
balance of the hydrostatic and osmotic pressures. Hydrostatic
pressures are created by the external loads acting upon the
disc, such as those from muscle and ligamentous tension.
Osmotic pressures are generated within the disc by proteogly¬
can molecules that have water-binding properties. Thus cyclic
loading in the presence of a normal concentration of proteo¬
glycans within the IVD creates a series of events that move tis¬
sue fluid in and out of the disc. Considering the numerous
variations in a person s posture from one moment to the next
and, therefore, changing loads over a 24-hour period, one can
see that the disc is constantly changing its shape and fluid con¬
tent. An interesting and clinically relevant application of this
process relates to diurnal variations in the fluid content of the
disc. During loading, the disc initially adapts by slight move¬
ments in the collagen fibers; however with sustained loading,
fluid is lost from the disc [70,71], resulting in a loss of vertical
dimension. During periods of reduced loading, such as recum¬
bency while sleeping, the osmotic gradient is greater, and fluid
travels back into the disc. This explains why persons are taller
(sometimes up to 2 cm!) in the early morning than in the
evening. This phenomenon is exaggerated by exposure to
weightless environments during space travel.

Clinical Relevance

THE FLUID CONTENT OF THE IVD AND ITS
RELATIONSHIP TO LOW BACK PAIN: Because of the
great importance of maintaining adequate hydration of the
disc, factors that adversely influence this may lead to disc
degeneration and spinal dysfunction. For examplethe syn¬
thesis of proteoglycans may be impaired by smoking or dur¬
ing prolonged immobilization [47]. Exposure to vibration
also has been postulated to cause this.

Considering the diurnal variations in the disc , Snook et al.
[61] describe a randomized clinical trial on persons with
chronic low back pain. Noting that most lifting injuries to
the low back occurred in the morning , the authors postulat¬
ed that flexion avoidance in the morning might help to
reduce pain. In their study , one group of persons with low
back pain avoided early morning lumbar flexion and had
much better outcomes than a second group that performed
stretching exercises in the early morning. The authors sug¬
gest that the elevated disc fluid volume early in the morning
predisposed the disc to injury during lumbar flexion. This is
an intriguing finding and may have implications for instruc¬
tion to patients performing exercises for low back pain.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

575

MECHANICAL PROPERTIES OF THE 1VP

Humzah provides an appropriate description of the IVD,
referring to it as a “flexible interspace” between the vertebrae
[23]. The IVDs extraordinary ability to absorb and transmit
forces is accomplished by developing a hydraulic effect dur¬
ing loading [13,30,70,76]. The IVD allows joint displacement
to occur by maintaining a separation between the vertebral
bodies (i.e., acting as a “spacer”) and by being capable of
deformation in all planes of motion. The uniqueness of the
mechanics of the IVD, coupled with its central role in the
generation of low back pain, make it one of the most highly
investigated musculoskeletal tissues. To understand the
mechanical properties of the IVD, it is first important to con¬
sider the external forces to which it is subjected. The basic
external stresses acting upon the IVD can be classified as
compression and tension, bending and rotation.

Compression

External forces that tend to approximate the vertebral bodies
exert compressive loads on the IVD. In general, the disc tol¬
erates these loads by converting vertically applied compres¬
sion into circumferentially applied tension by a phenomenon
known as hoop stress (Fig. 32.13) [41,53,76]. Pascal’s law
states that pressure applied to a liquid is distributed equally in
all directions. As the compressive load is applied, pressure
within the nucleus pulposus increases, but because water is
incompressible, the nucleus pulposus in turn exerts pressure
against the surrounding anulus fibrosus through a process

Load

Figure 32.13: An example of the "hoop stress" created within the
IVD during compressive load bearing. Compressive loading on
the nucleus pulposus causes it to exert radial stresses on the anu¬
lus fibrosus.

known as radial expansion. The anulus fibrosus then resists
this load through tension developed in its collagen fibers. The
nucleus pulposus also exerts pressure against the superior and
inferior vertebral endplates, thus serving to transmit part of
the load from one vertebra to the next. Enormous loads can
be tolerated in this fashion. Because of their association with
the vertebral body, the endplates do not deform unless large-
magnitude, damaging forces are applied.

Bogduk and Twomey [13] describe a second property of
the disc as the ability to store energy during loading and to
recoil elastically once the load is released. This mechanism is
critical to the load-bearing capacity of the motion segment and
the ability of trabecular bone in the vertebral body to function
as a shock absorber. This hypothesis is supported by recent
work that shows the presence of elastic fibers in the anulus
fibrosus and nucleus pulposus, implying a dynamic flexibility
to the IVD and the capacity for viscoelastic behavior [24,27].

The normal disc, therefore, functions hydrostatically, with
internal pressures increasing in relation to externally applied
forces. With dehydration or surgical excision of the nucleus
pulposus, the capacity of the IVD to tolerate compressive
loads is altered. Short applications of light compressive loads
to denucleated discs can be tolerated by the anulus fibrosus
alone; however, higher forces or prolonged application of
forces are problematic because of the inability of the disc to
develop internal fluid pressure and transform the compres¬
sive load to radial forces on the anulus fibrosus [76]. This
leads to excessive loading on the vertebral bodies, which often
results in further degenerative changes.

Bending

The behavior of the IVD during bending motions, such as
occur with many of activities of daily living, is of great interest
both as a mechanism to understand tissue injury and as a
strategy for exercise prescription. Consider that the nucleus
pulposus is not a rigid sphere but is capable of deformation in
three directions. In 1935, Steindler [63] postulated that the
nucleus pulposus deforms in the direction opposite to the
motion during sagittal or frontal plane motions, so that during
lumbar extension, the nucleus pulposus displaces anteriorly
and vice versa. This theory has been confirmed in several
studies using cadaveric material and living subjects, support¬
ing the notion of the intact nucleus pulposus functioning as a
ball bearing during spinal motion [56,76] (Fig. 32.14).

Clinical Relevance

DEFORMATION OF THE NUCLEUS PULPOSUS AS A
BASIS FOR BACK EXERCISES: The deformation of the
nucleus pulposus during lumbar motion forms the basis for
the repeated prone press-up exercises advocated by
McKenzie [40]. McKenzie's hypothesis is that as a patient
flexes his or her lumbar spine, the nucleus pulposus

( continued)

576

Part III I KINESIOLOGY OF THE HEAD AND SPINE

(Continued)

displaces posteriorly, while during lumbar extension, it dis¬
places anteriorly, away /roa? the pain-sensitive structures in
the vertebral and intervertebral foramina. Thus exercises
and postures are prescribed to influence the position of the
IVD. Both discography and magnetic resonance imaging
demonstrate this phenomenon in normal discs [7,56]. Thus
during lumbar bending, a normal nucleus pulposus deforms
in a direction opposite that of the applied load.

It is important to realize, however, that the nucleus pulposus
is deforming, not actually displacing or moving across bone,
during lumbar motions. As previously stated, the nucleus pul¬
posus is not a discrete structure, but actually represents an area
of the disc with greater hydration than the periphery [23].
Dehydration of the discs leads to an even less clear distinction
between the nucleus and anulus. Interestingly, in discs with evi¬
dence of degeneration or herniation, the nucleus pulposus has
been shown to have an inconsistent pattern of deformation [7[.
This may explain why certain patients with disc pathology
have symptom enhancement with lumbar extension and oth¬
ers do not. White and Panjabi [76] point out that during

bending, the anulus fibrosus is compressed on the side to
which the subject bends. For example, the posterior portion of
the IVD is compressed during extension but is exposed to tensiie
loading on the opposite side (Fig. 32.15). Therefore, with lumbar
extension, a posterior bulging of the anulus fibrosus and nucle¬
us pulposus may be present, especially in patients with degener¬
ative IVDs. The clinical significance of this is unknown.

Rotation

While well suited to withstand compressive loads, the disc is
much less able to tolerate torsional (rotational) forces. During
torsional stress on the IVD, the anulus fibrosus is loaded in
tension. Recall, however, that the anulus fibrosus is a series of
obliquely arranged fibers; thus during rotation of a vertebral
body, a portion of these fibers is not under tension [13] (Fig.
32.16). Therefore, only a portion of the anulus fibrosus is able
to resist a torsional stress. Fortunately for the lumbar spine,
the sagittal plane arrangement of the facet joints in this region
limits rotation and thus protects against these forces. This
protective mechanism is significantly reduced when the spine

Figure 32.14: The concept of the nucleus pulposus acting as a ball bearing during lumbar motion. This principle results in deformation
of the nucleus in the direction opposite the motion. During lumbar flexion, the nucleus pulposus tends to deform posteriorly; in lum¬
bar extension, the nucleus pulposus tends to deform anteriorly.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

577

Figure 32.15: As an individual bends backward, the posterior
aspect of the IVD sustains compressive forces while the anterior
aspect of the disc undergoes tensile loading.

is in flexion [13,19]; thus a common mechanism of disc injury
is combined rotational and forward-bending movements
[1,40,51,61].

IVD Pressures during Activities
of Daily Living

Of great interest clinically is the effect of various activities and
postures on the intradiscal pressure. While this mechanism

Figure 32.16: Stress on the fibers of the anulus fibrosus during
lumbar rotation. The criss-cross arrangement of the collagen
fibers results in only a portion of the fibers being loaded.

has been studied extensively [1,2,30,46,58], the classic work
was reported by Nachemson et al. [46]. Using a pressure sen¬
sor inserted into the nucleus pulposus of the L3 disc, these
authors demonstrated a linear relationship between intradis¬
cal pressure and the moment acting upon the disc. The
moment is the product of the superincumbent load, includ¬
ing the mass of the head, arms, and trunk plus anything being
lifted or carried, and the length of the moment arm of the
superincumbent load.

Activities that increase intradiscal pressure often involve
lumbar flexion from the upright position and/or increased
trunk muscle activity. For example, Nachemson reports that
lying supine results in 250 N (56 lb) of intradiscal pressure,
which increases to 500 N (112 lb) when standing erect [46].
Forward-bending 40°, which increases the moment arm of
the superincumbent weight, raises the pressure to 1000 N
(224 lb). Lifting 100 N (22.5 lb), which increases the superin¬
cumbent mass, elevates the pressure to 1700 N (382 lb), and
holding 50 N (11 lb) at arms length, increasing both the
moment arm and the superincumbent mass, raises the pres¬
sure to 1900 N (427 lb). Coughing (which requires contrac¬
tion of the trunk muscles) increases the pressure to 700 N
(157 lb). Clinically, increases in symptoms during forward¬
bending, lifting, or coughing are common findings in persons
with IVD pathology.

When sitting in an unsupported position and thus reducing
the normal lumbar lordosis, intradiscal pressure rises to 700 N
(200 N, or 45 lb, greater than standing). This decreases to 400
N (90 lb) when the lumbar spine is supported. Considering
this, it is not surprising that persons with symptomatic herni¬
ated lumbar IVDs have increased symptoms when sitting,
which is often considered to be light duty! Use of a lumbar
roll to maintain a normal lumbar lordosis and thus reduce
intradiscal pressure is often a very useful intervention for per¬
sons with discogenic low back pain [40].

Clinical Relevance

THE INTERVERTEBRAL DISC AS A SOURCE OF
SYMPTOMS OF LOW BACK PAIN: Disorders of the IVD

are one of the most common sources of low back symptoms
and lumbar nerve root compression . Numerous hypotheses
have been proposed. The three primary ways in which an
abnormal IVD may cause symptoms are (a) direct injury to
the pain-sensitive outer portion of the anulus fibrosus; (b) a
herniated disc in which nuclear material breaks through its
boundaries created by the anulus fibrosus and causes
mechanical pressure and chemical irritation of the pain-sen¬
sitive structures in the vertebral foramina, and (c) a degener¬
ative disc that loses vertical dimension and causes the verte¬
brae to approximate one another, leading to a reduction in
the stability of the segment

(continued )

578

Part III I KINESIOLOGY OF THE HEAD AND SPINE

(Continued)

Interestingly, not all herniated discs cause symptoms; and
most degenerative discs do not. Numerous factors such as
the ratio of disc abnormality to the size of the vertebral
canal [9], the degree of instability at the motion segment
[29], and various biochemical issues [41,47] must also be
considered. This lack of linearity between pathology and
symptoms makes the evaluation and treatment of persons
with low back pain extremely challenging [5,9].

MOTION OF THE LUMBAR SPINE

Motion occurring at the lumbar spine is critical to a persons
ability to perform the numerous tasks of daily living. Lumbar
motion can range from very small displacements providing
a “damping effect” during loading to very large arcs of motion
that occur with bending and reaching tasks. Abnormalities
of lumbar motion can manifest themselves as various combina¬
tions of reduced, excessive, or poorly timed joint displace¬
ments. These abnormalities are a primary source of symptoms
and often lead to tissue degeneration through repetitive, abnor¬
mal loading. In the clinical environment, accurate assessment
of impairment of lumbar motion and its relationship to a
person s symptoms and functional limitations is a critical com¬
ponent of the assessment process. In this section, the motion of
the lumbar spine is discussed from a variety of perspectives,
including gross motions, osteo- and arthrokinematics, and clin¬
ical measurement.

Gross Motion of the Lumbar Spine

When considering the lumbar spine as an entire unit, motions
traditionally are described using cardinal planes as a reference
(Table 32.2). This system is quite useful as a classification of
joint displacement, as it provides a conceptual framework for
lumbar ROM by creating common reference points. It is
important to note, however, that under the conditions of load¬
ing associated with daily activities, the lumbar spine is nearly
always undergoing multidirectional displacement. In fact, at
the level of the joint surface, pure single-plane movement may
not exist [19].

When determining the nature of gross motions of the lum¬
bar spine, the plane of the facet joints dictates the directions

TABLE 32.2: Gross Motions of the Lumbar Spine
Based upon Cardinal Planes

Motion

Cardinal Plane

Flexion (forward-bending)

Sagittal

Extension (backward-bending)

Sagittal

Side or lateral bending

Frontal

Rotation

Transverse

of displacement possible. For example, the sagittal plane
alignment of the lumbar facet joints favors flexion and exten¬
sion but greatly limits rotation. The height of the IVD acts to
maintain the alignment of the joint surfaces as well as tension
on the segmental ligaments. The vertical dimension of the
IVD space also is related to the available motion at a given
motion segment, since the deformation of the disc con¬
tributes to the motion between adjacent vertebrae. In the
lumbar spine, the normal disc spaces are larger than those of
the thoracic spine. This contributes to the relatively large arc
of motion that is possible in the lumbar spine. With disc space
narrowing such as occurs with disc degeneration, changes in
the positional relationships of the vertebrae to one another
can adversely affect joint mechanics [7,29,41,76].

LUMBAR FLEXION

Lumbar flexion (forward-bending) is achieved by a “flatten¬
ing” or perhaps a slight reversal of the normal lumbar lordo¬
sis. When a person bends forward from the standing position,
segments are recruited from rostral to caudal (i.e., the upper
lumbar segments move into flexion first followed by the mid¬
dle then lower segments) [13]. Lumbar flexion is lim¬
ited by tension in the posterior anulus fibrosus and
the posterior ligament system (Table 32.3).

A critical concept related to lumbar flexion is its relation¬
ship to anterior pelvic rotation, a phenomenon often referred
to as lumbopelvic rhythm [14]. This concept applies to a
person attempting to bend forward to touch his or her toes
while keeping the knees straight. The interplay between lum¬
bar spine motion and pelvic motion is integral to understand¬
ing how an individual moves. Although there appears to be a
variety of lumbopelvic rhythms exhibited by individuals, the
following, described by Calliet [14], is a commonly reported

TABLE 32.3:

General Trends for

Angular Displacement

at the

Segmental Levels of the Lumbar

Spine (in°)

LI-2

L2-3

L3-4

L4-5

L5-S1

Total

Flexion

6-8

7-10

7-12

8-13

7-9

35-52

Extension

4-5

3-5

1-6

2-7

5-6

15-29

Side bending

3-6

5-6

4-5

1-2

16-25

Rotation

1-4

1-3

5-16

Data from Grieve GP: Common Vertebral Joint Problems. New York: Churchill-Livingstone, 1981 and Pearcy M, Portek I, Shepherd J: Three dimensional x-ray analysis of
normal movement in the lumbar spine. Spine 1984; 9: 294-297.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

579

sequence of lumbar spine and pelvic movement. Initially the
trunk inclines forward as the lumbar lordosis flattens. Once
full lumbar flexion is achieved, the additional forward inclina¬
tion of the trunk occurs from the pelvis rotating anteriorly
upon the hip joints. The forward rotation of the trunk is in
turn typically limited by tension in the hamstring muscles.
Thus a persons ability to touch his or her toes relies upon
pelvic rotation and extensible hamstring muscles, as well as
the ROM of lumbar flexion.

Clinical Relevance

THE RELATIONSHIP OF INEXTENSIBLE HAMSTRING
MUSCLES TO LOW BACK PAIN: The concept of lumbo-
pelvic rhythm illustrates the potential relationship of inexten-
sihie hamstring muscles to excessive flexion forces on the
lumbar spine during forward-bending. For example, if a
patient has inextensible hamstring muscles; forward rotation
of the pelvis when standing may be prematurely restricted.

In an attempt to reach forward and down , a person may try
to compensate for this by increasing the amount of lumbar
flexion , often causing injury to the posterior lumbar struc¬
tures. Stretching the hamstring muscles; therefore; is often a
critical portion of treatment for low back pain.

LUMBAR EXTENSION

Lumbar extension, or backward-bending, occurs in a similar,
but opposite manner to lumbar flexion (i.e., it is an increase
in the lumbar lordosis). The overall magnitude of lumbar
extension is much less than that of lumbar flexion because of
the unique bony geometry of the lumbar vertebrae. As the
lumbar spine extends from a normal lordosis, the spinous
processes approach one another, and tension in the anterior
longitudinal ligament restricts motion.

The relationship of pelvic displacement to lumbar exten¬
sion is also limited. In the standing position, posterior pelvic
rotation is limited by the tension in the iliolumbar ligaments
and the hip flexor muscles that restrict hip extension and,
therefore, pelvic rotation. Clinically, inextensible hip flexor
muscles can act to hold the pelvis in anterior rotation, which in
turn increases the lumbar lordosis, especially when a person
attempts to extend the hip. The resulting excessive lumbar
extension places increased loads on the posterior elements of
the lumbar spine and may be associated with symptoms and
tissue degeneration.

Another commonly observed clinical phenomenon relates
to how a person returns to the upright position from a posi¬
tion of forward-bending. Typically, a person initially rotates
the pelvis posteriorly, followed by a return to the normal lum¬
bar lordosis. Occasionally, one may initially arch the back to
regain the lumbar lordosis while flexing the hip and knees and
“walking” the hands up the thighs. This abnormal lum-
bopelvic rhythm may indicate some lumbar spine segmental
instability [41].

LUMBAR ROTATION AND SIDE-BENDING

Lumbar rotation, or twisting, as previously described, is
potentially deleterious to the IVDs if it is excessive. Because
of the roughly sagittal plane alignment of the facet joints, the
transverse plane motion of rotation is quite restricted in the
lumbar spine, limited by the approximation of the facet joint
surfaces. Anatomically, the facet joints surfaces at L5-S1 tend
to have a more oblique arrangement than the other segments
of the lumbar spine. Because of this, authors have proposed
that more lumbar rotation occurs at this segment than in the
other segments of the lumbar spine [19,76]. Recent findings
by Pearcy et al. [50] have contested this. The mechanics of
the lumbosacral junction are discussed in greater detail in
Chapter 35.

Despite the very limited degree of lumbar rotation, most
persons are able to compensate by achieving a relatively large
arc of total trunk and neck rotation. For example, the amount
of total rotation necessary to back up ones car is provided by
the contributing motions of the thoracic and cervical spine.

Lumbar side-bending in the lumbar spine, displacement
in the frontal plane, has a larger ROM than rotation but sub¬
stantially less than the sagittal plane motions [50,65,68]. The
amplitude of motion appears relatively evenly distributed
over all segments except L5-S1, which is quite restricted by
bony geometry and tension from the iliolumbar ligament [4].
Side-bending cannot occur without some lumbar rotation
(and vice versa) because of the phenomenon known as joint
coupling. Joint coupling occurs when two motions are linked
together so that one cannot occur without the other.

Joint Coupling in the Lumbar Spine

In the discussion above, lumbar motion is described from a
“neutral” starting point. This neutral position can be consid¬
ered a normal lumbar lordosis with no appreciable rotation or
side-bending. In most activities of daily living, the spine moves
in and out of a neutral position. How does this change the
nature of spinal motion? Interestingly, lumbar rotation and
side-bending depend upon one another (i.e., their motions are
“coupled”). The degree of coupling is determined primarily by
two factors: the direction of the articular processes acts to
guide specific displacements at the joint surface, and the posi¬
tion of the spine determines the relative tension on various soft
tissue structures [19]. For example, in the neutral position,
rotation is limited by approximation of the articular processes
and by tension in the anulus fibrosus and posterior longitudi¬
nal ligaments [19,20]. Lumbar flexion and extension reduce
the available range of side-bending and rotation, while the
position of side-bending reduces the available range of flexion
and extension. When the lumbar spine is in a position of side¬
bending, rotation is greater to the opposite side (toward the
convexity) than to the same side (toward the concavity). Thus,
when the lumbar spine is in flexion, side-bending and rotation
occur to the same side (e.g., left rotation is accompanied by
left side-bending). When the lumbar spine is in a neutral or
extended position, side-bending and rotation occur opposite

580

Part III I KINESIOLOGY OF THE HEAD AND SPINE

one another (e.g., left rotation is accompanied by right side¬
bending) [19].

Segmental Motion of the Lumbar Spine

The preceding section describes motions that encompass the
entire lumbar spine. Movement between adjacent vertebrae
is also described. It is important for the clinician to clarify
which motions are being discussed. Movement occurring at a
single motion segment is called segmental motion, while
gross motion of the entire lumbar spine is a multisegmental
phenomenon.

To understand the complexity of spinal motion, it is
important to recall that the stability and mobility of the lum¬
bar spine result from an interplay of the bony elements and
their associated joint structures under the guidance of an ele¬
gant neuromuscular control system. As stated in Chapter 7,
joint motion is described in terms of osteokinematics (dis¬
placement of a bone) and arthrokinematics (displacement
occurring at specific joint surfaces). The combination of
these two events allows a joint to move through a given
ROM. As opposed to joints of the appendicular skeleton,
such as the hip, where a small number of relatively large
bones move around a single axis, spinal motion occurs as a
result of numerous small bones moving around several axes.
Conceptually, the elbow joint can be viewed as a lever pump¬
ing up and down, while the lumbar spine moves like an accor¬
dion opening and closing.

The multijoint system of the lumbar spine serves beauti¬
fully to absorb and attenuate forces and to make the myriad
of finely tuned adjustments required of the spine during
activities of daily living. It makes, however, the quantification
of spinal motion difficult. Each motion segment has the
potential to displace through angular (rotary) and linear
(translatory) motion in each of the three planes. This pro¬
duces 6 degrees of freedom. Because each displacement can
occur in opposite directions (e.g., anterior and posterior
translation in the sagittal plane), a motion segment has a total
of 12 possible movements (2 types of motion in 2 directions
in 3 planes) (Table 32.4).

Segmental motion of the spine occurs at the three-joint
complex of the motion segment or vertebral unit, which is
composed of two adjacent vertebrae and the tissues included
within them. The plane of its facet joints and the height of the
IVD influence the movements of a motion segment.

TABLE 32.4: The Twelve Motions of a Lumbar
Motion Segment

Axis of Movement

Type of Movement

Sagittal

Anterior and posterior translation
Anterior and posterior rotation

Frontal

Left and right translation

Left and right side rotation

Transverse

Distraction and compression

Left and right rotation

SEGMENTAL MOTION IN THE SAGITTAL PLANE

Because of the roughly sagittal plane alignment of the artic¬
ular processes in the lumbar spine, segmental motion in the
sagittal plane, grossly described as flexion and extension, is
the closest to occurring in a single plane of motion. During
flexion, each lumbar vertebra displaces by rotating in an
anterior direction. This is coupled with a slight anterior
translation, so that the facet joint surfaces of the inferior
articular processes of the superior vertebra slide superiorly,
reducing contact between the joint surfaces and allowing a
slight anterior translation to occur [13,59,60]. This anterior
displacement is limited by the bony geometry of the facet
joints [31], while the tension in the posterior anulus fibrosus
and posterior ligament system resist anterior rotation.

At the joint surfaces, extension occurs in a manner similar
to lumbar flexion; however, the unique bony geometry of the
lumbar vertebrae acts to restrict lumbar extension to a much
smaller ROM than lumbar flexion [19,50]. During extension,
the lumbar vertebrae rotate posteriorly accompanied by a
small posterior translation. As the superior articular processes
slide inferiorly during extension, the spinous processes of
adjacent vertebrae impact upon one another to restrict exten¬
sion. Further lumbar extension is limited by the approxima¬
tion of the articular processes and spinous processes [13].

Clinical Relevance

FLEXION VERSUS EXTENSION EXERCISES: With
increasing flexion, compressive load bearing is shifted ante¬
riorly away from the facet joints and the posterior IVD while
increasing the area of the vertebral foramen [49]. With
increasing extension, compressive load bearing is shifted
posteriorly away from the IVD toward the facet joints while
decreasing the area of the vertebral foramen. This principle
provides the basis for two major biomechanical treatment
approaches for persons with low back pain. For persons
with symptoms related to lumbar flexion, reducing pressure
on the IVD by limiting lumbar flexion is often a useful
approach. Conversely, for persons with symptoms during
lumbar extension, a useful treatment approach is to limit
extension and thereby reduce pressure on the facets joints
as well as prevent narrowing of the vertebral foramina.

Abnormal displacement of vertebrae, which occurs during
lumbar motion, is thought to be a primary contributor to
symptoms of low back pain and, in some cases, to transient
nerve compression (dynamic stenosis). If the facet joints are
unable to resist anterior translation during flexion or during
flexion combined with rotation, excessive movement may
occur, creating a condition known as segmental instability,
or hypermobility [29,41]. Conversely, it has been hypothe¬
sized that shortening of the facet joint capsule can lead to lim¬
ited displacement of a motion segment and result in symp¬
toms caused by premature end-range loading, a condition
known as segmental hypomobility [67].

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

581

SEGMENTAL MOTION IN THE TRANSVERSE
AND FRONTAL PLANE

Consistent with the morphology of the motion segment, joint
movement in the transverse plane, rotation, is quite restricted
throughout the lumbar spine. Because of their oblique align¬
ment, the collagen fibers within the anulus fibrosus are quickly
loaded in tension during lumbar rotation. Bogduk and
Twomey [13] report that stretching a collagen fiber beyond
4% of its resting length can lead to failure. These authors cal¬
culate that lumbar segmental rotation beyond 3° in a given
direction can lead to injury of the anulus fibrosus. Fortunately,
unilateral rotation rarely exceeds 3° under normal conditions
[50]. As previously stated, the mechanism providing the pri¬
mary restraint is approximation of the plane of the facet joints.
For example, if the vertebral body rotates to the left (left rota¬
tion of the motion segment), the joint surfaces of the right
facet approximate one another while the joint capsule of the
left facet is stretched, or loaded in tension. This restraint
mechanism is not as effective during lumbar flexion, which
may help explain the increased incidence of lumbar IVD
injuries that occur during combined flexion-rotation activities.

As noted earlier, segmental motion in the frontal plane,
side-bending, is coupled with rotation. There is more dis¬
placement in side-bending than in rotation, with the excep¬
tion of L5-S1, where both motions are limited. Tension in the
intertransverse ligament and the capsule of the contralateral
facet joint and approximation of the ipsilateral facet joint sur¬
faces all act to restrict side-bending.

Clinical Methods of Lumbar Range
of Motion Assessment

Assessment of ROM in the clinical setting is a fundamental
component of the physical examination. This section discusses
(a) the variables that must be considered to understand the
measurement of lumbar motion and (b) the general trends of
lumbar ROM on the basis of age and gender.

Numerous techniques have been described to assess lum¬
bar motion in the clinical setting, including observation, pal¬
pation of active and passive motion, and the use of instruments
such as goniometers, tape measures, inclinometers, and
spondylometers [11,21,32,34-37,43,44,50,57,64,65,68,69,75].
Recently, several types of computer-based motion analysis sys¬
tems have been described in the literature. Because of the cost
and lack of general accessibility of these systems, this discus¬
sion is limited to those methods commonly used in clinical
practice.

Two common procedures to assess lumbar motion are
goniometry and the fingertips-to-floor method. Unfortunately,
both of these techniques are problematic. A goniometer is a
single-axis device typically used to measure ROM in joints of
the extremities. However, its use for lumbar motion assess¬
ment is not appropriate (with the possible exception of lumbar
rotation) because spinal motion is the result of several joints
moving around numerous axes [44]. The fingertips-to-floor
method attempts to assess lumbar flexion by simply measuring

the distance of the fingertips to the floor when a person bends
forward from a standing position. This procedure is inexpen¬
sive and easy to perform but is of limited use because it does
not differentiate between lumbar flexion and forward inclina¬
tion of the pelvis. The distance to which a person can reach
toward the floor is a function of both lumbar flexion and pelvic
rotation. For example, as noted earlier in this chapter, a patient
might have normal lumbar flexion, but inextensible hamstring
muscles, limiting the distance he or she may reach the finger¬
tips to the floor. This can lead an examiner to conclude falsely
that the lumbar spine is restricted in forward-bending.
Conversely, someone with very extensible hamstring muscles
may be able to touch the floor easily, even with limited ROM
of the lumbar spine.

Considering that the limitations of the techniques above
are caused by the unique multisegmental motion of the lum¬
bar spine, it is not surprising that a common way to obtain
reliable and valid measures in the clinical setting is to break
down lumbar motion into two primary dimensions: linear dis¬
placement of spinous processes and angular displacement of
a given point on the trunk relative to the pelvis.

LINEAR DISPLACEMENT OF THE SPINOUS
PROCESSES OF THE LUMBAR SPINE

Clinicians can easily perform a gross assessment of the linear
displacement of the spinous processes occurring during sagit¬
tal motion. In 1937, Schober [57] described the following sim¬
ple procedure. The examiner places the tip of his or her little
finger over the posterior tubercle of SI and then places the
index finger over a spinous process approximately 10 cm supe¬
riorly. By having the subject bend forward, the distraction
between the spinous process and the posterior tubercle can be
appreciated. During lumbar backward-bending (extension),
an approximation or attraction of these bony prominences may
be felt, providing the examiner with a gross assessment of lum¬
bar motion. To quantify lumbar flexion using this principle,
Macrae and Wright [34] and Moll and Wright [43,44] use a
tape measure over the lumbar spine with the inferior land¬
mark 5 cm inferior to the lumbosacral joint and the superior
landmark 10 cm superior. Beattie et al. [11] use the same land¬
marks to assess lumbar extension (Fig. 32.17). Both of these
studies report that the tape measure technique (distraction
method for forward-bending and attraction method for back¬
ward-bending) yields reliable measures and is a simple, inex¬
pensive technique for clinical use.

ANGULAR DISPLACEMENT OF THE LUMBAR SPINE

Angular measures of lumbar ROM are obtained using a vari¬
ety of instruments that typically provide a single angle of trunk
displacement relative to the ground or to the sacrum. Reliable
measures are reported from the use of spondylometers
[21,64], flexible rulers [32], and inclinometers, which are fluid-
filled instruments measuring the angle in degrees that the
trunk makes with the vertical [36,37,75]. Of these, the incli¬
nometer appears to be the most inexpensive and easiest to use.

582

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 32.17: Landmarks used to perform measurements of lum¬
bar flexion (distraction method) and lumbar extension (attraction
method). This figure demonstrates the method for assessing flex¬
ion excursion. The landmarks are located when the subject is
standing in a neutral posture. The reference locations are the
lumbosacral region (L-S) at 0 cm, a second point (A) at 10 cm
superior to L-S, and a third point (B) located 5 cm inferior to L-S
that acts as an anchor point for the tape measure. At the end of
the subject's available lumbar flexion, the distance between the
reference point and the superior point is measured again. Ten
centimeters is subtracted from this new value to yield a linear
measurement of the subject's flexion. For example, if the second
measurement is 15 cm, the subject's flexion excursion is 5 cm
(15 cm - 10 cm = 5 cm) [7,43].

Figure 32.18: Use of an inclinometer to measure angular motion
of the lumbar spine. Two inclinometers are placed, one on the
sacrum and one at the proximal aspect of the lumbar spine. The
difference between the two measurements indicates the lumbar
spine angular motion.

Mayer et al. [37] describe the two-inclinometer method.
During this procedure, an examiner identifies a point on the
sacrum in the standing subject and places one inclinometer
over this area. A second inclinometer is placed over the spin¬
ous process of LI. The subject then performs forward-bend¬
ing (Fig. 32.18). The upper inclinometer indicates the total
anterior displacement of the trunk, while the lower incli¬
nometer indicates pelvic rotation. By subtracting the lower
value from the upper value, the degree of angular motion for
lumbar flexion is obtained. A similar procedure is used for

backward-bending. Because the pelvis does not typically dis¬
place laterally during standing side-bending, a single incli¬
nometer over the upper lumbar spine is adequate. Lumbar
rotation is not measured with an inclinometer.

Waddell et al. [75] report acceptable reliability for use of
the double inclinometer technique and identify differences
between nonpatients and persons with chronic low back pain
('Table 32.5). Patients with chronic low back pain exhibit sig¬
nificantly less motion for anterior rotation of the pelvis, total
flexion, total extension, and lateral flexion.

TABLE 32.5: Normal Values (95% Confidence Intervals) for Adults without LBP and with Chronic LBP (in°)

Motion

Normal Subjects (n = 70)

Patients (n = 120)

Lumbar flexion

42.4 (39.8-44.9)

48.7 (46.0-51.4)

Anterior rotation of pelvis

57.1 (54.1-59)

30.7 (27.4-34)

Total flexion

99.5 (96.2-102.8)

79.3 (74.7-83.9)

Total extension

26.5 (24.4-28.6)

18.4(17.0-19.8)

Lateral flexion

29.4 (27.9-31.0)

22.7 (21.3-24.1)

Adapted from Waddell G, Somerville D, Henderson 1, et al.:

: Objective clinical evaluation of physical impairment in chronic low back pain.

Spine 1992; 17: 617-628.

Chapter 32 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE LUMBAR SPINE

583

Normative Values for Lumbar Range
of Motion

A fundamental question is, “What is normal lumbar ROM for
the lumbar spine?” While general trends of motion are
known, exact values for normal lumbar ROM are difficult to
establish. To understand this, consider the concept of norma¬
tive data. Normative data can be thought of as a distribution
of measures obtained from a large number of persons to
determine an “average score” [55]. For these values to be
meaningful, several factors must be considered, including the
specific way in which the measures are obtained, the specific
characteristics of the population sampled, and the variation
within the distribution of measures. In other words, estab¬
lishment of normative values requires an evaluation proce¬
dure that yields reliable and valid measures that are obtained
from a clearly defined group of persons (consider age, gender,
pathology, etc.), and provide a mechanism to determine how
much variation from the average score (mean or other
description of central tendency) is considered “normal.” If
one considers the large number of measures used to describe
lumbar ROM as well as the numerous factors influencing
ROM, it is not surprising that a single set of normative values
has not yet been agreed upon. Thus it is very difficult in a clin¬
ical setting to identify a threshold for determining the pres¬
ence of abnormal motion of the lumbar spine.

Twomey and Taylor [68] report age and gender differences
in lumbar motion measured by a special instrument known as
a spondylometer, which reports an overall angular displace¬
ment in a given plane of motion. The results of the study are
described in Table 32.6 and reveal the following general
trends: (a) considerably more lumbar flexion-extension than
rotation is present throughout life; (b) teenage females have
more flexion, extension, and rotation than teenage males;
(c) young adult females have slightly more flexion-extension

TABLE 32.6: Approximate Mean Values for Lumbar
Range of Motion (in°) as Measured with a
Spondylometer

Motion

Age

Male

Female

Flexion

13-19

20-35

36-59

60+

Extension

13-19

20-35

36-59

60+

Rotation

13-19

20-35

36-59

60+

Adapted from Twomey LR: The effects of age on the ranges of motions of the
lumbar region. Aust J Physiother 1979; 25: 257-262.

than young adult males; and (d) older adults have less ROM
than younger adults or teenagers; however, there is little dif¬
ference between the genders in this group.

Several factors can influence the measurements obtained
over time by the same or different examiners, and these must
be considered when reviewing any measurement of lumbar
motion. Four primary concerns relative to a measure include
(a) device error, (b) human-device interface or procedural
error, (c) human performance variability, and (d) lack of train¬
ing among test administrators [36].

MANUAL ASSESSMENT OF PASSIVE
INTERVERTEBRAL MOTION

Determining the degree of passive motion available at an
individual motion segment is of interest to clinicians.
Although several variations have been described, the primary
technique for this involves either (a) applying varying degrees
of pressure to lumbar spinous processes to determine dis¬
placement or (b) passively moving the spine while palpating
changes in the size of the intersegmental spaces. Although
these techniques are still widely taught, recent work has
demonstrated low agreement between examiners relative to
the amount of motion available [35]. Because of the small
amounts of displacement and relatively large variability
among subjects, the validity of the manual assessment of seg¬
mental motion remains unproved.

RELATING THE OSTEOCARTILAGINOUS
LUMBAR SPINE TO FUNCTIONAL
DEMANDS

Throughout this chapter, the emphasis has been upon the
multiple factors that contribute to function (and dysfunction)
of the lumbar spine. While it is convenient to describe form
and function by considering one tissue and one motion at a
time, the lumbar spine, like the transmission in a car, relies
upon the proper functioning and interplay of numerous struc¬
tures. The osteocartilaginous lumbar spine is associated with
muscles, ligaments, and fascia arising from the pelvis, thoracic
spine, and extremities. The lumbar spine, therefore, provides
a series of mobile links within a kinetic chain that includes the
sacroiliac joints, pubic symphysis, and lower extremities as well
as the cervicothoracic spine and upper extremities (in
other words, everything). Mechanical loading of these
structures greatly influences the lumbar spine and vice
versa. Clinically, it is critical to assess these structures carefully
when working with persons who have lumbar spine problems.
For example, a limb length discrepancy resulting from tibial
shortening following a fracture can dramatically change the
loading patterns on the lumbar spine [8]. Conversely, excessive
lumbar lordosis associated with anterior pelvic rotation can
change loading patterns on the hip joint. Limited
ROM of the hip is a frequent finding in persons with
chronic low back pain [52].

584

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Recall that in addition to protecting neurovascular ele¬
ments, the primary function of the lumbar spine is to provide
stability while allowing adequate mobility for activities of daily
living. As with most joint systems this is really an issue of
dynamic stability. Stability in the presence of motion at any
given vertebral level is a function of bony architecture, disc
height and mechanics, facet joint orientation, ligamentous
support, and motor control in response to the load being
applied. When this system is working properly, the lumbar
spine is capable of withstanding large loads throughout the
ROM. However, when any component of the system
becomes impaired (e.g., fracture, disc herniation, soft tissue
trauma, or loss of motor control), relatively minor loads can
result in further trauma and symptoms (Box 32.1).

Finally, when applying the information from this chapter
in the clinical environment, the clinician must relate the
structure and mechanics of the lumbar spine to the physical

EXAMINING THE FORCES BOX 32.1

SUMMARY OF STRUCTURES THAT RESIST
LOADING IN THE LUMBAR SPINE AND
COMMON INJURIES THAT OCCUR FROM
EXCESSIVE LOADING

Compressive (Axial) Loading

Tissues resisting: Vertebral bodies and interverte¬
bral disc, abdominal mechanism

Injury: Vertebral body or endplate fracture

Rotational and Side-Bending Stress

Tissues resisting: Facet joints, pedicles, abdominal
mechanism, quadratus lumborum, superficial and
deep back muscles

Injury: Fracture of pars interarticularis, pedicle
Flexion Stress

Tissues resisting: Posterior ligament system, pos¬
terior anulus fibrosus, thoracodorsal fascia,
superficial and deep back muscles

Injury: Anular tear, disc herniation, muscle injury

Extension Stress

Tissues resisting: Anterior ligament system, poste¬
rior bony elements, abdominal mechanism

Injury: Pars fracture, traumatic spondylolisthesis

Protection of Neurovascular Elements

Tissues resisting: Fibro-osseous foramina

Injury: Entrapment of cauda equina and/or nerve
roots

and emotional stresses encountered by the patient. The prob¬
lem of low back pain and associated spinal disorders remains
a major public health consideration throughout the world.
Considerable suffering and loss of quality of life have resulted
from these conditions. Through an understanding of the
structure and function of the lumbar spine, the disorders of
the lumbar spine, and most importantly the persons who
must endure these conditions, clinicians have the opportunity
to make great contributions.

SUMMARY

This chapter examines the bones and joints of the lumbar
spine and describes how these structures influence the mobil¬
ity and stability of the region. The structures of the lumbar
spine are specialized to perform load-bearing functions, and
they sustain large loads during most activities. The ligaments
of the lumbar spine contribute to stability but also appear to
play a role in the motor control of the region by providing
important sensory feedback. The IVDs absorb and transmit
forces and work as dynamic spacers that allow more mobility
between lumbar vertebrae. The contribution of these struc¬
tures to common lumbar disorders is introduced.

Total lumbar motion as well as segmental motion is
reviewed. The lumbar spine exhibits greatest motion in the
sagittal plane as a result of the alignment of the facet joints.
Transverse and frontal plane motions are more limited and
coupled with each other. Normative data on the mobility of
the spine are reviewed. Mobility of the lumbar spine is
affected by age and gender.

The lumbar spine is a fascinating system of bones, joints,
ligaments, and fascia under the control of a very sophisticated
neuromuscular mechanism. Providing a protected pathway
for neurovascular structures, this complex interplay of mobil¬
ity and stability acts as a critical pivot in the center of the
human skeleton. The following chapter describes the mus¬
cles’ participation in the support and movement of the lum¬
bar spine.

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56. Schnebel RE, Watkins RG, Dillin W: A digitizing technique for
the study of movement of intradiscal dye in response to flexion
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57. Schober P. The lumbar vertebral column and backache.
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58. Schultz AR, Rrinckman P, Pope M, et al.: Riomechanical analyses
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59. Sharma M, Langrana NA, Rodriquez J: Role of ligaments and
facets in lumbar spinal stability. Spine 1995; 20: 887-900.

60. Shiraxi-Adi A: Riomechanics of the lumbar spine in sagittal/lat¬
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61. Snook S, Webster ES, McGorry RW, et al.: The reduction of
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62. Solomonow M, Ring-He Z, Harris M, et al.: The ligamento-mus-
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66. Taylor JR: The development and adult structure of lumbar
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CHAPTER

Mechanics and Pathomechanics of
Muscles Acting on the Lumbar Spine

STUART M. McGILL, PH.D.

CHAPTER CONTENTS

MUSCLE SIZE .587

MUSCLE GROUPS.588

Rotatores and Intertransversarii .588

Extensors: Longissimus, Iliocostalis # and Multifidus Groups .593

Abdominal Muscles.596

Special Case of the Quadratus Lumborum and Psoas Major.598

SUMMARY.599

T raditional description of the spine musculature is from a posterior vantage point, but many of the functionally
relevant aspects are better viewed in the sagittal plane. (For a nice synopsis of the sagittal plane lines of
action, see Bogduk et al. [3].) This traditional approach has hindered understanding of the many roles that
muscles play in lumbar mechanics. Furthermore, understanding of muscle function is typically obtained by simply inter¬
preting the lines of action and region of attachment, which may be misleading. Understanding the function and pur¬
pose of each muscle requires knowledge of muscle morphology, together with knowledge of activation of the muscu¬
lature over a wide variety of movement and loading tasks. Muscles create force, but these forces play roles in moment
production for movement and for stabilizing joints for safety and performance. Further, interpreting anatomy, mechanics,
and activation profiles is the only way to understand motor control system strategies chosen to support external loads
and maintain stability. This chapter enhances the discussion of anatomically based issues of the spine musculature and
blends the results of various electromyographic (EMG) studies to help interpret function and the functional aspects of
motor control. The purposes of this chapter are to

■ Present the current understanding of the functional roles of the muscles of the lumbar spine
■ Demonstrate the application of this knowledge in the design of exercises for the lumbar spine

MUSCLE SIZE

As noted in Chapter 4 on the mechanics of muscle, the phys¬
iological cross-sectional area (PCSA) of muscle determines
the force-producing potential, while the line of action and
moment arm determine the effect of the force in moment
production, stabilization, etc. It is erroneous to estimate force
on the basis of muscle volume without accounting for fiber

architecture or by taking transverse scans to measure anatom¬
ical cross-sectional areas [13], as has often occurred in inter¬
preting spine mechanics. In such cases, muscle forces are
underestimated, as a large number of muscle fibers are not
“seen” in a single transverse scan of a pennated muscle. Thus,
areas obtained from magnetic resonance imagery (MRI) or
computed tomography (CT) scans must be corrected for fiber
architecture and scan plane obliquity [14]. In Figure 33.1 ,

587

588

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 33.1: Lumbar musculature in cross section. Transverse scan of one subject (supine) at the level of (left to right) T9, LI (top) and
L4, SI (bottom); anterior is the top of each scan. (Reprinted with permission from McGill SM, Santaguida L, Stevens J: Measurement of the
trunk musculature from T6 to L5 using MRI scans of 15 young males corrected for muscle fiber orientation. Clin Biomech 1993; 8: 171-178.)

transverse scans of one subject show the changing shape of the
torso muscles over the thoracolumbar region, highlighting the
need to combine transverse scan data with data documenting
fiber architecture obtained from dissection. In this example,
the thoracic extensors seen at T9 provide an extensor moment
at L4 even though they are not “seen” in the L4 scan. Only
their tendons overlie the L4 extensors.

Raw muscle PCS As and moment arms [14] are provided in
Tables 33.1-33.3. Areas corrected for oblique lines of action
are shown in Table 33.4 for some selected muscles at several
levels of the thoracolumbar spine. Guidelines for estimating
true physiological areas are provided in McGill et al. [13].

Moment arms of the abdominal musculature reported in
CT- or MRI-based studies have recently been shown to under¬
estimate true values by 30%, given the supine posture required
in the MRI or CT scanner. This posture causes the abdominal
contents to collapse posteriorly under gravity [10]. In real life,
the abdominals are pushed away from the spine by visceral
contents when standing. In summary, muscle areas obtained
from various medical imaging techniques need to be corrected
to account for fiber architecture and contractile components

that do not appear in the particular scan level (only the tendon
passes the level). Further, moment arms for muscle line of
action obtained from subjects who are lying down need to be
adjusted for application to real life and upright postures.

MUSCLE GROUPS

Rotatores and Intertransversarii

MUSCLE ACTION: ROTATORES

Action

Evidence

Trunk rotation

Inadequate

Proprioception and position sense

Inadequate

MUSCLE ACTION: INTERTRANSVERSARII

Action

Evidence

Trunk lateral flexion

Inadequate

Proprioception and position sense

Inadequate

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589

a Abdominal wall includes external and internal oblique and transverse abdominis.
^Erector mass includes longissimus thoracis, iliocostalis lumborum, and multifidus.

TABLE 33.2: Raw Lateral Distances (mm) between Muscle Centroids and Intervertebral Disc Centroid (Standard Deviation)

590

Total area 0(2) 1(3) -2(4) -1(3) -1(4)

a Abdominal wall includes external and internal oblique and transverse abdominis.
b Erector mass includes longissimus thoracis, iliocostalis lumborum, and multifidus.

TABLE 33.3: Raw Anterior-Posterior Distances (mm) between Muscle Centroids and Intervertebral Disc Centroid (Standard Deviation)

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591

592

Part III I KINESIOLOGY OF THE HEAD AND SPINE

TABLE 33.4: Examples of Corrected Cross-Sectional Areas and A-P and Lateral Moment Arms Perpendicular
to the Muscle Fiber Line of Action Using the Appropriate Cosines: These Values Should Be Used in
Biomechanical Models Rather Than the Uncorrected Values Obtained Directly from Scan Slices

Muscle

Cross-Sectional
Area (mm) 2

Anterior-Posterior

(mm)

Lateral (mm)

Longissimus pars lumborum a

L3-4

644

Quadratus

lumborum

LI-2

358

L2-3

507

L3-4

582

L4-5

328

External oblique

L3-4

1121

110

Internal oblique

L3-4

1154

a Longissimus pars lumborum at the L4-5 level would have been listed here by virtue of their cosines, but were not, as they could not be distinguished on all scan slices.

Many anatomical textbooks describe the function of the small
rotator muscles of the spine, which attach to adjacent verte¬
brae, as creating axial twisting torque, consistent with their
nomenclature (Muscle Attachment Box 33.1). Similarly, the
intertransversarii are often assigned the role of lateral flexion
(.Muscle Attachment Box 33.2). There are several problems
with these proposals. First, these small muscles (Fig. 33.2)
have such small PCS As that they can only generate a few
newtons of force, and second, they work through such a small
moment arm that their total contribution to rotational axial
twisting and bending torque is minimal. It would appear that
they have some other function.

There is evidence to suggest that these muscles are highly
rich in muscle spindles, approximately 4.5-7.3 times richer
than the multifidus [16]. This evidence suggests that they are
involved as length transducers or vertebral position sensors at
every thoracic and lumbar joint. In some EMG experiments

MUSCLE ATTACHMENT BOX 33.1

ATTACHMENTS AND INNERVATION
OF THE ROTATORES

Inferior attachment: Superior and posterior portion
of the transverse process of one vertebra

Superior attachment: Inferior and lateral border of
the lamina of the vertebra immediately above the
inferior attachment. The rotatores lie deep to the
multifidus. The rotatores are less fully developed in
the lumbar than in the thoracic region.

Innervation: Dorsal rami of spinal nerves

Palpation: Not palpable.

performed in the authors laboratory a number of years ago,
some indwelling electrodes were placed very close to the ver¬
tebrae. In one case, there was strong suspicion that the elec¬
trode was in a rotator muscle. Isometric twisting efforts with
the spine untwisted (or constrained in a neutral posture) were
attempted in both directions but produced no EMG activity
from the rotator—only the usual activity in the abdominal
obliques, etc. However, when twisting was attempted in one
direction (with minimal muscular effort), there was no
response, while in the other direction, there was major activity.
It appeared that this particular rotator was not activated to
create axial twisting torque but acted in response to twisted
position change. Thus its activity, elicited as a function of
twisted position, was not consistent with the role of creating
torque to “twist” the spine.

MUSCLE ATTACHMENT BOX 33.2

ATTACHMENTS AND INNERVATION
OF INTERTRANSVERSARII

Inferior attachment: Transverse process of one
vertebra

Superior attachment: Transverse process of the ver¬
tebra above. In the lumbar region there are two
sets of muscles, medial and lateral, each lying poste¬
rior to the ventral ramus. The lateral lumbar portion
is further divided into ventral and dorsal sections.

Innervation: The nerve supply for the medial portion
of the muscle is the dorsal ramus of the associated
spinal nerve, while the lateral lumbar portion is inner¬
vated by the ventral ramus of the spinal nerve.

Palpation: Not palpable.

Chapter 33 I MECHANICS AND PATHOMECHANICS OF MUSCLES ACTING ON THE LUMBAR SPINE

593

Rotatores

muscles

Figure 33.2: The small rotator muscles of the lumbar spine, the
rotatores and intertransversarii, are seen crossing the joints of
the lumbar region.

Clinical Relevance

MANUAL THERAPY AND THE FUNCTION OF THE
ROTATORES AND INTERTRANSVERSARII: It is

suspected that these "muscles" are actually length transduc¬
ers, and thereby position sensors , sensing the positioning of
each spinal motion unit It is very likely that these structures
are affected during various types of manual therapy with
the joint at end range of motion.

Extensors: Longissimus, lliocostalis,
and Multifidus Groups

MUSCLE ACTION: EXTENSORS

Action

Evidence

Trunk extension

Supporting

Anterior shear support

Supporting

(longissimus and iliocostails)

MUSCLE ATTACHMENT BOX 33.3

ATTACHMENTS AND INNERVATION
OF THE LONGISSIMUS THORACIS PARS
LUMBORUM

Inferior attachment: Posterior superior iliac spine

Superior attachment: Transverse and accessory
processes of the lumbar vertebrae

Innervation: Dorsal rami of the lumbar spinal nerves

Palpation: Cannot be separately identified deep to
the thoracolumbar fascia.

The major extensors of the thoracolumbar spine are the longis¬
simus, iliocostalis, and multifidus groups (Muscle Attachment
Boxes 33.3-33.5). The longissimus and iliocostalis groups are
often separated in anatomy books, although it may be more
enlightening, in a functional context, to recognize the thoracic
portions of both of these muscles as one group and the lum¬
bar portions of these muscles as another group, since the
lumbar and thoracic portions are architecturally [3] and func¬
tionally different [12]. Bogduk [3] partitions the lumbar and
thoracic portions of these muscles into longissimus thoracis
pars lumborum and pars thoracis and iliocostalis lumborum
pars lumborum and thoracis.

MUSCLE ATTACHMENT BOX 33.4

ATTACHMENTS AND INNERVATION
OF THE ILIOCOSTALIS LUMBORUM

Iliocostalis lumborum pars thoracis:

Inferior attachment: Crest of the ilium from the
posterior superior iliac spine laterally approxi¬
mately 5 cm

Superior attachment: Angles of all 12 ribs

Palpation: Palpable with other erector spinae
along thoracic vertebrae.

Iliocostalis lumborum pars lumborum:

Inferior attachment: Iliac crest

Superior attachment: Transverse processes of the
first four lumbar vertebrae and thoracolumbar
fascia

Innervation: Dorsal rami of the thoracic and lum¬
bar spinal nerves

Palpation: Cannot be separately identified deep
to the thoracolumbar fascia.

594

Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 33.5

ATTACHMENTS AND INNERVATION
OF THE MULTIFIDUS

Inferior attachment: Posterior surface of the sacrum,
aponeurosis of the erector spinae muscles, posterior
superior iliac spine (PSIS) and sacroiliac ligaments,
and mamillary processes of the lumbar vertebrae

Superior attachment: Superficially to the third or
fourth vertebra above, intermediately to the second
or third vertebra above, and deeply to the vertebra
directly above the inferior attachment. The multi-
fidus muscles lie deep to the semispinalis and erec¬
tor spinae muscles.

Innervation: Dorsal rami of the spinal nerves
Palpation: Not palpable.

These two functional groups (pars lumborum, which attach
to lumbar vertebrae, and pars thoracis, which attach to thoracic
vertebrae) form quite a marvelous architecture for several rea¬
sons and are discussed in a functional context with this distinc¬
tion (i.e., pars lumbar vs. pars thoracic). Fiber-typing studies
note differences between the lumbar and thoracic sections, as
the thoracic sections contain approximately 75% slow twitch
fibers, while lumbar sections are generally evenly mixed [17].
The pars thoracis components of these two muscles attach to
the ribs and vertebral components and have relatively short
contractile fibers with long tendons that run parallel to the
spine to their origins over the posterior surface of the sacrum
and medial border of the iliac crests (Fig. 33.3). Furthermore,
their line of action over the lower thoracic and lumbar region is
very superficial, such that forces in these muscles have the
greatest possible moment arm and, therefore, produce the
greatest amount of extensor moment with a minimum of com¬
pressive penalty to the spine (Fig. 33.4). When seen on a trans¬
verse MRI or CT scan at a lumbar level, their tendons have the
greatest extensor moment arm, overlying the lumbar bulk—
often over 10 cm [13,14] (see Fig. 33.1).

The lumbar components of these muscles (iliocostalis lum¬
borum pars lumborum and longissimus thoracis pars lumbo¬
rum) are very different anatomically and functionally from
their thoracic namesakes. They connect to the mamillary,
accessory, and transverse processes of the lumbar vertebrae
and attach distally over the posterior sacrum and medial aspect
of the iliac crest. Each vertebra is connected bilaterally with
separate laminae of these muscles (Fig. 33.5). Their line of
action is not parallel to the compressive axis of the spine, but
rather has a posterior and caudal direction, which causes
them to generate posterior shear forces together with an
extensor moment on the superior vertebrae. These posterior
shear forces support any anterior reaction shear forces of the

Figure 33.3: A bundle of longissimus thoracis pars thoracis isolated
inserting on the ribs at T6 (probe A), with their tendons lifted by
the probes, course over the full lumbar spine to their sacral ori¬
gin (probe B). They have a very large extensor moment arm.
(Reprinted with permission from McGill SM: Biomechanics of the
thoracolumbar spine. In: Dvir Z, ed. Clinical Biomechanics. New
York: Churchill Livingstone, 2000.)

Axis of
rotation

Tendons of longissimus
thoracis pars thoracis

Figure 33.4: Tendons from the longissimus thoracis pars thoracis
have a large extensor moment arm as they cross the lumbar joints.
(Note that the actual muscle belly is in the thoracic region.) Fibers
of longissimus thoracis pars lumborum connect to each lumbar
vertebra, and these shorter muscles create extensor moments
together with posterior shear forces on the superior vertebrae.

Chapter 33 I MECHANICS AND PATHOMECHANICS OF MUSCLES ACTING ON THE LUMBAR SPINE

595

Figure 33.5: The lumbar extensor muscles, the iliocostalis lumborum
pars lumborum and longissimus thoracis pars lumborum, originate
over the posterior surface of the sacrum, follow a very superficial
pathway, and then dive obliquely to their vertebral attachments.
Thus, their line of action (A) is oblique to the compressive axis
(C), and they create posterior shear forces and extensor moments
on each successive superior vertebra. (Reprinted with permission
from McGill SM: Biomechanics of the thoracolumbar spine. In: Dvir Z,
ed. Clinical Biomechanics. New York: Churchill Livingstone, 2000.)

upper vertebrae that are produced as the upper body is flexed
forward in a typical lifting type of posture. A discussion of this
possible injury mechanism together with activation profiles
during clinically relevant activities is addressed in a following
section.

The multifidus muscles perform quite a different function,
particularly in the lumbar region where they attach posterior
spines of adjacent vertebrae or span two or three segments
(Fig. 33.6). Their line of action tends to be parallel to the com¬
pressive axis or, in some cases, runs anteriorly and caudal in an
oblique direction. But, the major mechanically relevant fea¬
ture of the multifidi is that since they span only a few joints,
their forces only affect local areas of the spine. Therefore, the
multifidus muscles are involved in producing extensor torque
(together with very small amounts of twisting and side-bending
torque) but only provide the ability for corrections or moment
support at specific joints that may be foci of stresses. An injury
mechanism involving inappropriate neural activation signals to
the multifidus is proposed in the next chapter, using an exam¬
ple of injury observed in the laboratory.

Clinical Relevance

EXERCISE FOR THE EXTENSOR MUSCLES OF THE
LOW BACK: The thoracic extensors (longissimus thoracis
pars thoracis and iliocostalis lumborum pars thoracis) that
attach in the thoracic region are the most efficient lumbar
extensors, since they have the largest moment arms as they

Figure 33.6. The multifidus consists of multiple bundles, or fasci¬
culi, that lie almost parallel to the lumbar spine, each bundle
spanning no more than a few lumbar motion segments.

course over the lumbar region. The clinical practice of "isolat¬
ing muscle groups," in this case the lumbar extensors for the
lumbar spine, needs to be revisited. Specifically, the "lumbar"
extensors located in the lumbar region only contribute a por¬
tion of the total lumbar extensor moment Training of the
lumbar extensor mechanism must involve the extensors that
attach to the thoracic vertebrae, whose bulk of contractile
fibers lie in the thoracic region but whose tendons pass over
the lumbar region and have the greatest mechanical advan¬
tage of all lumbar muscles. Thus, exercises to "isolate" the
lumbar muscles cannot be justified from either an anatomi¬
cal basis or a motor control perspective in which all "players
in the orchestra" must be challenged during training.

(continued )

596

Part III I KINESIOLOGY OF THE HEAD AND SPINE

(Continued)

Another important ciinicai issue is founded upon anatomi¬
cal features of the extensors. While the lumbar sections of the
longissimus and iliocostalis muscles that attach to the lumbar
vertebrae create extensor torquethey also produce large pos¬
terior shear forces to support the shearing loads that develop
during torso flexion postures. Some therapists
lAi unknowingly disable these shear force protectors by
having patients fully flex their spines during exercises ,
creating myoelectric quiescence in these muscles , or by recom¬
mending the subject maintain a posterior pelvic tilt during flex¬
ion activities such as lifting. Discussion of this functional
anatomy is critical for developing the strategies for injury pre¬
vention and rehabilitation and is described in the next chapter.

Abdominal Muscles

RECTUS ABDOMINIS

MUSCLE ACTION: RECTUS ABDOMINIS

Action

Evidence

Trunk flexion

Supporting

Rib depression

Supporting

While many classic anatomy texts consider the entire abdomi¬
nal wall to be an important flexor of the trunk, it appears that
the rectus abdominis is the major trunk flexor (and the most
active during sit-ups and curl-ups [4]) (.Muscle Attachment
Box 33.6). Muscle activation amplitudes obtained from both
intramuscular and surface electrodes, over a variety of tasks,
are shown in Table 33.5. It is interesting to consider why the
rectus abdominis is partitioned into sections rather than being

MUSCLE ATTACHMENT BOX 33.6

ATTACHMENTS AND INNERVATION
OF THE RECTUS ABDOMINIS

Inferior attachment: Pubic crest and adjacent
symphysis

Superior attachment: Fifth, sixth, and seventh costal
cartilages and anterior surface of the xiphoid process.
The rectus abdominis widens as it ascends and con¬
tains three transverse tendinous intersections that
adhere to the rectus sheath. One of the intersections
is at the umbilicus, one at the end of the xiphoid,
and the third one midway in between these two.

Innervation: Ventral rami of lower six or seven
thoracic spinal nerves

a single long muscle, given that the sections share a common
nerve supply and that a single long muscle would have the
advantage of broadening the force-length relationship over a
greater range of length change. Perhaps a single muscle would
bulk upon shortening, compressing the viscera, or be stiff and
resistant to bending. Not only does the “sectioned” rectus
abdominis limit bulking upon shortening, but also the sections
have a “bead effect” that allows bending at each tendon to facil¬
itate torso flexion-extension or abdominal distension or con¬
traction as the visceral contents change volume [2]).

Another clinical issue surrounds the controversy regarding
upper and lower abdominals. It appears that while the obliques
are regionally activated (and have functional separation

TABLE 33.5: Subject Averages of EMG Activation Normalized to 100% MVC—Mean and (Standard Deviation)

Abdominal Tasks

Quadratus

Lumborum

Psoas 1j

Psoas 2 ;

EOi

IOi

TAi

RAs

RFs

ESs

Straight leg situps

15(12)

24(7)

44(9)

15(15)

11(9)

48(18)

16(10)

4(3)

Bent knee situps

12(7)

17(10)

28(7)

43(12)

16(14)

10(7)

55(16)

14(7)

6(9)

Press heel situps

28(23)

34(18)

51(14)

22(14)

20(13)

51(20)

15(12)

4(3)

Bent knee curlup

11(6)

7(8)

10(14)

19(14)

14(10)

12(9)

62(22)

8(12)

6(10)

Bent knee leg raise

12(6)

24(15)

25(8)

22(7)

8(9)

7(6)

32(20)

8(5)

6(8)

Straight leg raise

9(2)

35(20)

33(8)

26(9)

9(8)

6(4)

37(24)

23(12)

7(11)

Isom, hand-to-knee LH-RK

16(16)

16(8)

68(14)

30(28)

28(19)

69(18)

8(7)

6(4)

RH-LK

56(28)

58(16)

53(12)

48(23)

44(18)

74(25)

42(29)

5(4)

Cross curlup RS-across

6(4)

5(3)

4(4)

23(20)

24(14)

20(11)

57(22)

10(19)

5(8)

LS-across

6(4)

5(3)

5(5)

24(17)

21(16)

15(13)

58(24)

12(24)

5(8)

Isom, side bridge

54(28)

21(17)

12(8)

43(13)

36(29)

39(24)

22(13)

11(11)

24(15)

Dyn. side bridge

26(18)

13(5)

44(16)

42(24)

44(33)

41(20)

9(7)

29(17)

Pushup from feet

4(1)

24(19)

12(5)

29(12)

10(14)

9(9)

29(10)

10(7)

3(4)

Pushup from knees

14(11)

10(7)

19(10)

7(9)

8(8)

19(11)

5(3)

3(4)

Note: Psoas channels, quadratus lumborum, external oblique, internal oblique, transverse abdominals are intramuscular electrodes; rectus abdominis, rectus femoris,
erector spina are surface electrodes. RH-LK, right hand-left knee; LH-RK, left hand-right knee; RS, right shoulder; LS, left shoulder.

Chapter 33 I MECHANICS AND PATHOMECHANICS OF MUSCLES ACTING ON THE LUMBAR SPINE

597

between upper and lower regions), all sections of the rectus
are activated together at similar levels during flexor torque
generation. It appears that there is not a significant functional
separation between upper and lower rectus [5] in most per¬
sons. Research reporting that there are differences in upper
and lower rectus activation sometimes suffer from the
absence of normalization of the EMG signal during process¬
ing. Briefly, raw amplitudes of myoelectric activity (in milli¬
volts) have been used to conclude that there is more, or less,
activity relative to other sections of the muscle, but the mag¬
nitudes are affected by local conductivity characteristics.
Thus, amplitudes must be normalized to a standardized
contraction and expressed as a percentage of this activity
(rather than in millivolts). Additional details regarding nor¬
malization of EMG are found in Chapter 4.

ABDOMINAL WALL

MUSCLE ACTION: EXTERNAL OBLIQUE ABDOMINAL
MUSCLE

Action

Evidence

Trunk flexion

Supporting

Contralateral trunk rotation

Supporting

Increase intraabdominal pressure

Supporting

Rib depression

Supporting

Spinal stabilization

Supporting

MUSCLE ACTION: INTERNAL OBLIQUE

Action

Evidence

Trunk flexion

Supporting

Ipsilateral trunk rotation

Supporting

Increase intraabdominal pressure

Supporting

Rib depression

Supporting

Spinal stabilization

Supporting

MUSCLE ACTION: TRANSVERSUS ABDOMINIS

Action

Evidence

Increase intraabdominal pressure

Supporting

Spinal stabilization

Supporting

The three layers of the abdominal wall (external oblique,
internal oblique, transverse abdominis) perform several func¬
tions (Muscle Attachment Boxes 33.7-33.9). The oblique
muscles are involved in flexion and appear to have the ability
to flex, enhanced by their attachment to the linea semilunaris
(Fig. 33.7 ) [9], which redirects the oblique muscle forces
down the rectus sheath to effectively increase their flexor
moment arm. Specifically, a large portion of the obliques does
not have an anterior bony attachment, but attaches to the rec¬
tus abdominis via the linea semilunaris. In this way, the rectus
abdominis actually carries some of the oblique muscle forces,
enhancing the flexor moment potential of the torso [14]. The
obliques are involved in torso twisting [7] and lateral bend¬
ing [8] and appear to play some role in lumbar stabilization,
since the obliques increase their activity, to a small degree,

MUSCLE ATTACHMENT BOX 33.7

ATTACHMENTS AND INNERVATION
OF THE EXTERNAL OBLIQUE

Inferior attachment: Anterior two thirds of the
outer lip of the iliac crest and aponeurosis

Superior attachment: Outer surfaces of lower eight
ribs, interdigitating with serratus anterior and latis-
simus dorsi. The external oblique runs in an inferior
and anterior direction and is the largest and most
superficial of the three muscles of the abdominal
wall (external oblique, internal oblique, and trans-
versus abdominis).

Innervation: Ventral rami of lower six thoracic spinal
nerves

Palpation: May be palpable in thin individuals with
well-developed muscles interdigitated with the
serratus anterior on the lateral side of the trunk.

when the spine is placed under pure axial compres-
r 54 sion [11]. This functional notion is developed in the
Xy next chapter.

The fibers of the transversus abdominis run transversely
and consequently produce little or no flexion force [15].
Rather the muscle is well aligned to contract with the oblique
abdominal muscles to increase intraabdominal pressure for
functions such as coughing, defecation, and childbirth. The
role of increased intraabdominal pressure in stabilizing the
low back is discussed in detail in Chapter 34. Isolated con¬
traction of the transversus abdominis is rare.

MUSCLE ATTACHMENT BOX 33.8

ATTACHMENTS AND INNERVATION
OF THE INTERNAL OBLIQUE

Inferior attachment: Thoracolumbar fascia, anterior
two thirds of the intermediate line of the iliac crest,
lateral two thirds of the inguinal ligament, and the
fascia on the iliopsoas muscle

Superior attachment: Inferior borders and tips of
the last three or four ribs and cartilage and the
aponeurosis. The internal oblique runs superiorly
and anteriorly and is thinner and lies beneath the
external oblique.

Innervation: Ventral rami of lower six thoracic and
first lumbar nerves

Palpation: Not palpable

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Part III I KINESIOLOGY OF THE HEAD AND SPINE

MUSCLE ATTACHMENT BOX 33.9

ATTACHMENTS AND INNERVATION
OF THE TRANSVERSUS ABDOMINIS

Superior attachment: Deep surfaces of the costal
cartilages of the lower six ribs, interdigitating with
the diaphragm, thoracolumbar fascia between iliac
crest and 12th, anterior two thirds of the inner lip
of the iliac crest, lateral one third of the inguinal
ligament, and fascia over the iliacus muscle

Inferior attachment: The pubic crest and the
aponeurosis that fuses with the posterior layers of
the aponeurosis of the internal oblique. The trans-
versus abdominis is the innermost of the three mus¬
cles of the abdominal wall.

Innervation: Ventral rami of lower six thoracic and
first lumbar spinal nerves

Palpation: Not palpable

Figure 33.7: The oblique muscles transmit force along their fiber
lengths and then redirect the force along the rectus abdominis,
via their attachment to the linea semilunaris, to enhance their
effective flexor moment arm. (Reprinted with permission from
McGill SM: J Biomech 1996; 29: 973-977.)

Clinical Relevance

ABDOMINAL MUSCLE EXERCISES: The functional divi¬
sions of the abdominal muscles justify the need for several
exercise techniques to enhance their multiple roles. While the
obliques are regionally activated , there appears to be no
functional separation of upper and lower rectus abdominis.
All parts are activated together at similar amplitudes in most
persons. This can be seen in the clinic if care is taken to nor¬
malize the channels of EMG as described briefly in Chapter 4.
Thus a curl-up exercise activates all of the rectus abdominis.
However ; upper and lower portions of the oblique abdominal
muscles are activated separately r , depending upon the
demands placed on the torso.

Special Case of the Quadratus Lumborum
and Psoas Major

While the psoas major, a muscle of the hip and lumbar spine,
has often been regarded as a good stabilizer of the lumbar
spine, it is the opinion of this author that such a role is unlikely
(see Chapter 39 for details on the psoas major). This issue is
highlighted by a comparison with the quadratus lumborum.
Like the lumbar portion of the psoas major, the quadratus
lumborum attaches to the lumbar vertebrae, but it also
exhibits several structural differences from the psoas major. It
has many more fibers cross-linking the vertebrae than does
the psoas; it has a larger lateral moment arm via the trans¬
verse process attachments; and it traverses the rib cage and
iliac crests (Muscle Attachment Box 33.10). Thus, while both

MUSCLE ATTACHMENT BOX 33.1

ATTACHMENTS AND INNERVATION
OF THE QUADRATUS LUMBORUM

Inferior attachment: Iliolumbar ligament, posterior
iliac crest, transverse processes of lower lumbar
vertebrae

Superior attachment: Medial one half of the lower
border of the 12th rib, transverse processes of
upper lumbar vertebrae and 12th thoracic vertebra.
The quadratus lumborum lies between the anterior
and middle layers of the thoracolumbar fascia, ante¬
rior to the erector spinae muscles and posterior to
the abdominal organs.

Innervation: Ventral rami of the 12th thoracic and
upper three or four lumbar spinal nerves

Palpation: Not palpable.

Chapter 33 I MECHANICS AND PATHOMECHANICS OF MUSCLES ACTING ON THE LUMBAR SPINE

599

muscles could buttress shear instability, the quadratus is more
effective in all loading modes, by design. In addition, psoas
activation profiles are not consistent with that of a spine
stabilizer [4] (see Table 33.5). These data indicate that the
role of the psoas major is primarily as a hip flexor and to pro¬
vide hip stiffness. In contrast, activation profiles support the
notion of the stabilizing role of the quadratus. It is active dur¬
ing a variety of flexion-dominant, extension-dominant, and
lateral-bending tasks of the lower back [1,11]. Further,
Andersson et al. [1] find that the quadratus lumborum does
not relax with the trunk extensors during the flexion—
relaxation phenomenon. The flexion-relaxation phenomenon
is an interesting task, since there is no substantial lateral or
twisting torque and the extensor torque appears to be sup¬
ported passively. Continued activity in the quadratus lumbo¬
rum suggests that the muscle plays a stabilizing role. An
experiment in which subjects stand upright but hold buckets
in either hand as a load is incrementally added to each bucket
reveals that the quadratus lumborum increases its activation
level (together with the obliques) as more stability is required
[11] (Fig. 33. 8). This task forms a special situation, since only

Figure 33.8: Role of active muscle to stabilize the spine. Loading the
upright vertebral column (as shown here with the person standing
while having weight loaded into baskets in each hand) requires guy
wire support from the musculature to prevent buckling. This is an
interesting test since muscles, in this posture, are recruited to pre¬
vent buckling and not to support moments. In this task, the motor
control system appears to choose the abdominal obliques and, to
some degree, the quadratus lumborum to provide stability.

compressive loading is applied to the spine in the absence of
any bending moments.

It is interesting to consider why the psoas major courses
over the lumbar spine. Why not just let the iliacus, another
hip muscle, perform the role of hip flexion? Without the
psoas major, the iliacus would rotate the pelvis upon hip
flexion, placing large bending stresses on the lumbosacral
junction (in the direction to cause excessive lordosis). The
psoas major disperses these stresses over the length of the
lumbar region.

Clinical Relevance

QUADRATUS LUMBORUM: Myoelectric evidencetogether
with anatomical analysis , suggests that the psoas major
acts primarily to flex the hip and that its activation is mini¬
mally linked to spine demands. The quadratus lumborum
appears to be involved with stabilization of the lumbar
spine with other muscles , suggesting clinical focus on the
quadratus lumborum may be warranted. Exercises empha¬
sizing activation of the quadratus lumborum are described
in the next chapter.

SUMMARY

This chapter provides an overview of the roles of the muscles
of the trunk in moving and stabilizing the lumbar spine. It
presents the posterior muscles in four large functional groups.
The deepest group appears to act as a position sensor rather
than as a generator of torque. The more superficial extensors
fall into three categories to (a) generate large extension
moments, (b) generate posterior shear, or (c) affect only one
or two lumbar segments. The roles of the abdominal muscles
in trunk flexion and in trunk stabilization are also discussed,
together with the roles of psoas and quadratus lumborum.
The abdominal muscles and quadratus lumborum appear to
play important roles in stabilizing the spine, but the psoas
major appears to be less important for lumbar stabilization. It
is clear from this discussion that many muscles play a large
role in protecting the low back from injury. Applications of
these findings to exercise regimens for individuals with low
back pain are presented in the following chapter.

Acknowledgment

The author wishes to acknowledge the contributions of several
colleagues who have contributed to the collection of works
reported here: Daniel Juker, M.D.; Craig Axler, M.Sc.; Jacek
Cholewicki, Ph.D.; Robert Norman, Ph.D.; Michael Sharratt,
Ph.D.; John Seguin, M.D.; and Vaughan Kippers, Ph.D. Also
the continual financial support from the Natural Science and
Engineering Research Council, Canada, has made this series
of work possible.

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Part III I KINESIOLOGY OF THE HEAD AND SPINE

References

1. Andersson EA, Oddsson LIE, Grundstrom H, et al.: EMG
activities of the quadratus lumborum and erector spinae muscles
during flexion-relaxation and other motor tasks. Clin Biomech
1996; 11: 392-400.

2. Belanger M: Personal communication, University of Quebec at
Montreal, 1996.

3. Bogduk N: A reappraisal of the anatomy of the human lumbar
erector spinae. J Anat 1980; 131: 525.

4. Juker D, McGill SM, Kropf P, Steffen T: Quantitative intramus¬
cular myoelectric activity of lumbar portions of psoas and the
abdominal wall during a wide variety of tasks. Med Sci Sports
Exerc 1998; 30: 301-310.

5. Lehman G, McGill SM: Quantification of the differences in
EMG magnitude between upper and lower rectus abdominis
during selected trunk exercises. Phys Ther 2001; 1096-1101.

6. Macintosh JE, Bogduk N: The morphology of the lumbar erec¬
tor spinae. Spine 1987; 12: 658.

7. McGill SM: Electromyographic activity of the abdominal and
low back musculature during the generation of isometric and
dynamic axial trunk torque: implications for lumbar mechanics.
J Orthop Res 1991; 9: 91.

8. McGill SM: A myoelectrically based dynamic 3-D model to pre¬
dict loads on lumbar spine tissues during lateral bending.
J Biomech 1992; 25: 395.

9. McGill SM: A revised anatomical model of the abdominal mus¬
culature for torso flexion efforts. J Biomech 1996; 29: 973.

10. McGill SM, Juker D, Axler CT: Correcting trunk muscle geom¬
etry obtained from MRI and CT scans of supine postures for use
in standing postures. J Biomech 1996; 29: 643-646.

11. McGill SM, Juker D, Kropf P: Quantitative intramuscular myo¬
electric activity of quadratus lumborum during a wide variety of
tasks. Clin Biomech 1996; 11: 170.

12. McGill SM, Norman RW: Effects of an anatomically detailed
erector spinae model on L4/L5 disc compression and shear.
J Biomech 1987; 20: 591.

13. McGill SM, Patt N, Norman RW: Measurement of the trunk
musculature of active males using CT scan radiography: dupli¬
cations for force and moment generating capacity about the
L4/L5 joint. J Biomech 1988; 21: 329.

14. McGill SM, Santaguida L, Stevens J: Measurement of the trunk
musculature from T6 to L5 using MRI scans of 15 young males
corrected for muscle fiber orientation. Clin Biomech 1993; 8:171.

15. McGill SM: Ultimate back fitness and performance. Backfitpro
Inc. www.backfitpro.com 2006.

16. Nitz AJ, Peck D: Comparison of muscle spindle concentrations
in large and small human epaxial muscles acting in parallel com¬
binations. Am Surg 1986; 52: 273-277.

17. Sirca A, Kostevc V: The fibre type composition of thoracic and
lumbar paravertebral muscles in man. J Anat 1985; 141: 131.

CHAPTER

Analysis of the Forces on the
Lumbar Spine during Activity

STUART M. McGILL, PH.D.

CHAPTER CONTENTS

NORMAL BIOMECHANICS AND PATHOMECHANICS OF THE LUMBAR SPINE .602

Loads on the Low Back during Lifting and Walking .602

BIOMECHANICS OF THE PASSIVE TISSUES OF THE LUMBAR SPINE .603

Functional Consideration for the Interspinous and Supraspinous Ligaments .604

Functional Consideration of the Vertebrae .608

Functional Consideration of the Intervertebral Disc .610

FUNCTIONAL CONSIDERATION FOR THE LUMBODORSAL FASCIA.610

SPINE STABILITY: MUSCLE STIFFNESS AND CO-CONTRACTION,

MOTOR CONTROL, AND THE LINK TO THE CLINIC .611

CLINICAL APPLICATION: USING BIOMECHANICS TO BUILD BETTER REHABILITATION PROGRAMS

FOR LOW BACK INJURY.612

Preventing Injury: What Does the Patient Need to Know?.612

Toward Developing Scientifically Justified Low Back Rehabilitation Exercises .612

Issues of Strength and Endurance.614

Exercises for the Abdominal Muscles (Anterior and Lateral) and Quadratus Lumborum.614

Exercises for the Back Extensors .616

Should Abdominal Belts Be Worn? .616

Beginner's Program for Stabilization.617

Notes for Exercise Prescription.617

SUMMARY .617

T his chapter examines the biomechanical evidence regarding the loads and loading mechanisms of the lumbar
spine available to date and uses it to assist clinicians in designing and prescribing better rehabilitative exer¬
cise on the basis of the best available scientific evidence. While clinicians strive to attain evidence-based prac¬
tice, too often the prescription of exercise falls short of this laudable objective.

The purpose of this chapter is to lay a scientific foundation upon which the real clinical issues pertaining to optimal
rehabilitation may be based. A description of relevant normal biomechanics of the lumbar spine, together with some
injury mechanics, is combined with the information from the previous chapter on muscle.

601

602

Part III I KINESIOLOGY OF THE HEAD AND SPINE

The specific objectives of this chapter are to

■ Discuss the forces developed in the low back tissues during selected activities

■ Review the biomechanics of the passive tissues (vertebrae, discs, ligaments, fascia)

■ Discuss the concepts of spine stability

■ Provide guidelines and caveats to assist the development of better rehabilitative exercise programs

The professional challenge for clinicians is to make wise decisions by blending laboratory evidence with clinical
experience.

NORMAL BIOMECHANICS AND
PATHOMECHANICS OF THE
LUMBAR SPINE

Loads on the Low Back during
Lifting and Walking

The common activities of lifting and walking generate forces
on the low back tissues that are introduced here to “calibrate”
the reader for the ensuing discussion. Tissue loads during lift¬
ing result from muscle and ligament tension required to sup¬
port the static posture while holding a load and to facilitate
movement. Lifting techniques modulate the distribution of
force among the tissues. Given all the possible techniques, it
is how these forces are distributed that is so important in

determining the risk of injury from excessive loading. The fol¬
lowing example demonstrates this concept.

A man is lifting 27 kg (approximately 60 lb) held in his
hands, using a squat lift style. This produces an extensor reac¬
tion moment in the low back of 450 Nm (332 ft-lb). The
forces in the various tissues that support this moment impose
a compressive load on the lumbar spine of over 7,000 N
(1,568 lb). Contributions to the total extension moment and
to the forces from the muscular components are detailed in
Table 34.1. These forces and their effects are predicted by a
sophisticated modeling approach that uses body segment dis¬
placement, spine curvature, and muscle electromyographic
(EMG) signals obtained directly from the subject. The inter¬
ested reader is urged to consult McGill [49] or Cholewicki
and McGill [17] for details. It should be noted here that 7,000
N (1,568 lb) of compression begins to cause damage in very

TABLE 34.1: Musculature Forces and Shear and Compressive Contributions to Spine Load during a Squat Lift
of 27 kg That Required a Lumbar Extensor Moment of 450 Nm

Muscle a

Force (N)

Moment (Nm)

Compression (N)

Shear(N)

Rectus abdominis

-2

External oblique 1

External oblique 2

-2

Internal oblique 1

-2

Internal oblique 2

-1

-16

Longissimus thoracis pars lumborum L4

862

744

-436

Longissimus thoracis pars lumborum L3

1514

1422

-518

Longissimus thoracis pars lumborum L2

1342

121

1342

Longissimus thoracis pars lumborum LI

1302

110

1302

lliocostalis lumborum pars thoracis

369

Longissimus thoracis pars thoracis

295

Quadratus lumborum

393

386

Latissimus dorsi L5

112

-2

Multifidus 1

136

134

Multifidus 2

226

189

124

Psoas LI

Psoas L2

Psoas L3

Psoas L4

a Muscles include both left and right sides of the body.

Note: Negative moments correspond to flexion; negative shear corresponds to L4 shearing posteriorly on L5.

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

603

weak spines, although the tolerance of the lumbar spine in an
average healthy young man probably approaches 12-15 kN
[1] (2,688-3,360 lb). In extreme cases, compressive loads on
the spines of competitive weight lifters have safely exceeded
20 kN (4,480 lb).

The individual muscle forces, their contribution to sup¬
porting the low back, and their components of compression
and shear force that are imposed on the spine are very useful
information. In this particular example, the lifter avoided full
spine flexion by flexing at the hip, minimizing ligament and
other passive tissue tension, and relegating the moment gen¬
eration responsibility to the musculature. An example in which
the spine is flexed is presented later in this chapter. As can be
seen, the pars thoracis extensors described in the preceding
chapter are very effective lumbar spine extensors, given their
large moment arm. Also, since the lifters upper body is flexed,
large reaction shear forces on the spine are produced (the rib
cage is trying to shear forward on the pelvis). These shear
forces are supported to a very large degree by the pars lum-
borum extensor muscles. Furthermore, it is clear that the
abdominal muscles are activated but produce a negligible con¬
tribution to moment-posture support. Why are they active?
These muscles are activated to stabilize the spinal column,
although this mild abdominal activity imposes a compression
penalty to the spine. A more robust explanation of stabilizing
mechanics follows in this chapter.

The preceding example demonstrates to the reader how
diffusely the forces are distributed and illustrates how proper
clinical interpretation requires anatomically detailed free-
body diagrams that represent reality (Fig. 34.1). It is the opin¬
ion of this author that oversimplified free-body diagrams have
overlooked the important mechanical compressive and, espe¬
cially, shear components of muscular force. This has compro¬
mised assessment of injury mechanisms and the formulation
of optimal therapeutic exercise.

During walking, thousands of low-level loading cycles are
endured by the spine every day. While the small loads in the
low back during walking suggest it is a safe and tolerable
activity, clinicians have found that walking provides relief to
some individuals but is painful to others. Recent work has
suggested that walking speed affects spine mechanics and
may account for these individual differences. During walking,
the compressive loads on the lumbar spine of approximately
2.5 times body weight, together with the very modest shear
forces, are well below any known in vitro failure load.

Clinical Relevance

WALKING AND LOW BACK PAIN: "Strolling" reduces spine
motion and produces static loading of tissues; whereas faster
walking, with arms swinging, causes cyclic loading of tissues
[14]. This change in motion may begin to explain the relief
walking provides some people who have low back pain. Arm

Figure 34.1: Free-body diagram of forward-bending while keep¬
ing the lumbar spine erect shows how the lumbar extensors sup¬
port the reaction shear of the upper body. Specifically, the lum¬
bar fibers of the longissimus and iliocostalis muscles (2) create
posterior shear to offset the reaction shear (Rs) of gravity on the
upper body. 1, the force from the pars thoracis muscles spanning
the lumbar segments; Rc, the compression reaction force; mg,
the superincumbent weight.

swinging, with all other factors controlled, results in lower lum¬
bar spine torques, muscle activity, and loading. Swinging the
arms may facilitate efficient storage and recovery of elastic
energy, reducing the need for concentric muscle contraction
together with reduction in upper body accelerations associated
with each step. It is interesting to note that fast walking has
been shown to be a positive cofactor in prevention of, and
more successful recovery from, low back troubles [55].

An appreciation of the magnitude and direction of loads sus¬
tained by tissues of the trunk is essential to understanding the
mechanisms of low back injury and repair. This brief discussion
of the spine and spine tissue forces will assist in building the
foundation needed for developing better clinical practice.

BIOMECHANICS OF THE PASSIVE
TISSUES OF THE LUMBAR SPINE

Interpretation of the forces described in the previous section
is limited to neutral spine postures, therefore only muscle con¬
tributions are considered. However, as the spine flexes, bends,

604

Part III I KINESIOLOGY OF THE HEAD AND SPINE

and twists, passive tissues are stressed, and their resultant
forces change the interpretation of injury exacerbation and/or
the discussion of clinical issues. For this reason, the mechan¬
ics of passive tissues is introduced below, followed by some
examples illustrating the effects on clinical mechanics.

Functional Consideration for the
Interspinous and Supraspinous
Ligaments

The interspinous and supraspinous ligaments are important
contributors to lumbar flexion mechanics. These ligaments
are often described as a single structure in anatomy texts,
although functionally they appear to have quite different
roles. The interspinous ligaments connect adjacent posterior
spines but are not oriented parallel to the compressive axis of
the spine. Rather, they have a very large angle of obliquity
(Fig. 34.2) [27], which is often shown incorrectly in anatomy
texts. Heylings [27] suggests that the interspinous ligaments
act like collateral ligaments similar to those in the knee, con¬
trolling the vertebral rotation throughout the flexion range.

Figure 34.2: The interspinous ligament runs obliquely (C) to the
compressive axis (A) and thus has limited capacity to check flex¬
ion rotation of the superior vertebral. Rather, the interspinous
may act as a collateral ligament, controlling vertebral rotation
and imposing anterior shear forces on the superior vertebrae. LI,
L2, and L3 indicate the spinous processes of the respective verte¬
brae. Interspinous ligament between LI and L2 is indicated by a,
b, and c. Anterior is to the left. (Reprinted with permission from
Heylings D: J Anat 1978; 123: 127-131.)

This control, in turn, assists the facet joints to remain in con¬
tact, gliding with rotation. Furthermore, with their oblique
lines of action, these ligaments protect against posterior
shearing of the superior vertebrae on its inferior partner. The
supraspinous ligament, on the other hand, is aligned parallel
to the compressive axis of the spine, connecting the tips of the
posterior spines. It appears to provide resistance against
excessive forward flexion.

Determining the roles of ligaments has involved qualita¬
tive interpretation using their attachments and lines of action
together with functional tests in which successive ligaments
are destroyed, and the joint motion reassessed. Early studies
to determine the relative contribution of each ligament to
restricting flexion, in particular, were performed on cadaveric
preparations that were not preconditioned prior to testing,
resulting in an abnormally large disc space. This suggests that
early data that described the relative roles of various liga¬
ments are incorrect. For example, upon death, the discs,
being hydrophilic, increase their water content and, conse¬
quently, their height. The “swollen” discs in cadaveric speci¬
mens result in an artificial preload on the ligaments closest to
the disc, causing the earlier studies to suggest that the capsu¬
lar and longitudinal ligaments are more important in resisting
flexion than is actually true in vivo. The work of Sharma et al.
[64] shows that the major ligaments for resisting flexion are
the supraspinous complex.

MECHANISMS OF INJURY

The lumbar spine is subjected to large compressive and shear
forces. However, the margin of safety is much larger in com¬
pressive loading than in shear loading since the spine can tol¬
erate well over lOkN in compression, but 1,000 N of shear
force causes injury with cyclic loading (one-time loading of
2,000 N [448 lb] is very dangerous.) As noted earlier in this
chapter, compressive loads arise from the weight of the head,
arms, and trunk and any loads being carried, but also from
contractions of the supporting trunk musculature. Although
compressive loading can produce injuries, it is likely that
more low back injuries result from shear loading.

In a previous section, lifting with the torso flexing about
the hips, rather than the spine, is analyzed. Now the exercise
is reexamined, but the lifter has elected to flex the spine suf¬
ficiently to cause the posterior ligaments to strain (Fig. 34.3).
This lifting strategy (spine flexion) has dramatic effects on
shear loading of the intervertebral column and the resultant
risk of injuiy. First, the dominant direction of the pars lum-
borum fibers of the longissimus thoracis and iliocostalis lum-
borum muscles noted in the previous chapter, causes these
muscles to produce a posterior shear force on the superior
vertebra. In contrast, with spine flexion, the interspinous lig¬
ament complex generates forces with the opposite obliquity
and, therefore, imposes an anterior shear force on the superior
vertebra (see Figs. 34.2 and 34.3). Thus, the posture, or
curvature, of the spine is important in influencing the inter¬
play between passive tissues and muscles that ultimately
modulates the risk of several types of injury.

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

605

Figure 34.3: Free-body diagram while bending forward with lum¬
bar spine flexion shows that the anterior shear forces can reach
dangerous levels from the upper body reaction (Rs), interspinous
ligaments (7 and 2), and reorientation of the lumbar extensor
fibers of longissimus and iliocostalis muscles (3), which decrease
their ability to support shear as they change their orientation
during lumbar flexion. Rc, compression reaction force; mg,
superincumbent weight.

If a subject holds a load in the hands with the spine fully
flexed, sufficient to achieve myoelectric silence in the
extensors (reducing their tension as the result of the flexion-
relaxation phenomenon) and with all joints held still so that
the low back moment remains the same, then the recruited
ligaments appear to contribute to the anterior shear force, so
that shear force levels are likely to exceed 1,000 N (224 lb).
Such large shear forces are of great concern from an injury
risk viewpoint. However, when a more neutral lordotic pos¬
ture is adopted, the extensor musculature is responsible for
creating the extensor moment and at the same time provides
a posterior shear force that supports the anterior shearing
action of gravity on the upper body and hand-held load. Thus
using muscle to support the moment in a more neutral pos¬
ture, rather than being fully flexed with ligaments supporting
the moment, greatly reduces shear loading (Table 34.2).

This example demonstrates that the spine is at a much
greater risk of sustaining shear injury (>1,000 N) (224 lb)
than compressive injury (3,000 N) (672 lb), simply because
the spine is fully flexed, or in a position at the end range of
motion (for a more comprehensive discussion see references
37, 42, and 43). As noted earlier the margin of safety is much

larger in the compressive mode than in the shear mode, since
the spine can safely tolerate well over 10 kN in compression,
but 1,000 N of shear force causes injury with cyclic loading.
This example also illustrates the need for clinicians to consider
more loading modes than simple compression. In this exam¬
ple, the real risk is anterior-posterior shear load.

Clinical Relevance

LIFTING POSTURE: Most individuals recognize that lifting
a load safely requires bending the knees. An understanding
of shear forces on the spine leads to the recognition that lift¬
ing safety is enhanced when the lifter maintains a neutral
spine. This posture allows the trunk extensors to exert a pos¬
terior pull to oppose the anterior shear forces from
the body weight and the additional shear forces
from the load.

Although lifting is a familiar mode of low back injury, falls
and other traumatic mechanisms can also produce injury.
Such injuries are characterized by their high velocities and
the resulting high rates of strain applied to the tissue.

King [29] notes that soft tissue ligamentous injuries are
much more common during high-energy traumatic events
such as automobile collisions and impact scenarios in athletics.
Our own observations on pig and human specimens loaded at
slow load rates in bending and shear forces most frequently
suggest that excessive tension in the longitudinal ligaments
results in avulsion or bony failure as the ligament pulls some
bone away from its attachment. Noyes and colleagues [54]
noted that slower strain rates (0.66%/sec) produced more lig¬
ament avulsion injuries, while faster strain rates (66%/sec)
resulted in more ligamentous failure of the fiber bundles in
the middle region of the ligament, at least in monkey knee lig¬
aments. (Chapter 6 discusses in detail the effects of loading
rate on connective tissue.)

Rissanen [60] reports that approximately 20% of cadaveric
spines possessed visibly ruptured lumbar interspinous liga¬
ments in their middle, not at their bony attachment. This
report also notes that dorsal and ventral portions of the inter¬
spinous ligaments, together with the supraspinous ligament,
remained intact.

Given the oblique fiber direction of the interspinous com¬
plex, a very likely scenario to damage this ligament would be
slipping, falling, and landing on ones behind, driving the pelvis
forward on impact and creating a posterior shearing of the
lumbar joints when the spine is fully flexed (Fig. 34.4). The
interspinous ligament is a major load-bearing tissue in this
example of high-energy loading in which anterior shear dis¬
placement is combined with full flexion. Given the available
data, it is the opinion of this author that damage to the liga¬
ments of the spine during lifting or other normal occupational

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Note: The extensor moment with full lumbar flexion is 171 Nm, producing 3,145 N of compression and 954 N of anterior shear. In the more neutral posture, an extension moment of 170 Nm produces 3,490 N of compression
and 269 N of shear.

608

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 34.4: Loads during a fall. Landing on the buttocks pushes
the pelvis anteriorly and creates a posterior shear on the lumbar
spine.

activities, particularly to the interspinous complex, is more
uncommon than common. Rather, it appears much more
likely that ligament damage occurs during a more traumatic
event, particularly landing on ones behind during a fall, and
leads to joint laxity and acceleration of subsequent arthritic
changes. As has been often said in reference to the knee joint,
“ligament damage marks the beginning of the end.”

Clinical Relevance

LOW BACK INJURIES FROM FALLS: As in lifting injuries ,
shear forces appear to be the culprit in low back injuries
from fails. Yet the direction of the damaging shear force is
in the opposite direction compared with lifting. Consequently ,
the motions that reproduce the symptoms as well as those
that reduce symptoms are likely to differ from those motions
in an individual with a lifting injury. Understanding the
mechanism of injury helps the clinician to identify strategies
to reduce pain.

Functional Consideration
of the Vertebrae

THE VERTEBRAL BODY

While many consider the vertebrae to be stiff, rigid struc¬
tures, in fact, they are not. The vertebral bodies themselves
may be likened to a barrel in which the round walls are
formed with relatively stiff cortical bone. However, the top
and bottom of the barrel are formed with a more deformable
cartilage plate (endplate), while the inside of the body is filled
with cancellous bone. The trabecular arrangement within the
cancellous bone is aligned with the trajectories of stress to

which it is exposed, dominated by compression and thus a
vertical arrangement. This is a very special architecture in
terms of how the vertebral bodies bear compressive load and
fail under excessive loading. Two major types of injury appear
to occur, endplate fracture and cancellous bone fracture with¬
in the body, both of which are discussed below.

While the walls of the vertebra appear to be rigid upon
compression, the nucleus of the disc is pressurized, as demon¬
strated by the classic work by Nachemson [50,53]. This pres¬
sure causes the cartilaginous endplates of the vertebra to bulge
inward, seemingly to compress the cancellous bone [10]. In
fact, under compression, it is the cancellous bone that fails first
[10], making it the tissue that determines the compressive
strength of the spine (at least when the spine is in a neutral
posture and not positioned at the end range of motion). It is
difficult to injure the anulus fibrosus under compressive load¬
ing. Mechanisms that lead to anular failure are discussed later
in this chapter. While this notion of compressibility of the ver¬
tebral endplate is contrary to the concept that the vertebral
bodies are rigid, the functional interpretation of this anatomy
suggests the presence of a very clever shock-absorbing and
load-bearing system. Farfan [21] proposes the notion that the
vertebral bodies act as shock absorbers of the spine, although
this theory is based on vertebral body fluid flow and not end¬
plate bulging. Since the nucleus pulposus is an incompressible
fluid, under compressive loading the vertebral endplates bulge
inward, suggesting fluid expulsion from the vertebral bodies,
specifically blood through the perivertebral sinuses [61]. This
suggests protective dissipation of stress during quasi-static and
dynamic compressive loading of the spine. The question is,
how do the endplates bulge inward into seemingly rigid can¬
cellous bone? The answer appears to be in the architecture of
the cancellous bone, which is dominated by the system of
columns of bone with much smaller transverse bony ties.
Upon axial compression, as the endplates bulge into the verte¬
bral bodies, these columns experience compression and
appear to bend in a buckling mode. Fyhrie and Schaffler [22]
demonstrate that under excessive load, these columns buckle
as the small bony transverse ties fracture (Fig. 34.5). In this
way, the cancellous bone can rebound back to its original
shape (at least 95% of the original unloaded shape) when the
load is removed, even after suffering fractures of the trans¬
verse ties. This architecture appears to afford superior elastic
deformation, even after marked damage, and allows the bone
to heal and regain its original structure and function.

Under excessive compressive loading, endplates bulge into
the vertebral bodies, causing radial stresses in the endplate
sufficient to cause fracture in a “stellate” pattern. These frac¬
tures or cracks in the endplate are sometimes large enough to
allow the nucleus pulposus to squirt through into the verte¬
bral body [53], forming Schmorls node. The classic
SchmorPs node is nuclear material found within the verte¬
bral body and surrounded by bone (Fig. 34.6). This type of
injury is associated with compression of the spine when the
spine is not at the end range of motion (i.e., neither flexed,
bent, nor twisted).

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

609

Figure 34.5: Trabecular bone fractures. Under compressive loading,
bulging of the endplate can cause buckling fractures in the vertical
trabeculae (A). These generate tensile stresses in the transverse tra¬
beculae that can produce tensile cracks (B). (Reprinted with permis¬
sion from Fyhrie DP, Schaffler MB: Failure mechanisms in human
vertebral cancellous bone. Bone 1994; 15: 105-109.)

Clinical Relevance

ENDPLATE FRACTURES: It is the opinion of this author
that endplate fractures, with the loss of nuclear fluid
through the crack into the vertebral body (often forming
Schmorl's nodes), are very common compressive injuries
and perhaps the most misdiagnosed. Loss of the nucleus
pulposus results in a flattened interdiscal space that when
seen on planar x-rays is usually diagnosed as a herniated
disc However, the anulus of the disc remains intact. It is
simply a case of the nucleus squirting through the end¬
plate crack into the cancellous core of the vertebra. True
disc herniation requires very special conditions, which are
described shortly.

POSTERIOR ELEMENTS OF THE VERTEBRA

The facet joint complex is described in Chapter 32. However,
a relevant biomechanical feature is that the neural arch made
up of the pedicles and laminae appears to be somewhat flexi¬
ble [7,20]. Failure of these elements together with facet dam¬
age leads to spondylolisthesis, an anterior displacement of
the superior vertebra on the inferior vertebra. It is often
blamed exclusively on anterior-posterior shear forces. There
is no doubt that excessive shear forces also cause injury to
these elements. Posterior shear of the superior vertebra can
lead to ligamentous damage but also to failure in the vertebra
itself as the endplate avulses from the rest of the vertebral
body, particularly in adolescent and geriatric spines. Further,

Figure 34.6: A. The stellate pattern of an endplate fracture.

B. Intrusion of nuclear material (shown at the tip of the scalpel)
into the vertebral body from compressive loading of a spine
in a neutral posture. Both photos are of porcine specimens.
(Reprinted with permission from McGill SM: Biomechanics of low
back injury: implications on current practice and the clinic. J
Biomech 1977; 30: 465-475.)

anterior shear of the superior vertebra has been documented
to cause pars and facet fracture leading to spondylolisthesis
[68], with a typical tolerance of an adult lumbar spine of
approximately 2,000 N (448 lb) [18]. This magnitude of force
may be created during a slip and fall, producing a posterior
shear, or during lifting with a fully flexed spine, producing an
anterior shear as noted in a previous section of this chapter. It
appears from both mechanical analysis and epidemiological
evidence that damage to these posterior elements may also be
associated with repeated, full range of motion, such as that
sustained by gymnasts and Australian cricket players [26].
These sorts of activities cause stress reversals in the pars with
each cycle of bending (full flexion and extension), causing
cracks to form and propagate, eventually fracturing the arch.
These fractures are examples of fatigue fractures. Thus, the
facet joint complex is susceptible to injury from activities that
produce excessive loading as well as from low load, high rep¬
etition activities.

610

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Functional Consideration
of the Intervertebral Disc

The ability of the disc to bear loads depends upon its anatom¬
ical structure together with the posture or curvature of the
spine. Twisting of the spine is a good example of this depen-
dance. As noted in Chapter 32, the collagen fibers within the
concentric rings of the anulus fibrosus are arranged with one
half of the fibers oblique to the other half (Fig. 34 . 7 ). In this
way, the anulus is able to resist twisting. However, only half of
the fibers are able to support this mode of loading, while the
other half become disabled, resulting in a substantial loss of
strength or ability to bear load with increasing twist.

From a review of the literature, one can make three gen¬
eral conclusions about anulus injuries and true disc hernia¬
tions. First, it appears that the disc must be bent to the full
end range of motion to herniate [2]. Typically, from a func¬
tional perspective, this means the spine must be at the end
range of flexion. Also, herniations tend to occur in younger
spines [3], with higher water content [4] and more-hydraulic
behavior. Older spines do not appear to exhibit “classic”
extrusion of nuclear material, but rather are characterized
by delamination of the anulus layers, and radial cracks that
progress with repeated loading. A nice review is provided by
Goel et al. [23]. Furthermore, disc herniation is associated
not only with extreme postures (either fully flexed or side-
bent), but also with repeated bending at least 20 or 30 thou¬
sand times, highlighting the role of fatigue as a mechanism
of injury [24,29]. Recent work has documented progressive
tracking of the nucleus through the posterior parts of the
anulus with continual full-flexion bending. Finally, epidemi¬
ological data link herniation with sedentary occupations and
the sitting posture [66]. In fact, Wilder et al. [67] document
anular tears in young calf spines from prolonged simulated

Figure 34.7: Collagen fibers of the anulus are arranged with one
half of the fibers being oblique to the other half so that during
twisting only half of the fibers bear load.

sitting postures and cyclic compressive loading (i.e., simu¬
lated truck driving).

Clinical Relevance

MECHANISMS OF TISSUE FAILURE: Damage to the
anulus fibrosus (herniation) appears to be associated with a
fully flexed spine. This has implications on posture correc¬
tion and exercise prescription. Prolonged sitting and abdom¬
inal exercises such as "crunches" are characterized by a fully
flexed lumbar spine. Damage to posterior bony elements of
the vertebrae appears to be associated with repeated cycles
of full flexion to full extension , such as what occurs during
gymnastic routines. Damage to ligaments is associated with
ballistic insults such as slips and falls or impacts in athletic
or other traumatic situations.

FUNCTIONAL CONSIDERATION
FOR THE LUMBOPORSAL FASCIA

Recent studies attribute various mechanical roles to the lum-
bodorsal fascia (LDF). In fact, there have been some
attempts to recommend lifting techniques based on these
hypotheses. Rut are they consistent with experimental evi¬
dence? Suggestions were originally made [25] that lateral
forces generated by the internal oblique and transverse abdo¬
minis muscles are transmitted to the LDF via their attach¬
ments to the lateral border, with claims that the fascia could
support substantial extensor moments. This lateral tension on
the LDF was hypothesized to increase longitudinal tension by
virtue of the collagen fiber obliquity in the LDF, causing the
posterior spinous processes to move together, resulting in
lumbar extension. This proposed sequence of events formed
an attractive proposition because the LDF has the largest
moment arm of all extensor tissues. As a result, any extensor
forces within the LDF would impose the smallest compres¬
sive penalty to vertebral components of the spine.

However, this hypothesis was examined by three studies,
all published about the same time, which collectively chal¬
lenge its viability: Tesh et al. [65], who performed mechanical
tests on cadaveric material; Macintosh et al. [32], who recog¬
nized the anatomical inconsistencies with the abdominal acti¬
vation; and McGill and Norman [48], who tested the viability
of LDF involvement with latissimus dorsi as well as with the
abdominals. These collective works show that the LDF is not
a significant active extensor of the spine. Nonetheless, the
LDF is a strong tissue with a well-developed lattice of colla¬
gen fibers, suggesting that its function may be that of an
extensor muscle retinaculum [8] or natures abdominal belt.
The tendons of longissimus thoracis and iliocostalis lumbo-
rum pass under the LDF to their sacral and iliac attachments.
It appears that the LDF may provide a form of “strapping” for
the low back musculature.

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

611

SPINE STABILITY: MUSCLE STIFFNESS
AND CO-CONTRACTION, MOTOR
CONTROL, AND THE LINK TO THE CLINIC

The concept of stability is being used in the clinic to enhance
rehabilitation outcomes and justify better injury prevention
strategies. In fact, “stability” is the foundation for the current
paradigm shift now occurring in rehabilitation. An earlier sec¬
tion of this chapter documents abdominal wall activity during
a lift. Why does the motor control system expend energy in
this way?

It is clear that abdominal activity during lifting is counter¬
productive for producing an extensor moment that is needed
to support the lifting posture. Consider that a spine without
muscular support fails under compressive loading in a buck¬
ling mode, at about 20 N (5 lb) [30]. In other words, a bare
spine is unable to bear compressive load! The spine can be
likened to a flexible rod that buckles under compressive load¬
ing. However, if the rod has guy wires connected to it, like the
rigging on a ship s mast, more compression is ultimately expe¬
rienced by the rod, but the rod is able to bear much more
compressive load as it is stiffened and becomes more resistant
to buckling (Fig. 34 . 8 ). The co-contracting musculature of the
lumbar spine performs the role of stabilizing guy wires to
each lumbar vertebra of the flexible column, bracing the
spine against buckling.

Understanding stability from a clinical perspective
requires several steps. First, there is a critical link between
muscle activation and stiffness. Activating a muscle increases
stiffness of both the muscle and the joint(s) [16]. Activating a
group of muscle synergists and antagonists in the optimal way
now becomes a critical issue. From a motor control point of
view, the analogy of an orchestra is useful. The orchestra must
play together, or in clinical terms, the full complement of the
stabilizing musculature must work together to achieve stabil¬
ity. One instrument out of tune ruins the sound. One muscle
with inappropriate activation or force-stiffness can produce
instability or at least unstable behavior will result at lower
externally applied loads.

It has been claimed for many years that intraabdominal
pressure (IAP) plays an important role in support of the lum¬
bar spine, especially during strenuous lifting. Although it was
thought that IAP directly reduced compressive loads on the
spine, it was found that the abdominal muscle activity needed
to create higher IAP actually increased spine compression
[47,52]. Despite adding additional compressive force to the
lumbar spine, IAP through contraction of the abdominal
muscles appears to stabilize the spine. The mechanism of this
increased stability remains controversial. Some suggest that
IAP produces an extensor moment that assists the erector
spinae in supporting the spine [19]. Others suggest that the
abdominal muscles with other trunk muscles serve to stiffen
the spine, effectively creating a flexible corset or air splint

Figure 34.8: Co-contracting muscles stabilize the spine to prevent buckling. A. Paraspinal muscles stiffen and stabilize the vertebrae
directly (a few are seen). B. The abdominal wall stabilizes the spine by its attachment to the rib cage and pelvis. Buckling can occur
when one or more muscles have an inappropriate amount of stiffness, determined by the activation level of the muscles.

612

Part III I KINESIOLOGY OF THE HEAD AND SPINE

around the spine [16,47]. Regardless of the mechanism, sta¬
bility of the spine is the result.

Clinicians are very aware of patients who co-contract their
torso muscles to stabilize a joint. This type of behavior makes
sense, and in fact, it is the only way to stabilize a joint actively.
However, the clinical question then becomes how much sta¬
bility is necessary? The concept of “sufficient stability” is
essential for clinicians to consider.

For a joint to bear larger loads, more stability is required.
In most activities only a modest amount of stability is required.
Too much stiffness from muscle activation imposes a severe
load penalty by increasing joint compression forces on the
joint. Excessive stiffness also impedes the joints motion. In
normal joints, with fit motor control systems, appropriate sta¬
bility is achieved. In addition to muscular sources of joint stiff¬
ness, individual joints have passive stiffness. Stiffness is
defined as the ratio between the force applied to an object and
the objects resulting change in shape (Chapter 2). Following
injury, passive tissue stiffness is reduced. Also, reports docu¬
ment that the motor system is altered, resulting in inappropri¬
ate muscle activation sequences. The biomechanists contribu¬
tion is to quantify the loss of passive stiffness and determine
how much muscular stiffness is necessary for stability. Once
this amount of stability is determined, the clinician will then
wish to add a modest amount of extra stability to form a mar¬
gin of safety. This is known as “sufficient stability.”

The stability concept is revolutionizing rehabilitation. The
biomechanists are beginning to provide clinicians with specific
target levels of muscle activation to achieve sufficient stability.
Interestingly enough, large muscular forces are rarely
required. Instead, low levels of muscular co-contraction are
required for sufficient stability in almost all tasks. This means
that a patient must be able to maintain sufficient stability get¬
ting on and off the toilet, in and out of the car, up and down¬
stairs, etc. This argument suggests that the margin of safety
when performing tasks, particularly the tasks of daily living, is
not compromised by insufficient strength but rather by insuf¬
ficient muscular endurance or muscle coordination. We are
beginning to understand the mechanistic pathway of those
studies showing the efficacy of endurance training, rather
than strength of the muscles that stabilize the spine. Having
strong abdominal muscles does not provide the prophylactic
effect that had been hoped for. However, recent work sug¬
gests that muscles with good endurance reduce the risk of
future back troubles [31].

Clinical Relevance

JOINT STABILITY AND CLINICAL PRACTICE: Stiffness
and stability of a spinal motion segment come from both
muscle contraction and the inherent passive stiffness of the
joint. Clinicians who practice manual therapy attempt to
identify spinal segments that are not moving correctly or are
"blocked" or "stiff." Recalling that the definition of stiffness

implies that a "stiff" joint requires increased force to move it,
a stiff joint actually is a more stable joint and requires a
very large perturbation to become unstable. In contrast, the
clinical expression "stiff joint" usually means that the joint
lacks range of motion. However, the joint that lacks mobility
often is supported by weaker tissue and is more susceptible
to injury from high loads. (Chapter 6 describes the effects of
immobilization on connective tissue.)

A common goal of therapy is to restore normal motion, but
more motion requires more stability. Clinicians should give due
consideration to enhancing spinal stability from muscular
sources following mobilization therapy. Furthermore, there
may be a peril in mobilization producing too much motion,
increasing the importance of specific training for muscular
endurance and motor control to enhance spine stability [41].

CLINICAL APPLICATION: USING
BIOMECHANICS TO BUILD BETTER
REHABILITATION PROGRAMS FOR
LOW BACK INJURY

Reducing the pain and improving function for patients with
low back pain involves two components: removing the stres¬
sors that create or exacerbate damage and enhancing activi¬
ties that build healthy supportive tissues. This section begins
with a brief listing of considerations for prophylaxis and then
focuses on issues relevant to wise exercise prescription.

Preventing Injury: What Does
the Patient Need to Know?

A few universal guidelines can be based on the foundation
laid in this and the previous chapters. Perhaps the single most
important guideline should be “Don’t do too much of any one
thing.” Either too much loading or too little is detrimental.
Other guidelines include (a) avoid repeated or prolonged
end-range lumbar motion that puts the disc at risk; (b) design
work to vary positions so that loads are rotated among the var¬
ious supporting tissues to minimize the risk of accumulated
tissue deformation; (c) allow time for tissues to restore their
unloaded rest geometry following the application of pro¬
longed loads when creep has occurred prior to performing
demanding tasks (such as in prolonged stooping); (d) don’t sit
too long (how long depends on the patient’s history and sta¬
tus); (e) keep the loads close to the low back. A much more
developed list may be found in McGill [46].

Toward Developing Scientifically Justified
Low Back Rehabilitation Exercises

The “art” of rehabilitation is to find the optimal physical
challenge—not too much and not too little. The “science” of
rehabilitation provides the foundation to find the optimum.

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

613

While it is outside the scope of this chapter, the interested
reader is urged to consult the literature for a description of
the scientific methods used to develop the following pro¬
gram [17,28,36,49].

A lot of the notions that clinicians consider principles for
exercise prescription may not be as well supported by data as
one might think. For example, most individuals are instructed
to perform sit-ups with bent knees. Why? Similarly, many cli¬
nicians emphasize performing a pelvic tilt when performing
many types of low back exercises. What is the scientific evi¬
dence for such recommendations? An examination of the lit¬
erature reveals that the scientific foundation upon which
many exercise notions are based is extremely thin.

Clinical Relevance

BENT-KNEE SIT-UPS: Several hypotheses to justify bent-
knee sit-ups have suggested that this disables the psoas
major and/or changes its line of action. Recent magnetic
resonance imagery (MRI)-based data [63] demonstrate that
the psoas major's line of action does not change because of
lumbar or hip posture (except at L5-S1), since the psoas
laminae attach to each vertebra and "follow" the changing
orientation of the spine. However ; there is no doubt that the
psoas major is shortened with the flexed hip, which decreases
its force production. But the question remains, is there a
reduction in spine load with the legs bent? McGill [38] found
no major difference in lumbar load as the result of bending
the knees with average moments of 65 Nm in both straight
and bent knees in 72 young men. The reported compression
loads are 3,230 N (723 lb) with straight legs and 3,410 N
(763 lb) with bent knees. Reported shear forces are 260 N
(58 lb) with straight legs and 300 N (67 lb) with bent knees.
Compressive loads in excess of 3,000 N (672 lb) certainly
raise questions of safety in both exercises.

This type of quantitative analysis is necessary to demon¬
strate that the issue of performing sit-ups using bent knees
or straight legs is probably not as important as the issue of
whether to prescribe sit-ups at all! There are better ways to
challenge the abdominal muscles. Furthermore, certain
types of low back injuries are characterized by very specific
tissue damage that may require quite different exercise reha¬
bilitation programs for different people. For example,
because flexion is a potent way to herniate the anulus the
individual with a posterior disc herniation would do well to
avoid full-spine flexion maneuvers, particularly with con¬
comitant muscle activity causing significant compressive
loading. Yet this spine posture is often unknowingly adopted
by patients or consciously advocated by clinicians who
demand a full pelvic tilt.

Several exercises are required to train all the muscles of the
lumbar torso, and the exercises that best suit the individual
depend on a number of variables, such as fitness level, training

goals, history of previous spinal injury, and other factors spe¬
cific to the individual. However, depending on the purpose of
the exercise program, several principles apply. For example,
an individual beginning a postinjury program is better advised
to avoid loading the spine throughout the range of motion,
while a trained athlete may indeed achieve higher perform¬
ance levels by doing so. Selection of the exercises described
in this chapter is biased toward safety, minimizing spine load¬
ing during muscle challenge. Therefore, a “neutral” spine
(neutral lordosis) is emphasized while the spine is under load;
that is, the spine is in neither hyperlordotic nor hypolordotic
posture. A general rule of thumb is to preserve the normal
low back curve similar to that of standing upright or some
variation that minimizes pain. The neutral spine is neither
flexed nor extended, but is in a position of elastic equilibrium
in which the passive tissues are in the least stressed confor¬
mation. Rotating the vertebrae from this neutral posture
increases the loading on the spine. Thus, performing a pelvic
tilt increases the stress within the spinal tissues and is not in
the best interest of minimizing loads during activities such as
exercise that places additional loads on the spine. A final
caveat for those in pain is to let pain guide small modifications
to the initial position of elastic equilibrium, allowing the pain-
free position to serve as their neutral spine. In the past, per¬
forming a pelvic tilt when exercising has been recommended.
However, it should be clear to the reader that this is not jus¬
tified, because the pelvic tilt increases spine tissue loading,
since the spine is no longer in static-elastic equilibrium. It
appears to be unwise to recommend the pelvic tilt when chal¬
lenging the spine.

ISSUES OF FLEXIBILITY

Training to optimize spine flexibility depends on the person s
injury history and exercise goal. There are two opposing con¬
siderations for the clinicians. First, training for flexibility can
lead to exacerbation of troubles, yet having spinal mobility
enables spine motion with lower stresses from passive tissues
whose role is to define the end-range. However, it is the opin¬
ion of this author that training for spine flexibility is overem¬
phasized. Generally, for the injured back, spine flexibility
should not be emphasized until the spine has stabilized and
has undergone strength and endurance conditioning. Some
individuals may never reach this stage! Despite the notion
held by some, there are few quantitative data to support a
major emphasis on trunk flexibility to improve back health
and lessen the risk of injury. In fact, some exercise programs
that have included loading of the torso throughout the range
of motion (in flexion-extension, lateral bend, or axial twist)
have had negative results [33,51]. In addition, greater spine
mobility has been, in some cases, associated with low back
trouble [11,51]. Further, having spine flexibility has been
shown to have little predictive value for future low back trou¬
ble [6,51]. The most successful programs appear to empha¬
size trunk stabilization through exercise with a neutral spine
[62] but emphasize mobility at the hips and knees. Bridger
et al. [9] demonstrate advantages for hip and knee flexibility

614

Part III I KINESIOLOGY OF THE HEAD AND SPINE

in sitting and standing, while McGill and Norman [49] outline
advantages for lifting.

For these reasons, specific torso flexibility exercises should
be limited to unloaded flexion and extension for those con¬
cerned with safety, but perhaps not those interested in specific
athletic performance. (Of course spinal flexibility may be of
greater desirability in athletes who have never suffered back
injury). A very conservative method is to have the patient cycle
through full flexion and extension in a slow, smooth motion,
while in a hands and knees posture (Fig. 34.9).

Issues of Strength and Endurance

The link between previous back injuries resulting in lower
muscle strength and endurance performance is well docu¬
mented. However, does less strength cause injury? The few

Figure 34.9: The flexion-extension exercise is performed by slowly
cycling through full spine flexion (A) to full extension (B). Spine
mobility is emphasized rather than "pressing" at the end range
of motion. This exercise provides motion for the spine with very
low loading of the intervertebral joints and reduces viscous
stresses for subsequent exercise.

studies available suggest that endurance has a much greater
prophylactic value than strength [31]. Furthermore, it appears
that emphasis placed on endurance should precede specific
strengthening exercise in a graduated progressive exercise
program (i.e., longer-duration, lower-effort exercises).

AEROBIC EXERCISE

The mounting evidence supporting the role of aerobic exer¬
cise in both reducing the incidence of low back injury [12]
and in treating patients with low back pain is compelling [55].
Recent investigation into the loads sustained by the low back
tissues during walking [14] confirm very low levels of sup¬
porting passive tissue load coupled with mild, but prolonged,
and beneficial activation of the supporting musculature.

Exercises for the Abdominal Muscles
(Anterior and Lateral) and Quadratus
Lumborum

The role of the abdominal muscles in stabilizing the low back is
discussed earlier in this chapter. The question remains, what is
the best way to train these muscles to perform their stabilizing
role? It is important to clarify first that all of the muscles of the
abdominal wall participate in stabilization [15,56,57]. Studies of
the transversus abdominis demonstrate its participation in sta¬
bilization, but clinicians are cautioned from attributing exclu¬
sive or unique roles to this muscle. Thus, exercises are needed
that elicit activity from each muscle of the abdominal wall to
educate individuals to utilize their stabilizing contributions.
Unfortunately, there is no single abdominal exercise that chal¬
lenges all of the abdominal musculature. Consequently, clini¬
cians must prescribe more than one exercise.

Gentle exercises to activate the abdominal wall have been
variously described as “bracing,” “hollowing,” and “pulling in.”
There appears to be significant confusion about the names and
form of these exercises. These exercises have come to mean
different things to different people. For the purposes of this
discussion, the following operational definitions are used:

Bracing: isometric contraction of the abdominal wall
resulting in IAP

Hollowing: visible hollowing of the anterior abdominal
wall with elevation (flaring) of the lower ribs

Pulling-in: concentric contraction of the abdominal wall
accompanied by a flattening of the abdomen and
depression of the lower ribs

Clinical Relevance

EXERCISES FOR MUSCLES OF THE ABDOMINAL
WALL: Contraction of the muscles of the abdominal wall
the internal and external obliques and the transversus
abdominis, contribute to spinal stability. Exercises to teach

(< continued )

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

615

(Continued)

an individual to increase motor control and endurance of
these muscles are important for prevention and rehabilita¬
tion of low back pain. However ; it is essential that the clini¬
cian teach the patient to perform the correct exercise to
recruit the intended muscles. Contraction of these
muscles can be verified by palpation for a stiffening
of the abdominal wall particularly laterally.

Calibrated intramuscular and surface EMG evidence
[28,38] suggests that the various types of curl-ups challenge
mainly the rectus abdominis muscles, with little activity in
the psoas major, internal and external obliques, and trans¬
verse abdominis. Sit-ups (both straight-leg and bent-knee)
are characterized by higher psoas major activation and higher
low back compression, while leg raises cause even higher
activation of the psoas major and also spine compression.

Several relevant observations are made regarding abdom¬
inal exercises in these investigations. The challenge to the
psoas major is lowest during curl-ups (Fig. 34.10), followed by

Figure 34.10: A. The curl-up is performed by lifting the head and
shoulders off the ground with the hands under the lumbar
region to help stabilize the pelvis and support the neutral spine.
B. A variation is to bend only one leg; the straight leg assists in
pelvic stabilization and preservation of a "neutral" lumbar curve.

Figure 34.11: The horizontal isometric side bridge or side bridge.
Supporting the lower body with the knees on the floor reduces
the demand further for those who are more concerned with
safety. Supporting the body with the feet increases the muscle
challenge, but also the spine load. Progression of the challenge
is indicated with the lowest in (A) and highest in (B).

higher levels during the horizontal isometric side support
(Fig. 34.11). Bent knee sit-ups are characterized by larger
psoas major activation than straight leg sit-ups, and the high¬
est psoas major activity is observed during leg raises and
hand-on-knee flexor isometric exertions. It is interesting to
note that the “press-heels” sit-up that has been hypothesized
to activate hamstrings and inhibit psoas major, actually
increases psoas major activation. (See normalized EMG data
in Table 33.5, Chapter 33.) One exercise not often performed,
but that appears to have merit, is the horizontal side
bridge. It challenges the lateral obliques without high
lumbar compressive loading [5]. In addition, this exer¬
cise produces high activation levels in the quadratus lumbo-
rum, which appears to be a significant stabilizer of the spine
[44], as noted in the previous chapter.

Graded activity in the rectus abdominis muscle and each
of the components of the abdominal wall changes with each
of these exercises, demonstrating that there is no single best
task for the collective “abdominals.” Clearly, curl-ups excel at
activating the rectus abdominis but produce relatively low
oblique activity. Several other clinically relevant findings from
Table 33.5 include notions that the psoas major activation is
dominated by hip flexion demands. Psoas major activation
is relatively high (greater than 25% maximum voluntary

616

Part III I KINESIOLOGY OF THE HEAD AND SPINE

contraction, MVC) during pushups, suggesting cautious con¬
cern for an individual with an injured low back. Psoas activity
is not linked with either lumbar sagittal plane moment or
spine compression demands. Thus the often-cited notion that
the psoas major is a lumbar spine stabilizer is questionable.
Quadratus lumborum activity appears consistent with sagittal
and lateral lumbar moments and compression demands, sug¬
gesting a larger role in stabilization.

A very wise choice for abdominal exercises, in the early
stages of training or rehabilitation, consists of several variations
of curl-ups for rectus abdominis and isometric horizontal side
support, with the body supported by the knees and the upper
body supported by one elbow on the floor. These exercises
challenge the abdominal wall in a way that imposes minimal
compressive penalty to the spine. The level of challenge with
the isometric horizontal side support can be increased by sup¬
porting the body with the feet rather than the knees. Specific
recommended abdominal exercises are shown: the curl-up
with the hands in the low back to stabilize the pelvis and sup¬
port a neutral lumbar spine together with one hip flexed to
assist in “locking the pelvis” to prevent rotation (Fig. 34.9)
and the horizontal isometric side support again with the spine
in a neutral posture using either the knees or feet for support
(Fig. 34.11) P

Exercises for the Back Extensors

The search for methods to activate the extensors with mini¬
mal spine loading [13] is difficult, since most traditional
extensor exercises are characterized by high spine loads that
result from externally applied compressive and shear forces.
It appears that the single leg extension hold, while on the
hands and knees (Fig. 34.12) minimizes external loads on the
spine but produces an extensor moment on the spine (and
small isometric twisting moments) that activates the extensors
(one side of lumbar approximately 18% MVC). Activation is
sufficiently high on one side of the extensors to facilitate train¬
ing, but the total spine load is reduced, since the contralater¬
al extensors are producing lower forces (lumbar compression
is less than 2,500 N (560 lb). Switching legs trains both sides
of the extensors.

In total, seven tasks have been analyzed to facilitate com¬
parison of various extensor tasks [13]. Simultaneous leg
extension with contralateral arm raise (the “birddog”)
increases the unilateral extensor muscle challenge (approxi¬
mately 27% MVC on one side of the lumbar extensors and
45% MVC on the other side of the thoracic extensors).
However, this exercise also increases lumbar compression to
well over 3,000 N (672 lb). The often-performed exercise of
lying prone on the floor and raising the upper body and legs
off the floor is contraindicated for anyone at risk of low back
injury or reinjury. In this task the lumbar region pays a high
compression penalty to a hyperextended spine (approximately
4,000 N (896 lb) or higher), which transfers load to the facets
and crushes the interspinous ligament, noted earlier as an
injury mechanism.

Figure 34.12: Extensor muscle exercises. (A) Single leg
extension holds, while on the hands and knees, produces
mild extensor activity and relatively low spine compression
(<2,500 N; 560 lb). (B) Raising the contralateral arm increases
extensor muscle activity but also spine compression to levels
exceeding 3,000 N.

Should Abdominal Belts Be Worn?

The average patient must be confused when observing both
Olympic athletes and those with back injuries wearing
abdominal belts. The following results are summarized from
a review of the effects of belt wearing [37]:

• Those who have never had a previous back injury appear
to have no additional protective benefit from wearing a
belt.

• Those who have had an injury while wearing a belt appear
to risk a more severe injury. Belts appear to give people the
perception they can lift more and may in fact enable them
to lift more. Belts appear to increase intraabdominal pres¬
sure and blood pressure.

• Belts appear to change the lifting styles of some people to
either decrease the loads on the spine or increase the loads
on the spine.

In summary, given the assets and liabilities to belt wear¬
ing, they are not recommended for routine exercise
participation.

Chapter 34 I ANALYSIS OF THE FORCES ON THE LUMBAR SPINE DURING ACTIVITY

617

Beginner s Program for Stabilization

Specific recommended low back exercises have been shown.
The following is an example of an exercise program based on
the scientific data reported in this chapter. This program
often forms a core set to which additional exercises can be
added as patients progress. During the typical rehabilitation
program the patient will experience setbacks. When these
occur, the patient should return to these core exercises,
reestablish slow improvement, and then build the program
once again. The four core exercises are:

• Flexion-extension cycles (Fig. 34.9) to reduce spine vis¬
cosity, followed by hip and knee mobility exercises [38].
Five or six cycles often suffice to reduce most viscous
stresses.

• Anterior abdominal exercises, in this case the curl-up with
the hands under the lumbar spine to preserve a neutral
spine posture (Fig. 34.10) and one knee flexed but with
the other leg straight to stabilize the pelvis on the lumbar
spine.

• Lateral musculature exercises are performed—namely, iso¬
metric side support, or side bridge, for quadratus lumborum
and the obliques of the abdominal wall for optimal stability
(Fig. 34.11). The upper leg-foot is placed in front of the
lower leg-foot to facilitate longitudinal “rolling” of the torso
to challenge both anterior and posterior portions of the wall.

• The extensor program consists of leg extensions and the
“birddog” exercises (Fig. 34.12).

“Normal” ratios of endurance times are reported for the torso
flexors relative to the extensors (0.99 for men, 0.79 for
women) and for the lateral musculature relative to the exten¬
sors (0.65 for men and 0.39 for women) [43] to help clinicians
identify endurance deficits in specific muscle groups. Finally,
as patients progress with these isometric stabilization exercises,
conscious simultaneous contraction of the abdominals is rec¬
ommended to enhance motor control and stability using the
deeper abdominal wall that includes the transverse abdomin¬
is and internal oblique [49,59].

Notes for Exercise Prescription

The exercise professional must design exercise programs to
meet a wide variety of objectives. The following is a list of gen¬
eral caveats to assist in achieving the best prescription [42].

1. While there is a common belief among some “experts” that
exercise sessions should be performed at least three times
per week, it appears that low back exercises have the most
beneficial effect when performed daily [35].

2. The “no pain-no gain” axiom does not apply when exer¬
cising the low back, particularly when applied to weight
training. Scientific and clinical wisdom suggests the oppo¬
site is true.

3. While specific low back exercises have been rationalized in
this chapter, general exercise programs that also combine
cardiovascular components (e.g., walking) have been

shown to be more effective in both rehabilitation and
injury prevention [55]. The exercises shown here only
make up a component of the total program.

4. Diurnal variation in the fluid level of the intervertebral
discs (discs are more hydrated early in the morning after
rising from bed) changes the stresses on the disc through¬
out the day. It is unwise to perform full range spine motion
while under load, shortly after rising from bed [1].

5. Low back exercises performed for maintenance of health
need not emphasize strength, with high-load low repeti¬
tion tasks. Instead, more repetitions of less-demanding
exercises assist in the enhancement of endurance and
strength. There is no doubt that back injury can occur dur¬
ing seemingly low level demands such as picking up a pen¬
cil and that the risk of injury from motor control error can
occur. While it appears that the chance of motor control
errors resulting in inappropriate muscle forces increases
with fatigue, there is also evidence documenting the
changes in passive tissue loading with fatigue lifting [58].
Given that endurance has more protective value than
strength [31], strength gains should not be overempha¬
sized at the expense of endurance.

6. There is no such thing as an ideal set of exercises for all
individuals. An individuals training objectives must be
identified (e.g., to reduce the risk of injury, optimize gen¬
eral health and fitness, or maximize athletic performance)
and the most appropriate exercises chosen. While science
cannot evaluate the optimal exercises for each situation,
the combination of science and clinical experiential “wis¬
dom” must be used to enhance low back health.

7. Be patient and stick with the program. Increased function
and pain reduction may not occur for 3 months [34].

SUMMARY

This chapter reviews the basic factors that explain the loads
sustained by the lumbar spine during activity, particularly lift¬
ing. In addition, this chapter describes the loading patterns on
the passive structures of the lumbar spine during activity as
well as the tissues’ response to loading. The position of the
spine affects the direction of the loads on the spine during
activity. Lifting with the lumbar spine extended tends to
increase the compressive loads on the spine, while lifting with
the trunk flexed increases shear forces on the spine. The lum¬
bar spine tolerates larger compressive forces than shear
forces, so strategies to reduce shear forces are discussed.

The information presented in Chapter 33 regarding the
muscles of the lumbar spine and the information from the
current chapter are applied to the clinical issues surrounding
exercise for people with and without a history of low back
pain. The best available biomechanical studies are used to
generate a list of guidelines for the clinician to follow when
developing individual exercise programs. Thus this chapter
provides convincing evidence of the clinical benefit of the
application of biomechanical analysis to clinical dilemmas.

618

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Acknowledgments

The author wishes to acknowledge the contributions of several
colleagues who have contributed to the collection of works
reported here: Daniel Juker, M.D.; Craig Axler, M.Sc.; Jacek
Cholewicki, Ph.D.; Michael Sharratt, Ph.D.; John Seguin,
M.D.; Vaughan Kippers, Ph.D.; and, in particular, Robert
Norman, Ph.D. The continual financial support from the
Natural Science and Engineering Research Council, Canada,
has made this series of work possible.

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CHAPTER

Structure and Function of the
Bones and Joints of the Pelvis

EMILY L. CHRISTIAN , P.T ., PH.D.

CHAPTER CONTENTS

OSTEOLOGY OF PELVIC AND ASSOCIATED STRUCTURES .622

Lumbar Spine and L5 Vertebra .622

Sacrum.624

Coccyx .626

Innominate Bone.626

Sexual Differences.633

PELVIC JOINTS AND PERIARTICULAR STRUCTURES .636

Lumbosacral Junction.637

Sacrococcygeal Junction.639

Sacroiliac Joint (SIJ).640

Symphysis Pubis.647

Pathology or Functional Adaptation?.648

SUMMARY .649

T he bones of the pelvic girdle consist of two innominate bones (os coxae, or hip bones) formed by the fusion
of three bones, the ilium, ischium, and pubis. In contrast to those of the upper limb, the girdle bones of the
lower limb are designed for stability, not mobility (Table 35.1). The two innominate bones join to the sacrum
dorsally and to one another in the anterior midline to form a robust osteoligamentous ring, the pelvis (Fig. 35.1).
The pelvis joins with the fifth lumbar vertebra above at the lumbosacral junction and below to two femurs (femora)
at the hip joints (Fig. 35.2).

The function of the bony pelvis is primarily locomotor and therefore somatic (i.e., pertaining to the limbs and body
wall). Serving in this capacity, the bony pelvis provides the attachment sites for trunk and lower limb muscles, trans¬
mits the superincumbent body weight to the lower limbs (in standing) or ischia (in sitting), and absorbs ground reac¬
tion forces in all standing and sitting activities. The bony pelvis functions in a visceral capacity in addition to its somatic
one, since several visceral tracts (the pelvic effluents) pass through its caudal end, involving it in micturition, defeca¬
tion, and, in females, sexual function and childbirth. The purposes of these three chapters on the bony pelvis are to
provide an understanding of how specific design features enable it to perform these seemingly divergent functions,
to describe changes that occur in pelvic structures over the life span that can have a deleterious effect on function,
and to analyze the loads sustained by the pelvis. The specific objectives of this chapter are to

620

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

621

TABLE 35.1: Comparison of Osteological Features of the Limb Girdles

Upper Limb (Pectoral) Girdle

Lower Limb (Pelvic) Girdle

Bones form by both intramembranous and endochondral methods
of bone formation

Bones form only by the endochondral method of bone formation

Formed from two elements on each side: clavicle and scapula

Formed from three elements on each side: ilium, ischium,
and pubis

The two elements on each side are distinct from each other

The three elements of each side fuse to form two innominate
bones

No direct ventral articulation between the two sides

The two innominate bones articulate ventrally at the symphysis
pubis

No dorsal articulation

Each innominate bone articulates dorsally with the sacrum

Articulations with the axial skeleton are ventral, small, and highly
mobile

Articulations with the axial skeleton are dorsal, robust, and nearly
immobile

Joint with proximal member of upper limb is relatively shallow
and permits a wide range of motion

Joint with proximal member of lower limb is deeper than that
of upper limb and permits a more limited range of motion

Designed for mobility; resilient to mechanical forces

Designed for stability; transmits mechanical forces between spine
and lower limb

■ Describe the bony features of the fifth lumbar vertebra, sacrum, coccyx, and innominate bones to
demonstrate how these features allow the pelvis to provide a stable support for the superincumbent
body weight and a sturdy base from which movement of the lower limbs is accomplished

■ Discuss the structure and ligamentous support of the lumbosacral articulation to understand its contribu¬
tion to the transfer of weight to the sacrum and how pathology contributes to its dysfunction

■ Discuss the structure, ligaments, and function of each of the articulations between the sacrum and
innominate bones and between the two innominate bones and to identify how each ensures stability
while permitting movement between specific skeletal elements

■ Describe structural alterations in pelvic articulations over time and in subpopulations and the effects
these changes impose on pelvic somatic function

■ Identify the amount of motion available between the axial and appendicular elements of the bony pelvis
as well as between the hemipelves and to discuss the sequelae of alterations in available motion

Figure 35.1: Anterior view of the osteoligamentous ring that forms
the pelvis. Elements of the ring include the sacrum and innominate
bones (os coxae). They are joined posteriorly by two sacroiliac joints
and anteriorly at the symphysis pubis. The three bones that fuse to
form each innominate bone are indicated.

Innominate bone

Sacroiliac joint (os coxae)

622

Part Ml

KINESIOLOGY OF THE HEAD AND SPINE

■ Describe the alignment of the axial and appendicular elements of the bony pelvis to gain an understand¬
ing of how alterations in the normal alignment can result in impaired function and the imposition of
potentially harmful loads on adjacent structures

■ Compare the bony pelvis of the male and female

■ Discuss the role of the bony pelvis in visceral function of the pelvis

OSTEOLOGY OF PELVIC AND
ASSOCIATED STRUCTURES

The bony pelvis, consisting of the two innominate bones
and the sacrum, provides the transition from the trunk to
the lower limb. Motion of the pelvis consists of movement
of both innominates as a unit in relation to the sacrum
(symmetrical motion), antagonistic movement of each
innominate bone with relation to the sacrum (asymmetri¬
cal motion), and rotation of the spine and both innomi¬
nates as a unit around the femoral heads (lumbopelvic
motion). Detailed knowledge of the bony pelvis is essen¬
tial to the clinicians understanding of pelvic motion and
appreciation of the clinical problems associated with its
mechanical dysfunction. A description of each involved
bony structure follows.

Lumbar Spine and L5 Vertebra

Bones of the lumbar spine are discussed in detail in
Chapter 32. Their size increases from cranial to caudal,
reflecting their role in transmitting the superincumbent

body weight to the pelvis for transmission to the lower
limbs. Typically, they are wider from side to side than from
front to back, are taller anteriorly than posteriorly, and have
long, thin transverse processes and short, almost horizontal
spinous processes. With the exception of L5, the facets
(zygapophyses) of the superior articular processes of the
lumbar vertebrae are vertical and directed medially and
slightly posteriorly, while those of their inferior articular
processes are vertical and directed laterally and slightly
anteriorly; the facet (zygapophyseal) joint cavities are ori¬
ented, therefore, predominately in the sagittal plane and
facilitate flexion and extension. The wedge-shaped (taller
anteriorly) vertebral bodies are responsible for the lordosis
(dorsal concavity) formed by the upper lumbar spine, but
the lordotic curvature in the lower part is attributed to both
the vertebrae and intervertebral discs (IVDs), both of
which are taller anteriorly [136,147].

The fifth lumbar vertebra is transitional from the lumbar
to the sacral region and atypical (Table 35.2). Several of its
features reflect its role in transmitting the weight of the head,
upper limbs, and trunk to the sacrum. The most robust of the
lumbar vertebrae, it has massive transverse processes that are

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

623

TABLE 35.2: Atypical Osteological Features of the
Fifth Lumbar Vertebra and Attached Structures

Osteological Features

Attachments

Body: largest and heaviest

Ligaments: anterior and posterior
longitudinal, supraspinous

Muscles: thoracolumbar fascia,
psoas major

Body height: greatest
discrepancy between anterior
and posterior heights

IVDs: thickest of all discs in spine

Articular processes: facets of
inferior articular processes
oriented in a nearly frontal plane

Zygapophyseal joint capsule

Transverse processes: in contact
with entire lateral surface of the
pedicle and body; project upward
and posterolaterally; shortest

Ligaments: iliolumbar,
lumbosacral, intertransverse

Muscles: thoracolumbar fascia,
multifidi, intertransversarii,
erector spinae

Spinous process: smallest

Ligaments: interspinous,
supraspinous

Muscles: interspinales, erector
spinae

in contact with the entire lateral surface of the pedicle and side
of the body (Fig. 35.3). The contrast between the anterior and
posterior heights of the vertebral body is greatest at L5; this
feature, as well as a greater anterior than posterior height of
the 5th lumber IVD, contributes to the angle formed at the
lumbosacral junction (Fig. 35.4). The superior articular

Figure 35.3: The fifth lumbar vertebra. A. Superior view.

B. Inferior view. Note the near-sagittal orientation of the
superior articular processes and the near-coronal orientation
of the inferior articular processes of this transitional vertebra.

Figure 35.4: Lateral view of the lumbosacral spine. Note that the
vertical height of the fifth lumbar vertebral body and interverte¬
bral discs are greater anteriorly than posteriorly. Both of these
features contribute to the lumbosacral angle.

processes of L5 are typical, but facets on its inferior articular
processes are vertical and project anteriorly and slightly later¬
ally to articulate with superior articular processes of the base
of the sacrum (Fig. 35.3); this orientation places the lum¬
bosacral facet joint cavities predominately in the coronal
plane. This abrupt change in predominate orientation from
the sagittal to the coronal plane contributes significantly to
lumbosacral integrity by resisting the shearing stress between
the fifth lumbar vertebra, the lowest IVD, and the base of the
sacrum [59,136,147].

Clinical Relevance

OBLIQUE RADIOGRAPH OF LUMBAR VERTEBRAE:

When viewed on an oblique radiograph , parts of the verte¬
bral arch of L5 (as well as other lumbar vertebrae) and its
processes take on the classical appearance of a Scottie dog
[101136] (Fig. 35.5). The oblique view is useful in imaging

(continued)

624

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 35.5: Posterior oblique view of the fifth lumbar vertebra
demonstrates the parts of the radiographic Scottie dog. The
muzzle is formed by the transverse process ( 1 ); the eye is the
pedicle viewed end on ( 2 ); the ear is the superior articular
process ( 3 ); the neck is the isthmus ( 4 ); the body is the lamina
and spinous process ( 5 ); the foreleg is the inferior articular
process ( 6 ); the hindleg is the contralateral inferior articular
process ( 7 ); and the tail is the contralateral lamina and contralat¬
eral superior articular process ( 8 ).

(Continued)

the pars interarticularis (isthmus), the part of the vertebral
arch connecting the superior and inferior articular processes
[109]; it coincides with the neck or collar of the Scottie dog.
Although not recognized as a vertebral part in the Nomina
Anatomica (N.A.) [45], its clinical importance and the signifi¬
cance of the oblique radiograph of L5 will become apparent
in a later discussion of spondylolisthesis.

Sacrum

The os sacrum was the sacred bone to the Romans, being the
last bone in the body to disintegrate and necessary for resur¬
rection [156,157]. Also known as the vertebra magna, the
most atypical of all the vertebrae is an inverted triangle
formed from the fusion of five sacral vertebral segments
(Table 35.3). Its broad base projects anterosuperiorly to artic¬
ulate with the fifth lumbar vertebra at the lumbosacral junc¬
tion, and its blunted apex projects posteroinferiorly to articu¬
late with the first coccygeal segment at the sacrococcygeal
junction (Fig. 35.6). The whole of the sacrum is convex dor-
sally and concave ventrally. Its ventral (pelvic) surface con¬
tributes to the posterior wall of the pelvic cavity, while the
dorsal surface is subcutaneous.

The sacrum possesses no intervertebral foramina for
emerging spinal nerves and has instead four sets of separate
dorsal and ventral (pelvic) foramina for passage of the dorsal
and ventral primary rami of spinal nerves Sl-4. A sacral canal
passes through its core and opens at the apex as the sacral hia¬
tus, the site of emergence of the fifth sacral (S5) and coc¬
cygeal (Col) spinal nerves. Sacral bodies are fused along
transverse lines in the central third, and fused transverse
processes and costal elements form lateral parts that run lon¬
gitudinally on each side. The piriformis originates from the
ventral surface of the sacrum, around the emerging ventral
primary rami. Muscles originating from its dorsal surface
include the multifidus, erector spinae, and gluteus maximus.

BASE

The base of the sacrum is its broadest part and represents
the superior surface of SI. The anterosuperior lip of SI juts
forward as the sacral promontory (Fig. 35.7). Facets of the
superior articular processes are vertical and project cranially,

TABLE 35.3: Osteological Features of the Sacrum and Coccyx and Attached Structures

Osteological Features

Attachments and Associated Structures

Sacrum

Consists of 5 fused vertebral segments

Ligaments: anterior and posterior longitudinal, dorsal and ventral sacroiliac,
sacrotuberous, sacrospinous

Muscles: piriformis, gluteus maximus, multifidus, erector spinae, coccygeus

A superior base: opening is the sacral canal and facets project
superiorly in a nearly coronal plane

Zygapophyseal joint capsules

An inferior apex: opening is the sacral hiatus and sacral cornua
project interiorly to articulate with coccygeal cornua

Ligaments: dorsal, ventral, and lateral sacrococcygeal

No intervertebral foramina: dorsal and ventral sacral
foramina instead

Dorsal and ventral primary rami of spinal nerves pass through individual
foramina

Coccyx

Consists of 3-4 rudimentary vertebrae

Ligaments: sacrotuberous, sacrospinous, dorsal sacroiliac, anococcygeal

Muscles: gluteus maximus, levator ani, coccygeus, external anal sphincter

Coccygeal cornua of Col project superiorly to articulate with
sacral cornua

Ligaments: dorsal, ventral, and lateral sacrococcygeal, intercoccygeal

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

625

Figure 35.6: The sacrum. A. The ventral surface
is concave. B. The dorsal surface is convex.

Base

Superior

articular

process

Base Sacral canal

Articular

surface

Sacral

foramina

Sacral

hiatus

Coccyx

Ventral

Dorsal

posteriorly, and slightly medially to articulate with the facets of
the inferior articular processes of L5. This orientation of the
lumbosacral facets is important in stabilizing the lumbosacral
junction, the point of greatest stress on the entire spine.

LATERAL PART

The lateral part of the sacrum, or ala, is formed by the fusion
of sacral transverse processes and costal elements with one

Tuberosity
of sacrum

Figure 35.7: Lateral view of the sacrum. Sacral vertebral seg¬
ments are numbered 1-5; the auricular surface and sacral
tuberosity of segments 1-3 participate in the formation of the
sacroiliac joints.

Clinical Relevance

POSITION OF THE SACRAL CORNUA: Sacral cornua are
landmarks useful in locating the sacral hiatus for the pur¬
pose of injecting an anesthetic agent into the epidural space
in caudal epidural blocks [113].

another, with the remainder of the vertebra, and with each
successive level. Each ala is wide at the base of the sacrum
and narrow at its apex. In the vast majority of individuals
[149], the lateral surface of the combined upper three sacral
segments bears an L-shaped auricular surface covered with
cartilage for articulation with an L-shaped area on the ilium,
also described as auricular, and covered with cartilage.
Immediately posterosuperior to the auricular surface is a
roughened area, the sacral tuberosity; it approximates an area
of similar shape and name on the ilium. Auricular surfaces
and tuberosities of both the sacrum and ilium contribute to
the formation of the sacroiliac joint (SIJ). The lateral surfaces
of the fourth and fifth sacral segments are usually nonarticu-
lar; however, S4 may form a part of the sacral auricular sur¬
face and contribute to the SIJ [12,149].

APEX

The caudal aspect of the fifth sacral segment, the apex of the
sacrum, bears a facet for articulation with the first coccygeal
segment. Sacral cornua project caudally on either side of the
sacral hiatus, a defect in the vertebral arch of S5 that permits
passage of S5 and Col spinal nerves (Fig. 35.6B). Along with
vascular elements, the sacral hiatus also transmits the coc¬
cygeal ligament, the caudal anchor of the spinal cord, formed
by contributions of the pial filum terminale and arachnoid-
dural filum of the dura [24].

626

Part III I KINESIOLOGY OF THE HEAD AND SPINE

OSSIFICATION

Primary centers of ossification of the bodies, vertebral arches,
and costal elements of the sacrum appear prenatally between
the third and eighth months [70,147]; several secondary cen¬
ters appear later. Fusion of the various parts of each vertebral
segment begins at 5 years and continues over the next 18 to
25 years; IVDs between segments may not ossify completely
until middle age [147]. Development and ossification of the
sacrum occurs later than that of the ilium.

Clinical Relevance

LUMBOSACRAL ANOMALIES: Being an area of transi¬
tion from one region of the spine to another ; the L5-S1
junction is subject to an unusually high degree of varia¬
tion and malformation. One author has referred to this
region's physiological unstableness [82], and others have
described it as being ontogenetically restless [Ml]. A mul¬
titude of anomalous morphological features have been
observed. Those that may be tolerated poorly by the indi¬
vidual include partial or complete sacralization of L5
(fusion of L5 to the sacrum) or iumbarization of the first
sacral segment (separation of the first sacral segment from
the remaining fused segments) [59,136,147] (Fig. 35.8),
congenital absence of a pedicle [117], inequities in the
height of the two sides of the base of the sacrum [59],
accessory laminae [10], dysplasia of the pars interarticu-
iaris (isthmus of the Scottie dog) [141] (Fig. 35.5), aplasia
or dysplasia of the superior zygapophyses of the first
sacral segment [59,141], and a change in the orientation
of one or both of the zygapophyseal facets from coronal
to sagittal [28,53] (Fig. 35.9).

Figure 35.8: Ventral view of anomalous features of lumbar and
sacral vertebrae. A. Partial Iumbarization of the first sacral
vertebra. B. Partial sacralization of the fifth lumbar vertebra.

Figure 35.9: Base of the sacrum. The main varieties of superior
articular process facet orientation are shown. A. Both superior
articular facets are flat. B. Both superior articular facets are
curved. C. One facet is flat, the other is curved.

Coccyx

The coccyx, a remnant of the skeleton of the tail [114], is a
beaklike bone (Gr. kokkyx , cuckoo) represented by three to
five fused rudimentary vertebrae, with four being the most
common number [113] (Fig. 35.10). Its curvature usually fol¬
lows that of the sacrum (i.e., ventrally concave). The first coc¬
cygeal segment has a facet for articulation with the apex of the
sacrum and cranially projecting coccygeal cornua for articula¬
tion with sacral cornua. A rudimentary IVD is present
between the sacrum and coccyx [136].

The coccyx (along with the last two sacral segments) does
not transmit weight from above. These bones do, however,
provide sites for attachment of several muscles (gluteus max-
imus, levator ani, coccygeus, sphincter ani externus) and liga¬
ments (sacrospinous, sacrotuberous, long dorsal sacroiliac).

Innominate Bone

Each innominate bone is formed from the union of three
separate bones, the ilium, ischium, and pubis (Fig. 35.1).

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

627

Figure 35.10: The coccyx. A. Dorsal surface. B. Ventral (pelvic)
surface.

The three parts unite at a central point, the acetabulum, from
which each of the three bones expand; the ilium superiorly,
the ischium posteroinferiorly, and the pubis anteroinferiorly
(Table 35.4). They are connected by hyaline cartilage until
20-25 years of age, after which they become one bone, the
innominate (Fig. 35.11). The largest of the three is the ilium,
and the smallest is the pubis. Each part has a body; the ala is
the upper expanded part of the iliac body, the ischial ramus
curves inferiorly and then anteriorly from the ischial body,
and superior and inferior pubic rami project posterosuperior-
ly and posteroinferiorly from the pubic body (Fig. 35.12). A
large, somewhat oval, obturator foramen is present in the
inferior part of the innominate bone. The largest foramen in
the body, it is closed completely by the obturator membrane
except for a small defect at its anterosuperior margin, the

TABLE 35.4: Osteological Features of the Innominate Bone and Attached Structures

Osteological Features

Attachments and Associated Structures

Ilium

Iliac crest with outer lip, intermediate area, and inner lip

ASIS; AIIS

PSIS; PIIS

Lateral surface of body

Lateral surface of wing with inferior, anterior, and posterior
gluteal lines

Superior 2/5s of acetabulum and its rim

Medial surface of wing with iliac fossa

Arcuate line

Iliac tuberosity

Auricular surface

External oblique, internal oblique, transversus abdominis

Inguinal ligament and sartorius; straight tendon of rectus femoris

Gluteus maximus; sacrotuberous ligament

Reflected tendon of rectus femoris

Glutei minimus, medius, and maximus

Capsule of hip joint, ligament of head of femur
lliacus

ISIL

Articular cartilage

Ischium

Ischial spine

Ischial tuberosity

Ischial ramus (conjoint ischiopubic ramus when joined
with inferior pubic ramus)

Posteroinferior 2/5's of acetabulum and its rim

Body

Sacrospinous ligament, superior gemellus, levator ani, coccygeus

Sacrotuberous ligament, semimembranosus, semitendinosus, biceps femoris,
quadratus femoris, adductor magnus, inferior gemellus

Obturator externus, adductor magnus, deep transverse perineus,
ischiocavernosus

Capsule of hip joint, ligament of head of femur

Obturator internus

Pubis

Iliopubic eminence

Superior pubic ramus

Pubic tubercle

Pubic crest

Inferior pubic ramus

Pecten

Obturator crest and sulcus

Body

Anteroinferior 1/5 of acetabulum and its rim

Psoas minor

Pectineus

Inguinal ligament

Rectus abdominis

Obturator externus, adductor magnus, gracilis, adductor brevis, adductor
longus, deep transverse perineus, ischiocavernosus, arcuate pubic ligament
Pectineus

Roof of obturator canal

Superior pubic ligament, interpubic disc, pyramidalis

Capsule of hip joint, ligament of head of femur

Foramina, Canal, and Notches

Obturator foramen

Obturator canal

Greater sciatic notch

Lesser sciatic notch

Formed by ischium and pubis; largely covered by obturator membrane and
obturator internus

Transmits obturator nerve, artery, and vein

Between PIIS and ischial spine; transmits pyriformis, gluteal nerves, and vessels
Between ischial spine and tuberosity; transmits obturator internus and its nerve,
pudendal nerve, and vessels

628

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Convenient for assessing the height of the iliac crests in the
standing postureit is also useful for locating the usual site
of lumbar puncture [112,136] (Fig. 35.13).

Figure 35.11: Lateral view of the right innominate bone of a
child, indicating ossification centers. Note the Y-shaped triradiate
cartilaginous stem connecting the three bones of the os coxae in
the acetabulum. Cartilage is also present at the ischiopubic junc¬
tion and along the superior margin of the iliac crest.

obturator canal. Each innominate bone is united to the
sacrum posteriorly and to its opposite member anteriorly to
form the bony pelvis. The two innominate bones and sacrum
receive muscular attachments from the segments above
(trunk) and below (lower limbs), while sheltering and sup¬
porting the visceral contents of the pelvis.

ILIUM

The ilium is flattened in the sagittal plane and has lateral
(gluteal) and medial (pelvic) surfaces. The expanded upper
end is the ala (wing), and the lower end is the body. The ala
receives fibers from a number of trunk and lower limb mus¬
cles. The upper edge of the ala, the iliac crest, represents the
caudal limit of the waist. Three ridges—the outer, middle, and
inner lips—curve along its upper border and serve as attach¬
ment sites for the obliquus extemus abdominis, obliquus inter-
nus abdominis, and transversus abdominis, respectively

Clinical Relevance

TRANSVERSE PLANE OF ILIAC CRESTS: The transverse
plane of the iliac crests, or supracristal plane ; is a horizon¬
tal one that passes through the IVD between L4 and L5.

Medial Surface

The medial (pelvic) surface of the ala bears a concavity, the iliac
fossa, which gives rise to the iliacus muscle. The posterior part
of the medial surface of each ilium has a pair of prominences
that mark the site of the SIJ: an anteroinferior L-shaped auric¬
ular surface covered with cartilage and a posterosuperior iliac
tuberosity. Both the auricular surface and tuberosity articulate
with areas of similar shape and name on the ala of the sacrum
to form the SIJ. An oblique ridge of bone divides the iliac
tuberosity into upper (posterosuperior) and lower (anteroinfe¬
rior) parts. The interosseous sacroiliac ligament attaches to the
lower part; two trunk muscles, the erector spinae and medial
fibers of the quadratus lumborum, attach to the upper part.
Descending anteriorly from the edge of the auricular surface is
the arcuate line; it joins the ilium to the pubis at the iliopubic
(iliopectineal) eminence, a roughened area that receives the
insertion of the psoas minor when it is present. The inferior
end of the medial surface of the body of the ilium marks the
position of the upper two fifths of the acetabulum.

Lateral Surface

Three oblique gluteal lines mark the lateral surface of the ala
and subdivide it into four areas (Fig. 35.12A). Beginning pos¬
teriorly, the gluteus maximus originates between the posterior
gluteal line and posterior border of the iliac wing; the gluteus
medius from the area between the posterior and anterior
gluteal lines; the gluteus minimus (posteriorly) and tensor fas¬
ciae latae (anteriorly) from bone between the anterior and
inferior gluteal lines; and the reflected tendon of the rectus
femoris from the area of bone inferior to the inferior gluteal
line, immediately above the acetabulum (Fig. 35.12C). The
inferior end of the lateral surface of the body of the ilium
forms the upper two fifths of the acetabulum.

Anterior Border

The most anterior point of the iliac crest is the anterior supe¬
rior iliac spine (ASIS), which receives the inguinal ligament
above and the sartorius muscle below. Moving caudally, the
anterior border is concave and ends in a large roughened
area, the anterior inferior iliac spine (AIIS), which serves as
the origin of the tendon of the rectus femoris.

Clinical Relevance

POSITION OF THE ASIS: The ASIS is an important land¬
mark for measuring leg length. A suspected discrepancy in
leg length can be assessed by measuring the distance
between the ASIS and the ipsilateral medial or lateral malle

(< continued )

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

629

Anterior gluteal line

Posterior
gluteal
line

Iliac tuberosity

Lateral

Latissimus
dorsi

Medial

Internal abdominal oblique

External abdominal
oblique

Tensor fasciae
latae

Inguinal
ligament
Sartorius
Rectus femoris

Auricular
surface

Obturator crest
and canal (arrow)

Ischium

Transversus

abdominis

Quadratus lumborum

Erector

spinae

Superior
gamellus

Inferior
gamellus

Semimembranosus
Semitendinosus
and biceps (long head)

Psoas minor
Pectineus

Adductor

longus

Gracilis
Adductor brevis

Quadratus

femoris

Adductor

magnus

Obturator

externus

Interosseus
sacroiliac
ligament

Coccygeus

Levator ani

Sacrotuberous

ligament

Deep transverse
perineus and
sphincter urethrae

Ischiocavernosus
and superficial
transverse perineus

Figure 35.12: Right innominate bone of an adult with osteological features indicated on the lateral (A) and medial surfaces (B). Sites
of attachment of muscles and ligaments are shown on the lateral (C) and medial surfaces (D).

(Continued)

olus in the supine position and comparing it to the same
measurement obtained from the contraiaterai limb [98].

Posterior Border

The most posterior point of the iliac crest is the posterior
superior iliac spine (PSIS); inferiorly and slightly forward of

the PSIS is the posterior inferior iliac spine (PUS). Fibers of
the sacrotuberous ligament are attached to the PSIS and
PUS, while those of the dorsal sacroiliac ligament proceed to
the PSIS and posterior end of the medial lip of the iliac crest.
Caudal to the PUS is a deep concavity, the greater sciatic
notch, positioned just above the acetabulum; a few fibers of
the piriformis originate from its superior margin. Only the
superior half of this deep notch is formed by the ilium; its
inferior half is formed by the posterior border of the ischium.

630

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 35.13: The horizontal line through the highest points of
the iliac crests passes through the intervertebral disc located
between the fourth and fifth lumbar vertebrae; this is the
supracristal plane.

A dimple in the skin is observable just medial to the PSIS and
is especially prominent in obese individuals.

ISCHIUM

The body of the ischium forms the posteroinferior two fifths
of the acetabulum. Extending first inferiorly and then anteri¬
orly from its body is the ischial ramus; its union with the infe¬
rior pubic ramus is marked by a roughened area, the ischiop-
ubic juncture. The two rami together form the conjoint
ischiopubic ramus. The body and ramus of the ischium con¬
tribute to the posterolateral wall of the pelvic cavity.
Projecting from the posterior border of the ischial body is the
sharp ischial spine above and the large, bulbous ischial
tuberosity below; between the two is the lesser sciatic notch,
a shallower concavity than its partner above. The two sciatic
notches are converted into foramina (greater and lesser sciatic)
by the sacrospinous and sacrotuberous ligaments, dense liga¬
ments that attach the sacrum and coccyx to the ischium and
ilium, thereby reinforcing the union between the axial and
appendicular skeletal elements (i.e., SIJ) (Fig. 35.14). The
sacrospinous ligament is anchored to the ischial spine, and the
sacrotuberous ligament to the ischial tuberosity. In addition to
these ligamentous structures, many muscles of the pelvis,
buttock, and posterior thigh originate from the spine and
tuberosity of the ischium. Most of the structures exiting the
pelvic cavity pass through the greater sciatic foramen, along
with the piriformis; only the obturator internus and its nerve
supply, along with the pudendal nerve and internal pudendal
vessels, traverse the lesser sciatic foramen.

Ischial Spine

The coccygeus and levator ani originate from the medial
surface of the ischial spine (Fig. 35.12D). The superior and

Greater sciatic

tuberosity ligament

Figure 35.14: Dorsal surface of the pelvis with the three parts
of the innominate bone shaded differently; the greater sciatic
notch is transformed into a foramen by the sacrospinous liga¬
ment; and the lesser sciatic notch is transformed into a foramen
by the sacrotuberous ligament.

inferior gemelli originate from the lateral surface of the
spine and tuberosity, respectively, immediately on either
side of the lesser sciatic notch (Fig. 35.12C).

Ischial Tuberosity

This large, posteroinferiorly projecting prominence of the
ischium has multiple functions; it serves as the origin of sev¬
eral large muscles of the buttock and thigh, the site of attach¬
ment of one extensive ligament that reinforces the SIJ, a shel¬
ter for the major nerve of the perineum (pudendal), and a
support for the body weight in sitting. Transverse and vertical
ridges divide the posterior surface of the tuberosity into four
unequal quadrants (Fig. 35.15): superolateral for the semi¬
membranosus; superomedial for the semitendinosus and long
head of the biceps femoris; inferolateral for the posterior
fibers of the adductor magnus; and inferomedial, where it is
covered by adipose tissue and the gluteus maximus bursa, for
its weight-bearing function. The upper, lateral surface of the
ischial tuberosity serves as the origin of the quadratus
femoris. The sacrotuberous ligament attaches to the entire
medial edge of the tuberosity and extends forward along the
ischial ramus as the falciform process (Fig. 35.16).

PUBIS

The body of the pubis is compressed in the sagittal plane
and has two rami projecting from it, one posterosuperiorly
(superior ramus) and one posteroinferiorly (inferior ramus)
(Fig. 35.12). The superior ramus is joined to the body of the
ilium at the iliopubic (iliopectineal) eminence and the infe¬
rior ramus to the ischial ramus at the ischiopubic junction.
The bodies of the two pubes project toward the midline as

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

631

Figure 35.15: Posterolateral aspect of the left os coxae. Two
ridges subdivide this caudal part of the ischium: a transverse
ridge separates it into upper and lower halves, and a longitudi¬
nal ridge divides the lower half. The long hamstrings originate
from the upper half; the posterior fibers of the adductor magnus
originate from the lateral part of the lower half; and the medial
portion of the lower half, used to support the sitting weight, is
covered by fat and fibrous connective tissue and a bursa for the
gluteus maximus. The upper, lateral surface of the ischial
tuberosity gives origin to the quadratus femoris. A ridge along
the medial edge of the tuberosity continues forward along the
ischiopubic ramus; fibers of the sacrotuberous ligament attach
here to form the falciform process.

roughened symphyseal surfaces. A fibrocartilaginous disc
interposed between the two surfaces is part of the anterior
joint between the two innominate bones, the symphysis
pubis. The posterior surface of the two pubic bodies and
interpubic disc form the anterior wall of the pelvic cavity
The adductor longus originates from the anterior surface of
the body of the pubis.

Superior Pubic Ramus

The posterior end of the superior pubic ramus, where it joins
with the body of the ilium, forms the anteroinferior one fifth
of the acetabulum. On the posteroinferior end of the ramus is
a slight groove, the obturator sulcus, guarded by a ridge of
bone, the obturator crest; the sulcus, crest, and a defect in the
obturator membrane, or obturator canal, all mark the site of
passage of the obturator nerve and vessels from the pelvic
cavity into the medial thigh.

Extending forward along the internal surface of the supe¬
rior pubic ramus, continuous with the arcuate line of the
ilium, is a sharp ridge of bone, the pecten; the arcuate line
and pecten together form the linea terminalis. The pecten

Greater sciatic
foramen

Ischial

spine

Obturator
crest and
canal

Obturator

foramen

Sacro-
spinous
ligament

'Sacrotuberous
ligament

Lesser sciatic
foramen
Falciform process of
sacrotuberous ligament

Figure 35.16: Medial view of the right innominate bone. Note
the extension of the sacrotuberous ligament along the medial
surface of the ischial ramus as the falciform process.

terminates at the pubic tubercle; from the tubercle a ridge
passes medially, the pubic crest, and terminates at the sym¬
physeal surface. The sacral promontory and ala, linea termi¬
nalis, and pubic crest on each side form the pelvic brim; it
divides the true pelvis (pelvis minor) below from the false
pelvis (pelvis major) above. The pecten serves as the origin of
the pectineus muscle.

Inferior Pubic Ramus and Ischial Ramus

The inferior ramus of the pubis and ramus of the ischium meet
at a point that is approximately equidistant between the ante¬
rior limit of the pubis and posterior limit of the ischium; the
two parts together form the conjoint ischiopubic ramus. With
the inferior margin of the pubic symphysis, the paired inferior
pubic rami and ischial rami form the pubic arch (Fig. 35.1).
The pubic bodies and ischiopubic rami have a pelvic (medial,
posterior) surface that serves as the bony origin for several
pelvic and perineal muscles: the levator ani of the pelvic
diaphragm from the pubic body, and the urogenital
diaphragm, superficial transverse perineal, and ischiocaver-
nosus from the ischiopubic ramus. Their lateral (anterior) sur¬
faces are roughened and mark the site of origin of the medial
(adductor) thigh muscles: the anterior fibers of the adductor
magnus from the ischiopubic ramus, and the gracilis and
adductor brevis from the inferior ramus and body of the pubis.

632

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Clinical Relevance

SACROTUBEROUS LIGAMENT AND PUDENDAL
CANAL: The sacrotuberous ligament that attaches to the
medial edge of the ischial tuberosity extends along the
medial side of the ischiopubic ramus as the falciform
process [134,147]. This forward extension of the ligament
forms the floor of the pudendal canal (Alcock's canal) and
shelters the contents of the canal, the pudendal nerve and
internal pudendal vessels, the primary neurovascular ele¬
ments of the perineum (Fig. 35.17). Nonetheless, contents of
the pudendal canal are intimately juxtaposed to the ischio¬
pubic ramus and can be jeopardized when this ramus is
fractured and bony fragments impinge upon or sever the
neurovascular contents of the canal. More commonly, pres¬
sure on the pudendal canal, as experienced in long distance
cycling when the seat presses against the medial border of
the ischiopubic ramus, can lead to temporary or protracted
erectile dysfunction and even impotence in males
[4,38,116,148]. Whether these aspects of male sexual dys¬
function are vasculogenic (trauma to the internal pudendal
vessels), neurogenic (trauma to the pudendal nerve), or a
combination of the two continues to be debated [93,143].

OBTURATOR FORAMEN AND
OBTURATOR MEMBRANE

The large, somewhat oval, obturator foramen in the inferior
part of the innominate bone is located below the acetabulum
and is formed by the body and rami of the ischium and pubis.
The obturator foramen is closed almost entirely by the obtu¬
rator membrane, except for a small anterosuperior defect,
the obturator canal, which allows the obturator neurovascu¬
lar elements to exit the pelvic cavity. The obturator externus

originates from most of the obturator membrane s lateral
(external) surface, as well as surrounding bone of the pubis
and ischium; the obturator internus originates from most of
the membranes medial (internal) surface, along with sur¬
rounding bone of the pubis, ischium, and ilium.

PALPATION OF BONY PROMINENCES
AND JOINTS OF THE PELVIS

Careful assessment of the low back and pelvis requires pre¬
cise palpation in an attempt to identify the source of a
patients complaints. Readily palpable structures of
the bony pelvis include the following:

• Spinous and transverse processes of L5

• Dorsal sacroiliac ligament

• Dorsal surface of the sacrum

• Coccyx

• ASIS

• Iliac crest

• PSIS

• Sacrotuberous ligament

• Ischial tuberosity

• Conjoint ischiopubic ramus

• Symphysis pubis

• Pubic tubercle

• Superior pubic ramus

OSSIFICATION

Ossification of the innominate bone begins prenatally by
three primary centers, one each for the ilium, ischium, and
pubis. The center for the ilium appears rostral to the greater
sciatic notch in the ninth week, the ischial center in its body
by the fourth month, and the center for the pubis in its supe¬
rior ramus between the fourth and fifth month of intrauterine
life [7,37,88]. Significant parts of each of the three bones of
the os coxae remain cartilaginous at birth. Most notably, the

Figure 35.17: Coronal section through the posterior part of the pelvis. Note the relationship of the pudendal nerve and internal
pudendal vessels to the medial surface of the ischial ramus.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

633

acetabulum is a cartilaginous cup that has expanding from its
center a triradiate stem of cartilage with prongs projecting
toward the ilium, ischium, and pubis [147] (Fig. 35.11).
Secondary centers of ossification appear at varying times post-
natally. Several secondary centers for the ilium, ischium, and
pubis appear at puberty and fuse sometime between 15 and
25 years of age. Three secondary centers for the acetabulum
join between 16 and 18 years [6,7].

Clinical Relevance

OSSIFICATION AND FUSION OF THE INNOMINATE
BONE: Owing to the lateness of fusion of the bones of the
pelvis; certain conditions and activities may be particularly
hazardous to adolescents and young adults. Teenage preg¬
nancies are particularly risky , considering the trauma of
fetal passage through the pelvic birth canal [150]. In addi¬
tion ; serious consideration should be given to participation
by adolescents in certain activities that require sudden accel¬
eration and deceleration , such as sprinting , soccer ; football
and basketball [112]. During these activities , avulsion frac¬
tures can occur at sites of attachment of muscles to apophy¬
ses (a bony prominence without a secondary ossification
center), such as the AS IS, AIIS , ischial tuberosity , and
ischiopubic ramus.

Sexual Differences

A higher degree of sexual dimorphism is apparent in the
bones of the pelvis than in other bones of the body (Fig. 35.18).
Distinctive sex characteristics appear prenatally as early as the
third month [18]; pelves are poorly marked prior to puberty
but fully developed afterward [86]. Differences between the

bony pelves of males and females are related to a number
of factors, including, but not limited to, relative differences
in stature and body composition resulting from actions of
the sex hormones and functions of the pelvis [8,136,147]
(Table 35.5).

TABLE 35.5: Differences between Female and Male Pelvis That Represent Adaptations for Childbearing

Features of the Male Bony Pelvis

Features of the Female Bony Pelvis

Concavity more conical

Cavity more cylindrical

Sacrum longer and narrower

Sacrum shorter and wider

Sacral concavity shallower

Sacral concavity deeper

>1/3 of the sacral base = the body

>2/3 of the sacral base = the ala

Anterolateral wall of pelvis narrower

Pubic tubercles closer together

Distance from the symphysis pubis to the anterior lip of the
acetabulum = diameter of the acetabulum

Anterolateral wall of pelvis wider

Pubic tubercles further apart

Distance from the symphysis pubis to the anterior lip of the
acetabulum > diameter of the acetabulum

Greater sciatic notch narrower

Greater sciatic notch wider

Pubic arch < 90°

Pubic arch ~ 90°

Ischiopubic rami robust and everted

Ischiopubic rami delicate

Ischium relatively and absolutely longer than the pubis

Ischiopubic index a < 90°

Pubis relatively and absolutely longer than the ischium

Ischiopubic index > 90°

a lschiopubic index = (length of pubis X 100) -r length of ischium

634

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 35.19: The pubic tubercles and anterior superior iliac spines
(ASIS) are aligned vertically; the upper surface of the symphysis
pubis, the ischial spine, and the tip of the coccyx are aligned hori¬
zontally; the plane of the pelvic inlet is approximately 60° off the
horizontal; and the plane of the pelvic outlet is approximately 15°
off the horizontal. The axis of the pelvic cavity (arrow) is oblique,
passing through the centers of the inlet and outlet.

In general, estrogen secretion in females stimulates for¬
mation of an individual with shorter and lighter bones, lower
weight, and lower lean body mass than males [63]; and the
requirements of childbirth necessitate a roomier pelvis in
females [147]. Subsequently, the sacrum and the innominate
bones of females are lighter; bony protuberances are less
prominent; and relative widening of the sacral base and pubic
body, increasing the angle of the pubic arch, everting the
ischial tuberosities, and increasing the forward inclination of
the sacrum all contribute to enlargement of the diameters of
the pelvic inlet and outlet, thus facilitating parturition. Other

Anterior edge
of sacral ala

Sacral promontory

Figure 35.20: The pelvic inlet is bordered by the pelvic brim,
formed by the promontory and ala of the sacrum, pecten of the
pubis, arcuate line of the ilium (together termed the iliopectineal
line, or linea terminalis), and the pubic crest. The junction of the
ilium and pubis is marked by the iliopubic eminence.

Conjugate

Diagonal

conjugate

Anteroposterior
diameter
of pelvic outlet

Anteroposterior
diameter
of pelvic cavity
(median sagittal)

Axis of
pelvic canal

Figure 35.21: A. Diameters of the superior aperture (pelvic inlet).
B. Anterosuperior diameters of the true (minor) pelvis.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

635

differences can be seen in the pelvic joints: the acetabulum
and auricular surfaces of the ilia and sacrum are smaller
[20,149], and the symphyseal surface of the pubic body is
shorter [164].

Clinical Relevance

PELVIC DIAMETERS AND TYPES OF PELVES:

Assessment of the overall size of the female pelvis and the
dimensions of its apertures is important in obstetrical prac¬
tice. Two openings are significant: one is situated above and
serves as the pelvic inlet (superior aperture), and the other
is positioned below and functions as the pelvic outlet (inferi¬
or aperture) (Fig. 35.19). The size of the pelvic cavity posi¬
tioned between these two bony boundaries is the limiting
factor in obstetric considerations; no attempts are made to
account for the fascial and muscular contributions made to
this space [135].

The pelvic brim borders the pelvic inlet (Fig. 35.20). On
each side , it is formed posteriorly by the sacral ala; laterally
by the arcuate line of the ilium and pecten of the pubis
(together forming the tinea terminalis), and anteriorly by the
pubic crest. The circle is completed in the posterior midline
by the sacral promontory and anteriorly by the symphysis
pubis. The border of the pelvic outlet is formed anteriorly by
the pubic arch, laterally by the ischial tuberosities and
sacrotuberous ligaments; and in the posterior midline by the
coccyx. The plane of the pelvic inlet is approximately 60° off
the horizontal while the plane of the outlet is nearly hori¬
zontal [135] (Fig. 35.19). Owing to the different orientations

of the two apertures; the axis of the pelvic cavity , running
through the centers of the inlet and outlet follows a curved
course that nearly parallels the sacrococcygeal curvature.
During parturition (childbirth), the fetus follows this curva¬
ture in its passage through the pelvic cavity.

For obstetrical purposes; three diameters of the superior
aperture of the pelvis are measured commonly to deter¬
mine the prospective mother's pelvic type prior to delivery
(Fig. 35.21). The true conjugate diameter is the distance
between the superior border of the symphysis pubis to the
sacral promontory; the diagonal conjugate diameter is
similar but has a starting point inferior to the symphysis
pubis. The former is measured radiographically , but the
latter can be assessed during the vaginal examination. The
transverse diameter, the greatest distance between sym¬
metrical points on the pelvic brim, is deduced from exter¬
nal pelvic dimensions or the distance between the ischial
spines, which are palpable through the vagina. Four types
of pelves are described on the basis of the ratio between
the transverse and conjugate diameters (Fig. 35.22). These
are gynecoid (female), anthropoid (ape), android (male),
and platypelloid (flat). The transverse diameter is greater
than the conjugate in the gynecoid , android , and platypel¬
loid pelves , while the opposite is true in the anthropoid
pelvis. Gynecoid and android pelves predominate in
Caucasian females, while gynecoid and anthropoid types
are more common in Negroid females; few females have
platypelloid pelves [8,147]. All pelvic types except the
gynecoid type hamper engagement of the fetal head
during labor [135].

Figure 35.22: Shapes of four major types of pelves are based on the ratio between the transverse and conjugate diameters.
A-C. The transverse diameter is greater than the conjugate. D. The opposite is true.

636

Part III I KINESIOLOGY OF THE HEAD AND SPINE

PELVIC JOINTS AND PERIARTICULAR
STRUCTURES

In the erect posture, the superincumbent weight of the head,
upper limbs, and trunk is transmitted onto the sacrum
through the last lumbar vertebra and its disc. Weight is
further transmitted through the paired SIJs and distributed to
the ischial tuberosities in sitting or the femora in standing.
Completed anteriorly with the union of the pubic bodies at
the symphysis pubis, this osteoligamentous ring is subdi¬
vided into two anatomical and functional arches to describe
the transmission of forces in the standing posture [3,9,
80,81,147]; a coronal plane passing through the acetabula
separates the bony pelvis into anterior and posterior arches
(Fig. 35.23). The upper three segments of the sacrum and
paired pillars of iliac bone passing from both SIJs to the pos-
terosuperior acetabula form the posterior arch, which serves
mainly to transfer weight from above to the lower limbs. The
anterior arch is a tie beam or counter arch and consists of the
superior pubic rami, pubic bodies, and interpubic disc; it
serves the dual function of connecting the anterior ends of
the iliac pillars to prevent separation of the posterior arch at
the SIJs, as well as acting as a compression strut against the
ground reaction forces from the femora below. The sitting
arches are somewhat different. Weight is transmitted from
above through the SIJs, inferior parts of the iliac pillars, and
then to the ischial tuberosities. The tie beam or counter arch
for the sitting arch includes the ischial tuberosities, ischiopu-
bic rami, pubic bodies, and interpubic disc [9]. The highest

Figure 35.23: In standing, the posterior arch passes through the
sacrum, paired iliac pillars, posterosuperior acetabula, and femora,
while the anterior (counter) arch passes through the femora,
superior pubic rami, pubic bodies, and interpubic disc (solid lines).
In sitting, the posterior arch passes through the sacrum, inferior
part of the iliac pillars, to the ischial tuberosities, while the
anterior (counter) arch passes through the ischial tuberosities,
ischiopubic rami, pubic bodies, and interpubic disc (broken lines).

Standing

transfer

Figure 35.24: Bony trabecular system of the right innominate
bone and proximal femur. The transfer of weight via the SIJ is
through the arcuate line to the acetabulum in standing, and
through the arcuate line to the ischial tuberosities when sitting.

stresses and greatest bone densities of the pelvic bones occur
along the lines of the these anterior and posterior arches
[34,35,75] (Fig. 35.24).

The bones and joints of the pelvis are inherently stable.
The line of transmission of both the trunk force from above
and the ground reaction forces from below pass anterior to
the SIJs (Fig. 35.25). The former force tends to tilt the sacrum
forward, and the latter tends to rotate the innominate bones
backward; both are resisted by the numerous strong liga¬
ments of the pelvic joints, as well as, in the SIJs particularly,
the inherent morphology of their articulating surfaces
[30,147]. Together, the two forces provide a self-locking,
screw-home mechanism for maximal stability [3,59,81,147].
The level of pelvic stability achieved by this arrangement
requires thousands of pounds of force to disrupt [30,137].
Furthermore, when large forces are applied to the pelvis,
either in vivo or in vitro, the sacrum or ilium often fractures
before the ligaments rupture or avulse.

The reader should be warned that many aspects of pelvic
joint morphology, mechanics, and pathology have been debated
heavily over the years. Most of the questions surrounding
these joints appear to have been adequately probed to allow
most clinicians and scientists to arrive at well-documented
conclusions regarding the issues around which controversy has
thrived. Although nagging questions persist, especially as
regards the amount and type of motion present at these joints,
only a few diehards remain who are skeptical about the other
issues. Two things account for the changing perspective
toward these joints: first, technological advances in research
techniques have produced less-equivocal evidence, and sec¬
ond, the body of evidence from numerous well-designed

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

637

Body weight force

Figure 35.25: Medial aspect of the left hemipelvis. In standing,
the sacral promontory tends to tilt down and forward while the
ilia tend to tilt backward because the center of gravity passes
anterior to the sacroiliac joints (SIJs) and posterior to the hip
joints. These tendencies are resisted by the interosseous sacroiliac,
sacrotuberous and sacrospinous ligaments, and the inherent
morphology of the SIJ.

Figure 35.26: The lumbosacral angle (a) is formed by the intersec¬
tion of lines drawn between the long axis of the fifth lumbar
vertebra and the sacrum. It results from a forward sacral inclina¬
tion (b) and wedge-shaped lower lumbar intervertebral discs and
bodies. As the sacral inclination and lumbar lordosis increase, the
lumbosacral angle decreases, and vice versa. The sacral inclina¬
tion is greater in the female, while the lumbosacral angle is
greater in the male. W, superincumbent weight.

scientific studies has found its way into clinical and basic sci¬
ence textbooks. Inclusion in this section of all of the studies
that have contributed to the knowledge base of the pelvic
joints is neither possible nor appropriate. Following a descrip¬
tion of each of the joints, the studies most critical to our
understanding are offered.

Lumbosacral Junction

The basic components of the L5 and SI articulations do not
differ significantly from the other intervertebral unions. The
bodies are joined by an amphiarthrodial symphysis, which
consists of thin layers of hyaline cartilage on either side of the
largest fibrocartilaginous disc in the spine; the disc is taller
anteriorly than posteriorly, a matching feature found in the
L5 body The synovial zygapophyseal joints have facets ori¬
ented in the coronal plane whose surfaces are more widely
separated than those above [147]. The sacrum sits below L5
with its base tilted forward and its apex backward. The sacral

inclination thus formed consists of the base of the sacrum
being tilted forward off the horizontal by approximately 30°
(Fig. 35.26). There is tremendous variability, however, with a
reported range of 20 to 90° [58,59,81]; it is greater in
the female [9,147]. The sacral inclination, as well a wedge-
shaped L5 vertebral body and IVD, each contribute to
the lumbosacral angle (between the long axes of L5 and the
sacrum) [81,136]; it is greater in males.

Clinical Relevance

LUMBOSACRAL ANGLE: The sacral inclination and the
lumbosacral angle are intimately related to the lumbar lor¬
dosis. An increase in the sacral inclination along with a
decrease in the lumbosacral angle necessitates an increase
in the lumbar lordosis.

638

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Many muscular and ligamentous entities cross and rein¬
force the lumbosacral junction. The muscles belong to the
trunk and lower limbs. Essentially any muscle that moves the
trunk across the joints of the lumbar spine stabilizes the joint,
including the anterior and anterolateral abdominal wall mus¬
cles, posterior abdominal wall muscles, and lumbar deep back
muscles. These are described in greater detail in Chapter 33.

Ligamentous support is provided by continuation of the
vertebral ligaments that are found at higher levels of the spine
and include the anterior and posterior longitudinal, inter-
transverse, interspinous, and supraspinous ligaments, along
with the ligamentum flavum and zygapophyseal capsular ele¬
ments at the L5-S1 interspace. In addition, the iliolumbar
ligaments reinforce the junction laterally (Fig. 35.27). Each
extends from the tip of the transverse process of L5 (and fre¬
quently L4) and spreads laterally to connect to the pelvis by
way of two bands, both of which pass anterior to the SIJ. An
upper band attaches to the iliac crest, where it is continuous
above with the thoracolumbar fascia; a lower band (some¬
times referred to as the lumbosacral ligament, though not
recognized in the N.A. [45]) passes to the upper surface of the
sacral ala, where it blends with the anterior sacroiliac liga¬
ment [27,91,94,123,147].

The iliolumbar ligament is not present in the newborn; it
develops over the first two decades by metaplasia of fibers of
the quadratus lumborum and undergoes degeneration from
the fourth decade on [94]. Researchers theorize that the liga¬
ment develops when the lumbosacral junction is stressed by
assumption of the upright posture [27,94] and suggest that
the different bands of the ligament serve different functions
[27,91]. The lower band is positioned in the coronal plane; it
serves to square L5 on the sacrum and thus control lateral

4th vertebra

Figure 35.27: Iliolumbar ligaments, both passing anterior to the
sacroiliac joint, are shown connecting both the fourth and fifth
lumbar vertebrae to the ilium. Although not recognized by the
Nomina Anatomica, a lumbosacral ligament is shown.

flexion. The upper band runs obliquely backward; it exerts a
posterior pull on L5 to prevent anterior slippage during
weight bearing, and it controls flexion. The iliolumbar liga¬
ments, as a whole, appear to also control axial rotation [180].
Apparently, the ligament assumes greater functional signifi¬
cance in contributing to lumbosacral stability when the lum¬
bosacral disc degenerates; it may protect the disc from exces¬
sive torque, particularly if the facet joints are defective [27].

As already indicated, the lumbosacral junction is a region
of high variability, as well as the point of greatest stress in the
entire vertebral column. Being one of the levels most subject
to internal derangement [33], Kapandji refers to it as the
weak link [81]. As a result of the body weight bearing down
on L5 and the anterior inclination of the sacrum, an anteroin¬
ferior shear stress is produced at the L5-S1 junction; the
resultant force vector, acting through the pars interarticularis,
is an anterior one [81] (Fig. 35.28). Subsequently, L5 tends to
slide forward on the sacral promontory. This tendency is resis¬
ted, and L5 is restrained, however, by the vertebras bony
hook, formed by its pedicles, pars interarticulares, and inferior
articular processes, fitting over the superior articular processes
of the sacrum below [59] (Fig. 35.28).

Figure 35.28: The bony hook of L5 consists of its pedicle, pars
interarticularis, and inferior articular process; it fits over the
superior articular process of the sacrum below. A. Disruption of
the bony hook mechanism between L5 and SI can be caused by
fracture of the pars interarticularis (spondylolysis) and can result
in spondylolisthesis. B. Pars interarticularis defect seen from
above L5.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

639

Clinical Relevance

PARS INTERARTICULARIS DEFECTS: Various anom¬
alies and pathological or congenital conditions, over time and
under stress, may weaken or destroy the integrity of the resist¬
ing hook mechanism; such defects include congenital aplasia
(or dysplasia) of the sacral facets, near-sagittal orientation of
one or both of the lumbosacral facet joints (Fig. 35.9), exces¬
sive anterior tilt of the sacrum resulting in increased lum¬
bosacral shear, and spondylolysis. Disruption of the pars
interarticularis (spondylolysis) can occur unilaterally (up to
30%), with or without slipping (olisthesis), and although it
has been observed at L3, L4, and L5, it is most frequent at L5
[59,60,100,136]. Although 5% of individuals with this condi¬
tion are asymptomatic [100], spondylolisthesis can be a seri¬
ous consequence of spondylolysis. The Belgian obstetrician
Herbineaux [72] is credited with describing the first cases of
spondylolisthesis when he noted that, on occasion, a bony
prominence on the anterior surface of the sacrum interfered
with labor. Because of the location of the spondylolytic defect,
the body, pedicles, and superior articular processes slip for¬
ward, leaving the inferior articular processes, laminae, and
spinous process in their normal position.

Spondylolisthesis is diagnosed on the oblique radi¬
ographic projection; the disrupted pars interarticularis (isth¬
mus) appears as a translucent area in the vicinity of the
neck of the Scottie dog, described earlier in this chapter
[101] (Fig. 35.5). The degree of slip of L5 on the sacrum is
assessed on the lateral radiographic view as a percentage
based on a grading system of Myerding [115]: grade 1, 25%;
grade 2, 25-50%; grade 3, 50-75%; grade 4, 75-100%
overhang (Fig. 35.29). Spondyloptosis occurs when the
posterior edge of L5 moves anterior to the sacral promontory
[100]. IVD degeneration, cauda equina compression, and
severe pain are serious potential sequelae of this disorder
[59,61,100,101].

Sacrococcygeal Junction

A symphysis consisting of a small fibrocartilaginous IVD
sandwiched between thin layers of hyaline cartilage unites the
apex of the sacrum and the superior surface of the first coc¬
cygeal segment (Fig. 35.7). Anterior, lateral, and posterior
sacrococcygeal ligaments complete the union. Successive seg¬
ments are nodular and generally fused to one another; occa¬
sionally, there is a synovial joint between the second and third
segments. In the young, all intercoccygeal joints are symphy-
seal, but they fuse in adulthood, earlier in males than in
females; with advancing age, the sacrococcygeal joint fuses
[80,147]. The tip of the coccyx is anchored to the overlying
skin and can be palpated easily in the intergluteal cleft [113].
Dorsal primary rami of S4-5 and Col anastomose with one
another and form loops that innervate the sacral apex and
coccyx, as well as the overlying skin [147].

Figure 35.29: Spondylolistheses is graded on the basis of the
amount of forward movement of L5 on the sacrum. In grades 1,
2, 3, and 4, some 25, 50, 75, and 100% of the body of L5 is posi¬
tioned anterior to the sacral promontory, respectively.

Clinical Relevance

COCCYX AND SACROCOCCYGEAL JUNCTION: During
childbirth, the coccyx moves posteriorly, allowing an
increase in the diameter of the pelvic outlet, thus facilitating
movement of the fetus through the birth canal [113].
Presumably, the movement is passive and secondary to pas¬
sage of the fetus and made possible by relative relaxation of
the ligaments surrounding the coccyx. It is stimulated by an
increase in circulating sex hormones and the hormone
relaxin (more on this later) [97,113].

Most injuries to the coccyx occur in women, probably
owing to its more posterior position in the broader pelvic
outlet of the female [33]. Injuries can occur during obstetri¬
cal and gynecological maneuvers, but most result from
other traumas [107,119]. An extension strain or fracture of
the coccyx can result from childbirth; flexion injury, fracture,
or a direct contusion can result from a fall on an uneven
surface or in the half-sitting position (allowing some of the
force to be absorbed by the coccyx away from the ischial
tuberosities) [33,49,80]. In extremely lean individuals without
sufficient gluteal mass, the coccyx may be vulnerable in sit¬
ting [130]. Coccygodynia (coccygeal pain; also known as
coccydynia, coccyalgia) from any of the aforementioned
mechanisms is always localized to the coccyx. Pain can be
experienced with activation of muscular fibers attaching to
the coccyx (i.e., gluteus maximus, iliococcygeus and coc-
cygeus). Consequently, walking, especially uphill or upstairs,
and sitting, particularly going from standing to seated, are
usually painful; defecation and coitus may be noxious in

640

Part III I KINESIOLOGY OF THE HEAD AND SPINE

some patients. Muscle pull from the gluteus maximus and
anococcygeal musculature may predispose some fractures
to nonunion and a coccygectomy may be indicated
[119,171]. The procedure is, however, highly controversial
and not without risk [11,127].

Sacroiliac Joint (SIJ)

The joints of the pelvic closed kinematic chain have been
steeped in controversy since they were first described by
Meckel in 1816 [102]. The controversy, involving the SIJ pre¬
dominately, has raged for centuries and centers around sev¬
eral arthrological features, most significantly its classification,
cartilage type, innervation, propensity for movement, and
predilection for causing pain. Interest in these joints was gen¬
erated first by obstetricians, who measured changes in pelvic
diameters with different body positions [33,84,167]. With the
passage of time, the joints uniting the pelvic ring gained a
peripheral place in trauma medicine and rheumatology, but
the attention devoted to them as a primary source of mechan¬
ical dysfunction and pain has waxed and waned [59]. The SIJs
especially fell into disfavor as producers of pain in 1934 when
Mixter and Barr demonstrated the key role of the IVD in
back pain [110].

Protagonists of the argument that the SIJs are capable of
motion and producing pain contend that this joint, being syn¬
ovial, has a nerve and vascular supply consistent with other

joints of its type, is subject to inflammation and radiographi¬
cally measurable degeneration [47,103], and is capable of lim¬
ited motion and therefore subject to mechanical dysfunction
[5,43,44,59,89,151,173,175]. Others point out the necessity
for clinically significant motion somewhere in the pelvic ring
to permit widening of the pelvis during delivery [163,176].
Antagonists of these arguments contend that motion at the
joint is nearly impossible, considering the complexity of its
topography, the magnitude of force required for its disrup¬
tion, the referred nature of the pain attributed to it, and
the flawed nature of the analyses of its motion [47,176].
Clearly, this is an important issue for clinicians, for without
motion there will be no dysfunction and no need for manual
therapy techniques. What facts, then, can be brought to bear
on this enigmatic joint?

STRUCTURE

For centuries, the SIJ was classified variably as a cartilaginous
joint (amphiarthrosis) [56,71], a synchondrosis that is ulti¬
mately replaced by bone [55], a diarthroamphiarthrodial joint
[160], and a cross between a synarthrosis and diarthrosis
[129]. Some concluded that the joint is synovial (diarthrodial)
but becomes an amphiarthrosis under certain pathological
conditions [20,138]. Researchers as early as the 18th and
19th centuries demonstrated that the SIJs are true synovial
joints consisting of a joint cavity, synovial membrane, and fluid
[40,84,105,165]; nonetheless, some continued to refer to the
joint as an amphiarthrosis [50]. In the first three quarters of
the 20th century, the SIJ was considered exclusively synovial

Iliac auricular
surface

Iliac and sacral Sacral auricular

Figure 35.30: Auricular surfaces and tuberosities of the ilium (A) and sacrum (B) form the sacroiliac joint. The joint has been opened,
as a book, to expose each of the bony surfaces that participate in the joint. Sacral segments are numbered 1-5.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

641

[2,32,36,48,52,125,129,142,159,172], a classification reflected
by that assigned to it in the 1983 N.A. [45]. The trend today
is to include both the iliac and sacral auricular surfaces
and tuberosities in the make-up of the SIJ [21,58,59,
147,169]. That is, the auricular surfaces form a synovial
joint, with a capsule and a cavity filled with fluid, and the
tuberosities, connected by an interosseous ligament, consti¬
tute a fibrous form of a synarthrosis (Fig. 35.30). Although
the synovial part of the joint is usually classified as plane [57],
its joint surfaces are far from flat or smooth. More prevalent
in males than females [161], accessory SIJ articulations are
formed frequently from posteriorly positioned supernumer¬
ary articular facets [19,44, 64,142,149,161].

The anteroinferior auricular surfaces are complementary
L-shaped surfaces, while the tuberosities are paired irregular,
pitted areas positioned posterosuperior to the auricular sur¬
faces. The tuberosities are connected by the massive
interosseous sacroiliac ligament (Fig. 35.31). The L-shaped
auricular surfaces have two limbs that point posteriorly and
embrace a dorsal concavity [19,149]. The shorter, more verti¬
cally oriented cephalic limb consists of the first segment on
the sacral side, while the caudal limb is longer, more horizon¬
tal, and composed of the second and third segments on the
sacral side. The auricular surfaces are strikingly variable, how¬
ever, both between subjects and from side to side within the
same subject; shapes include C- or L-shaped; limbs are long
or short and equal or unequal; and the angle formed between
the limbs is obtuse or perpendicular [19,21,142,149]. The
overall orientation of the joint is obliquely vertical, making it
the only weight-bearing joint that is not transverse to the
transfer of weight [106].

By all accounts, the cartilage covering the sacral auricular
surface differs from that of the ilium. In the vast majority of

Dorsal sacroiliac ligament

Figure 35.31: Horizontal section through the SIJ, a synovial artic¬
ulation enclosed by a joint capsule and a fibrous articulation at
the interosseous ligament. The short, stout ISIL is the major con¬
tributor to sacroiliac integrity.

reports, the sacral cartilage is hyaline and the iliac cartilage is
fibrous [19,131,138,142,159,170]. The one notable and
definitive exception is by Paquin et al. [122]. Using micro¬
scopic and biochemical methods, they examined the extra¬
cellular matrix of cartilage from sacral and iliac auricular sur¬
faces, as well as that from femoral condyles, for comparison.
Based on each samples metachromatic staining (indicating
high glycosaminoglycan content), collagen fibril diameter,
and exclusive presence of type II collagen peptides [79], they
concluded that both sacral and iliac cartilages are hyaline.
Cartilage from the two sites differs, however, in the organi¬
zation of the collagen fibers at the superficial versus the mid¬
dle and deep zones of the cartilage layer. The latter findings
are consistent with other reports indicating a denser aggre¬
gate of collagen fibers between chondrocytes in the iliac car¬
tilage than in the sacral side [19,138,170], and probably
explain why, using only light microscopic assessment, most
past researchers concluded that iliac cartilage is fibrous and
not hyaline. The findings of Paquin et al. should not be all
that surprising, the literature notwithstanding, when one
considers the development of synovial joints. Bones that
develop from a cartilaginous anlage (model) have their artic¬
ulating surfaces covered with hyaline cartilage, while mem¬
brane bones develop fibrocartilage at these sites [147]. Both
the sacrum and ilium form by endochondral mechanisms,
and therefore, their articular surfaces should be covered by
hyaline cartilage.

In addition to the differences in their collagen fiber organ¬
ization, sacral and iliac auricular cartilages are dissimilar in
gross appearance, thickness, and the extent to which they
undergo degenerative change across their life span. Sacral
cartilage is smoother and two to five times thicker than iliac
cartilage [19,21,25,122,142,147,163]. The development of the
SIJ and its age-related degenerative changes have been well
documented [19,20,96,131,138,142,163,164]. The joint
develops somewhat differently from other synovial joints,
because the ilium significantly antedates the sacrum in devel¬
opment [31,147]. In addition, joint cavitation, which is com¬
plete by 12 weeks in most synovial joints [118], begins later
and progresses more slowly in the SIJ [142]. A joint cavity
appears in the mesenchymal mass between the sacrum and
ilium by 7 weeks of intrauterine life, but it does not reach its
full extent until 7 or 8 months; a joint capsule is lined by a syn¬
ovial membrane at 37 weeks [19,31,142]. At birth, joint sur¬
faces are flat and smooth, and the capsule is thin and pliable
[19]. During the first 10 years, the auricular surfaces remain
flat and, along with a still-pliant capsule, permit gliding
motions in all directions. In the teen years, the capsule thick¬
ens and complementary unevenness starts to develop on the
two auricular surfaces [19,142]. By the early twenties, a con¬
vex iliac ridge and a concave sacral depression have formed;
they run centrally along the length of the joint surface
[19,149]. Although the congruency of the opposing joint sur¬
faces is usually high, eminences are more frequent on the
ilium and “almost every conceivable combination of grooves,
ridges, eminences, and depressions” [142] is apparent.

642

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Auricular surfaces of females are smaller and flatter than
those in males [20,149,151,163].

Starting in the middle of the third decade, surfaces of the
synovial part of the SIJ begin to show signs of degeneration
[19]. Joint degeneration progresses from the fourth through
the eighth decade and is characterized by thickening and stiff¬
ening of the capsule, severe loss of cartilage thickness, sub¬
chondral bone erosion, increasing surface irregularity, intraar-
ticular fibrosis of joint surfaces and, in a few individuals, total
ankylosis. The degenerative changes that develop on the iliac
side appear first and are more severe than those on the sacral
side [19,20,168]; furthermore, they appear at an earlier
age and advance more rapidly in males than females
[20,21,29,96,131,151,168]. One author [138] reports severe,
advanced degenerative changes in over 90% of the SIJs from
aged males (over 80 years old).

SUPPORTING STRUCTURES OF THE SIJ

The SIJ is reinforced by some of the strongest and most mas¬
sive ligaments in the body [6,147,169,175]. Three ligaments
are in intimate contact with the joint, and three others,
though better termed “accessory,” make important contribu¬
tions to the joint s integrity.

The SIJ capsule is closely attached to the joints margins;
ventral and dorsal sacroiliac ligaments (VSIL, DSIL) cross
the joint, and the interosseous sacroiliac ligament (ISIL)
connects the sacral and iliac tuberosities (Figs. 35.31 , 35.32).
The VSIL is little more than a thickening of the anterior joint
capsule; the cranial part is thin and reinforced by iliolumbar
ligament fibers, while the caudal half is well developed below
only as far as the iliac arcuate line [147,159]. It assists the sym¬
physis pubis in resisting separation or horizontal movement of
the innominate bones at the SIJs. The DSIL is heavier and

Dorsal sacroiliac
ligament

Sacrotuberous

ligament

Sacrospinous

ligament

Sacrospinous

ligament

Sacrotuberous

ligament

Lumbosacral

ligament

Ventral

sacroiliac

ligament

Figure 35.32: Ligaments of the sacroiliac joint. A. Dorsal view. B. Medial view. C. Ventral view.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

643

more extensive than its companion on the ventral surface and,
for descriptive and functional purposes, is divided into short
and long fibers. Short DSIL fibers are deep and pass infero-
medially from the PSIS to the back of the lateral part of the
first and second sacral segments. Positioned more superficially,
fibers of the long DSIL connect the PSIS to the same area of
the third and fourth sacral segments; these fibers are continu¬
ous inferolaterally with the sacrotuberous ligament and super-
omedially with the posterior lamina of the thoracolumbar
fascia [147,159,172]. During incremental loading of the
sacrum, the DSIL becomes tense when the base of the sacrum
moves backward (counternutation) and slackens with move¬
ment in the opposite direction (nutation) [162]. This is oppo¬
site to the tension that develops in the sacrotuberous ligament
during movement of the sacrum in the sagittal plane. The ISIL
is the major bond between the posterior two thirds of the joint
[147]; it connects and fills the space between the sacral
and iliac tuberosities. Dorsal primary rami of spinal nerves and
blood vessels ramify between the short fibers of the DSIL and
the ISIL (Fig. 35.33).

The accessory (vertebropelvic) ligaments are the iliolum¬
bar ligament (described earlier in this chapter) connecting L5
to the ilium, and the sacrotuberous and sacrospinous liga¬
ments, passing from the sacrum to the ischium (Fig. 35.32).
The sacrotuberous ligament blends with the DSIL as it fans
out from the ischial tuberosity, moving upward and medially
toward the PSIS, lower sacrum, and coccyx; some of its fibers
extend along the ischial ramus as the falciform process. Deep
fibers of the gluteus maximus originate from the dorsal sur¬
face of this ligament; biceps femoris fibers attach to it as both
muscle and ligament fibers anchor themselves to the ischial
tuberosity [159]. Deep to, and blended with, the sacrotuber¬
ous ligament at its medial attachments, the sacrospinous liga¬
ment passes from the ischial spine to the lower sacrum and
coccyx. On its deep surface, fibers of the coccygeus (part of

Figure 35.33: Dorsal primary rami of sacral nerves 1-4 ramify
between the fibers of the dorsal sacroiliac and ISILs and inner¬
vate the joints.

the pelvic diaphragm) blend with it. Both the sacrotuberous
and sacrospinous ligaments convert the greater and lesser sci¬
atic notches into foramina and resist forward movement of
the base of the sacrum under load.

MOTION

Inarguably, the most contentious issue regarding the SIJ is
related to the presence, degree, and type of motion available
to this joint. The argument has raged for over 2000 years!
Hippocrates (460-377 bc) is credited with being first to
believe that the SIJ is capable of motion [21,30], but only dur¬
ing pregnancy. Duncan [40,41] was first to provide indirect
evidence of SIJ mobility after observing the surface morphology
of the joint; he postulated that the sacrum rotated (nutated or
nodded) around a horizontal axis in the vicinity of the iliac
tuberosities. Later, gynecologists made manual measure¬
ments of pelvic diameters [84,167]; they attributed the reduc¬
tion in conjugate diameters from a recumbent to a standing
posture to motion at the SIJ. Goldthwaite and Osgood were
the first to suggest that the presence of motion at the SIJs was
a normal condition for both men and women [52,54] and that
the SIJ is a primary source of pain independent of pregnancy
[53]. Colachis et al. [30] were the first to make direct meas¬
urements of SIJ motion in living subjects; they observed
movement of Kirschner wires embedded in the PSISs of the
pelvis in nine different body positions.

A multitude of studies designed to assess SIJ mobility were
performed during the 19th and 20th centuries; methodolo¬
gies include morphological observation [19,20,41,149,172],
clinical observation [14], mechanical testing [54,92,108],
inclinometry [125], conventional radiography [46,173], cin¬
eradiography [133], computed tomography [145], stereoradi¬
ography [46,132,154], stereophotogrammetry [43], hologra¬
phy [166], the Metrecom skeletal analysis system [144,145],
mathematical modeling [51], biomechanical modeling [153],
and theoretic consideration [81,176]. Table 35.6 summarizes
findings from a sampling of 20th century studies performed
to assess motion of the sacrum relative to the innominates,
motion of the innominates relative to one another, and the
axes of these motions.

Perusal of the voluminous literature on the subject of SIJ
mobility leads to several conclusions:

• SIJs are capable of small amounts of motion.

• Rotation of the sacrum in the sagittal plane between the
two innominate bones ranges from 1 to 8°, with the mean
between 2 and 3°.

• Translation of the sacrum caudally between the two
innominate bones ranges from 0.5 to 8 mm, with a mean
between 2 and 3 mm.

• Tremendous variation exists in the amount of motion
reported to be available to the SIJs and probably results
from a variety of contributive factors, including age, sex,
joint surface topography, side-to-side asymmetries in joint
structure, ligamentous integrity, degree of joint degenera¬
tion, and last but not insignificant, measurement error.

644

Part III I KINESIOLOGY OF THE HEAD AND SPINE

TABLE 35.6: Movement of Sacroiliac Joint

Author(s)

Method(s)

Subjects

Joint Motion Conclusions

Pitkin and

Pheasant

1936

Inclinometry

Living subjects

Unilateral antagonistic movement of the ilium around
transverse axis through the symphysis pubis averaged

11° (3-19°), or 5.5° on each side

Strachan et al.

1938

Mechanical testing of sacral
rotation

Cadavers

During trunk movements, sacral rotation was 1-5°
when one ilium was immobilized and the other was
fixed to the sacrum

Weisl 1955

Movement of sacral
promontory via radiography

Living subjects

Max ventral movement of the sacral promontory was

5.6 ± 1.4 mm with standing from recumbent Axis of
angular movement was 5-10 cm below the sacral
promontory

Mennell 1960

Changes in distance between

PSISs via palpation

Living subjects

PSISs came 0.5 in. closer in horizontal plane

Colachis et al.

1963

Measured distance between

Kirschner wires implanted in PSISs

Living subjects

Maximum movement of PSISs was 5 mm with flexion
from standing

The axis was not fixed

Kapandji 1974

Theorized based on writings of
Farabeuf and Bonnaire

None

In nutation the ilia approximate and the iliac
tuberosities separate

Opposite in counternutation

Frigerio 1974

Biplanar radiography

Cadavers and
living subjects

Maximum movement between ilium and sacrum was

12 mm (mean ~2.7 mm)

Maximum movement between innominates was 15.5 mm

Egund et al.

1978

Roentgen stereophotogrammetry

Living subjects
with hypo-
or hypermobile SIJs

Maximum rotation was 2°

Axis of sacral rotation was through the iliac tuberosities
at the level of S2

Translations were ~2 mm

Wilder et al.

1980

Theoretical best-fit axes of
rotation based on topographic
analysis of joint surfaces

Dried bony
specimens

Joint rotation cannot occur exclusively about any
previously proposed axis

An important function of the SIJ may be to absorb energy

Reynolds 1980

Stereoradiography

Cadaver

Sacral rotations were 1-2°

Miller et al.

1987

Mechanical testing with one
or both ilia fixed

Cadavers

Both ilia fixed: 1.9° rotation, 0.5 mm translation One ilium
fixed: rotation 2-7.8x greater and translation

3x greater

Scholten et al.

1988

Biomechanical model

Model

Model relative pelvic motions rarely exceeded 1-2°
rotation and 3 mm translation

Sturesson et al.
1989

Stereoradiography

Living subjects

Mean rotation 2.5° ± 0.5°

Mean translation 0.7 mm (0.1-1.6 mm)

Smidt et al.

1995

Metrecom skeletal analysis
system

Living subjects

Composite sacroiliac motion (relative motion between

R/L innominates) was 9° ± 6.5° in oblique sagittal plane
and 5° ± 3.9° in transverse plane

Smidt et al.

1997

Computed tomography

Cadavers

Sagittal plane sacral rotation was 7-8°

Translation was 4-8 mm

PSISs, posterior superior iliac spines; SIJ, sacroiliac joint.

• In the absence of trauma, the greatest amount of SIJ
motion is present in the young, especially the young preg¬
nant female [12].

• The physiological and clinical significance of SIJ motion
has been, in the main, ignored by all except obstetricians
and clinicians who regularly deal with SIJ syndromes.

In general, three types of motion are available to the
innominate bones: symmetrical motion is movement of
both innominates as a unit in relation to the sacrum; asym¬
metrical motion consists of antagonistic movement of each
innominate bone with relation to the sacrum, which includes

movement at the symphysis pubis; and lumbopelvic motion
consists of rotation of the spine and both innominates as a
unit around the femoral heads.

Symmetrical Motion

Symmetrical trunk and hip movements result in paired, sym¬
metrical movements at the SIJs [43,138,173,175]. During
trunk flexion or bilateral hip flexion, the sacrum nutates (L.
nutatio, nodding) or rotates anteriorly, so that the promontory
moves ventrocaudally while the apex moves dorsocranially
(Fig. 35.34). The sacrum countemutates, or moves in the

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

645

Figure 35.34: Sagittal plane motion of the sacrum. In nutation,
the base of the sacrum moves ventrocaudally and its apex moves
dorsocranially; this occurs when the sacrum is loaded from
above, in trunk flexion, or in bilateral hip flexion. The base of
the sacrum moves in the opposite direction during trunk exten¬
sion and bilateral hip extension, when it counternutates.

opposite direction, during trunk extension or bilateral hip
extension. Nutation and countemutation are accompanied by
several millimeters of translation. In this type of motion, the
innominate bones move symmetrically, as a unit, in the absence
of motion at their anterior union, the symphysis pubis [67].

The sacral base always moves further than the apex [173].
Furthermore, it occurs about an instantaneous axis located
5-10 cm below the sacral promontory (Fig. 35.35). The com¬
bination of rotation and translation is angular movement of the
sacrum, during which the iliac crests move closer together
while the iliac tuberosities move further apart (Fig. 35.36).
Sagittal plane angular sacral movement is essentially the same
in males as in females, except during pregnancy, when it
increases in females. The greatest amount of movement, as
much as 5.6 ± 1.4 mm, occurs when going from recumbent to
standing and reverses in direction when moving from standing
to recumbent [173]. Rotation is accompanied by translation,
which results in increased ligamentous tension and absorption
of energy [176]. The SIJs thereby function as shock absorbers.

Asymmetrical Motion

A second type of motion occurs at the SIJs when asym¬
metrical forces are applied to the pelvis, as in static one-
legged stance and the one-legged stance that occurs during
gait and asymmetrical falls. Application of unbalanced
forces to the pelvis results in asymmetrical and antagonis¬
tic movements at the SIJs [20,125,175], resulting in pelvic

Farabeuf

Figure 35.35: Medial view of the innominate bone shows three
primary sites proposed as the location of the axis of rotation
between the sacrum and the ilium.

torsion. These movements are always accompanied by
movement at the symphysis pubis [67,125], despite the
fact that very little movement occurs in the symphysis
pubis, except in pregnancy. The experienced clinician can
assess manually the end position that results from

Figure 35.36: Movement of the innominate bones during nuta¬
tion of the sacrum. The ilia move closer together, and the ischia
move farther apart.

646

Part III I KINESIOLOGY OF THE HEAD AND SPINE

movement of one innominate bone relative to the other
by palpating the relative prominence of the right and
left ASISs and PSISs [33,39,58,90,99,104,152,179]. For
instance, if the left ASIS moves upward, the right ASIS
and the left PSIS become more prominent while the left
ASIS and right PSIS become less prominent (Fig. 35.37).
Alternating, asymmetrical forces are applied transiently to
the pelvis during each gait cycle [20]. The proposed axis
for pelvic torsion is transverse and passes through the sym¬
physis pubis [125], though this remains equivocal.
Abnormal mobility or instability in either the SIJ or sym¬
physis pubis often is accompanied, however, by a second¬
ary stress lesion in the other [69].

Lumbopelvic Rhythm

The lumbar spine and innominate bones can also move as a
unit. Inasmuch as movements of the spine are coupled with
those of the pelvis, a lumbopelvic rhythm (discussed in
Chapter 32), similar to the scapulothoracic rhythm, has been
postulated [22] (Fig. 35.38). The specific rhythm varies among
individuals, but flexion of the trunk from standing combines
flexion of the lumbar vertebrae and at the lumbosacral junction
with forward rotation of the pelvis on the fixed femora
[3,139]. Disturbances in the lumbopelvic rhythm can
contribute to low back pain [3,121,139].

Figure 35.37: Application of unbalanced forces on the pelvis, as
in static one-legged stance on the left, results in asymmetrical,
antagonistic movement at the SIJs along with movement at the
symphysis pubis. This type of movement can be assessed clinically
by palpating movement of the ASIS and PSIS.

Figure 35.38: Common lumbopelvic rhythm. A. Normal standing posture. B. During the first 45° of trunk flexion, most motion results
from lumbar and sacral flexion causing the sacrum to nutate and the lumbar curve to flatten. C. In extreme trunk flexion, the lumbar
spine continues to flatten and the pelvis rotates about the femoral heads, while the sacrum paradoxically counternutates.

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

647

Clinical Relevance

INFLUENCE OF HORMONES ON MOTION: Several of
the sexually dimorphic features of the SIJs have already
been mentioned; none are more functionally and clinically
significant than those associated with pregnancy and
childbirth. Obstetricians were the first clinicians to show
interest in the pelvic joints, noting that during pregnancy
and for a period of time following delivery; these joints
became more mobile [16,95,105,167]. The pregnancy-
induced increase in pelvic mobility lasts up to 4 months
postpartum, and the joints become more stable with invo¬
lution of the uterus [20].

The changes in pelvic joint mobility are related to ligamen¬
tous relaxation stimulated by increased levels of circulating sex
hormones during pregnancy and, albeit to a lesser extent, dur¬
ing menstruation [26,39,125]. The hormonally induced
changes in pelvic mobility have been confirmed radiographi¬
cally [1,146,181]. Increased levels of sex hormones as well as
the peptide hormone relaxin produced by the corpus luteum
during pregnancy and menstruation are credited by some with
the relaxation of pelvic joint ligaments [23,85,97,128].
Controversy continues, however, regarding the exact role of
relaxin in contributing to ligamentous relaxation, since serum
levels of relaxin do not always correlate with increased periph¬
eral joint laxity [140,174].

Whatever the cause, relaxation of SIJ ligaments results in
a less effective interlocking mechanism between the sacrum
and ilia, thereby permitting freer movement at the SIJs and,
ultimately, an increase in the diameter of the pelvis. This
hormonal effect is not limited to the ligaments of the SIJ; the
sacrococcygeal joint and symphysis pubis are affected as
well. The result of relaxation at all three sites is a 10-15%
increase [113] in the diameter (predominately transverse) of
the pelvis and facilitation of movement of the fetus through
the pelvic canal [81,95,126,147]. Expediting parturition by
increasing pelvic joint mobility does not come without a
price, however. That price, according to some, is greater tor¬
sional and shear stress, particularly at the SIJ, during preg¬
nancy and for a period of time each month around the
menses [3[. In contrast, some researchers and clinicians are
convinced that the periodic and cyclical increase in mobility
at the female SIJ favors slower development of degenerative
changes in the joint [20,138].

INNERVATION

Descriptions of the innervation of the SIJ vary. Hilton’s law of
nerves states that the nerve that innervates a particular mus¬
cle also innervates the joint moved by that muscle [158]. This
leads to a quandary On the one hand, the SIJ should have a
rich nerve supply, considering its classification as a synovial
joint. On the other hand, however, based on the above law, the

question is raised as to whether it is innervated to a significant
degree, since no muscles are intrinsic to the joint or act
on it directly [3,57,59,175]. The classical anatomy texts
[147,159,170] and various authors [15,59,74,76,77,87,111,124,
149,177] report innervation of the SIJ from a variety of com¬
binations of cord segments and peripheral nerves; the range
for the dorsal part of the joint is by branches of dorsal primary
rami of L5-S1-2 and for the ventral part of the joint by the
superior gluteal and obturator nerves and branches of ventral
primary rami of L4-5-S1-3. More-recent assessment of the
joints innervation, however, using a triad of techniques that
include gross and microscopic dissection, routine histology,
and immunocytochemistry, on both adult cadaveric and abort¬
ed fetal specimens, reveals that the SIJ is innervated exclu¬
sively by fine branches from the dorsal primary rami of SI-4
(Fig. 35.33) [62,83]; no branches from the sacral plexus, obtu¬
rator nerve, or superior gluteal nerve are revealed on the ven¬
tral surface of the joint, in spite of the intimate relationship
between the two as the upper part of the lumbosacral plexus
crosses the caudal part of the joint ventrally and is anchored
there by fibrous connective tissue [42]. Unmyelinated, finely
myelinated, and thickly myelinated fibers are evident, indicat¬
ing the potential presence of the full spectrum of joint recep¬
tors, including those that are activated by painful and mechan¬
ical stimuli [13,66,74,87,111].

Clinical Relevance

INNERVATION OF THE SIJ: Documentation of the Sirs
synovial classification and demonstration of its abundant
nerve supply have important clinical application. Clearly,
this joint can be a generator of pain. Reports in the litera¬
ture regarding the precise source of nervous elements to
and from the SIJ, however, remain contradictory. On the
one hand, one must consider the possibility of individual
variation in the innervation pattern of the SIJ. On the other,
innervation variability may explain the inconsistent pattern
of pain referral observed in individuals with disorders of the
SIJ [62,83].

Symphysis Pubis

The symphysis pubis is a median joint that consists of a
pair of oval bony surfaces joined by a fibrocartilaginous
disc (pubic disc) and reinforced by a pair of closely adher¬
ent ligaments (Fig. 35.39). The symphyseal surfaces of the
pubic bodies, shorter and broader in the female than the
male, are covered by a thin layer of hyaline cartilage as
they project toward one another in the anterior midline to
form this fibrocartilaginous amphiarthrosis [147,159].

648

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 35.39: Anterior view shows that the symphysis pubis is
reinforced by the superior pubic ligament and the arcuate pubic
ligament.

Upon removal of the hyaline cartilage, a subchondral con¬
tour is exposed that, like the SIJ, consists of complementary
ridges and papillae capable of resisting shearing forces.
The consistency with which the ridges and papillae increase
with age enables forensic scientists to age skeletons on the
basis of irregularities of the symphyseal surfaces of pelvic
specimens [120,155]. The pubic disc is firmly anchored to
each of the hyaline-covered symphyseal surfaces and is usu¬
ally thicker anteriorly than posteriorly [159]. Thicker over¬
all in the female than the male, the reported normal range
is 4-10 mm [175]. In half of specimens, the disc contains
an incomplete nonsynovial cavity that rarely appears
before the beginning of the second decade and is better-
developed in females [147,170]; the cavity may be an area
of resorption [147].

Two ligaments reinforce the joint: a superior pubic liga¬
ment between the two pubic tubercles crosses and is firmly
adherent to the disc superiorly; a more robust arcuate (infe¬
rior) pubic ligament borders the pubic arch as it passes
along the discs inferior edge, between the inferior pubic
rami. Several layers of interlacing collagenous fibers derived
from aponeuroses of the rectus abdominis and obliquus exter-
nus abdominis reinforce the disc anteriorly Like other
amphiarthrodial joints, the symphysis pubis is poorly inner¬
vated. Plexuses of afferent nerve terminals penetrate the
periphery of the pubic disc only, derived from one or more of
the pubic branches of the iliohypogastric, ilioinguinal, and
genitofemoral nerves of the lumbar plexus [147].

In most circumstances, movement at the symphysis
pubis is slight [147]. Based on elementary physics, however,
movement of the symphysis pubis must accompany any
unpaired antagonistic movement at one SIJ, unless the axis
of SIJ motion is a transverse one that runs through the body
of the pubis [125] (Fig. 35.40). If the postulated axis does,
indeed, pass through the pubic body, unilateral innominate
sagittal plane motion would result in only slight torsion at
the symphysis pubis.

Figure 35.40: Transverse section through the pelvis shows that any
transverse movement of the pubes away from each other could
contribute to sacroiliac instability. Unpaired antagonistic move¬
ment at the SIJs would necessitate some symphysial movement.

Clinical Relevance

SYMPHYSIS PUBIS DYSFUNCTION: Symphysis pubis
disruption can occur during pregnancy and for a period of
time postpuerperium (the period of 42 days foiiowing child¬
birth) [168,178,1811 following obstetric and other traumas
[17,54,65,78,95], and in athletes, because of repetitive trau¬
ma [69,73]. Loss of symphyseal integrity with resultant
hypermobility, if significant, contributes further to SIJ insta¬
bility, even more so in women already experiencing hor¬
monally stimulated SIJ ligamentous laxity [150,169,175].
Malalignment of the symphysis pubis is readily identified
radiographically [130]. Horizontal separation of the symph¬
ysis pubis in excess of 10 mm and vertical movement of one
pubic body in relation to the other by more than 5 mm is
considered pathological [65].

Pathology or Functional Adaptation?

In closing this chapter on pelvic osteology and arthrology, it is
appropriate to raise the question of whether certain features
of pelvic joint structure and degeneration are functional adap¬
tations or changes in a continuum of changes that lead to
pathology. Although the evidence is not unequivocal, several
authors support the former, that is, that certain sexually

Chapter 35 I STRUCTURE AND FUNCTION OF THE BONES AND JOINTS OF THE PELVIS

649

dimorphic features of the pelvis represent functional adapta¬
tions to the different roles of the pelvis in males and females
[20,21,25,138,163,164].

Two types of evidence support this premise. The first is that
the auricular surfaces of SIJs in males have a coarser texture
and more ridges and depressions than those in females. Even
in advanced age, the ridges and depressions are complemen¬
tary and generally are covered by intact cartilage [163].
Furthermore, these surface irregularities do not appear until
puberty [20], marked by an increase in weight during the
growth spurt of adolescence [163]. Furthermore, the male SIJ
would be subject to greater torque and therefore higher loads
because a male s center of gravity passes more ventral to the
SIJ than does the females [163]. Since these features enhance
friction [164], a reasonable conclusion might be that they rep¬
resent adaptations to the increased body weight and work load
generally experienced by males.

The second type of evidence is that the degenerative
changes observed in SIJs are more prevalent, are more exten¬
sive, and occur earlier in males than in females. As indicated
earlier, the SIJ undergoes a progressive loss of mobility with
increasing age, with some joints becoming completely or par¬
tially ankylosed. In one study of 210 specimens, ankylosis in
advanced age was limited to male joints (n = 105), where the
incidence was 37%; none of the 105 female joints in this study
was ankylosed [20]. In the female, apparently, strength at the
SIJ is sacrificed for mobility. The joints ligaments and capsu¬
lar elements remain comparatively lax in adaptation to its
function (i.e., to allow an increase in the pelvic diameter and
thereby facilitate the vaginal delivery) [25,169].

SUMMARY

This chapter examines the osteological and arthrological fea¬
tures of the bony pelvis that contribute to its ability to provide
a stable support for the body weight while allowing for suffi¬
cient motion for the functional requirements of the trunk and
lower limbs. The fifth lumbar vertebra is the most robust and
angular of the lumbar vertebrae. Its coronally aligned inferior
facets and the thick iliolumbar ligaments support the lum¬
bosacral junction against large anterior shear forces. The SIJs
are inherently more stable but allow systemic motion, includ¬
ing rotation and translation. Strong ligamentous support and
irregular joint surfaces limit mobility and stabilize the SIJ.
The pubic symphysis exhibits slight mobility.

The high degree of sexual dimorphism apparent in the
bones of the pelvis is described and related to the require¬
ments of childbirth in the female. The female pelvis exhibits
a larger medial-lateral diameter and a more posteriorly
angled sacrum. Clinically relevant osteological and arthrolog¬
ical variations, malformations, gender differences, and age-
related changes of the bony pelvis are detailed, with an
emphasis on the controversy involving the SIJ, most signifi¬
cantly its classification, cartilage type, innervation, propensity
for movement, and predilection for causing pain.

The following chapter discusses the muscular, nervous,
and visceral structures that contribute to the numerous and
distinct functions of the pelvis.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity in the Pelvis

EMILY L. CHRISTIAN, P.T., PH.D.

JULIE E. DONACHY, P.T., PH.D.

CHAPTER CONTENTS

DEVELOPMENTAL ANATOMY OF THE PELVIC FLOOR.655

MUSCLES OF THE PELVIS AND PERINEUM .656

Pelvic Muscles Associated with Somatic Function.656

Pelvic Muscles Associated with Visceral Function.656

Perineal Muscles .658

Somatic (External) Sphincters.658

Functional and Metabolic Properties of Pelvic and Perineal Muscle Fibers.660

Central Tendon of the Perineum.663

NERVOUS CONTROL OF MUSCLES OF THE PELVIS AND PERINEUM.664

Spinal Centers .664

Supraspinal Centers.665

Degenerative Diseases .665

SPECIFIC FUNCTIONS OF PELVIC AND PERINEAL MUSCULATURE.667

Urinary Continence and Micturition .667

Anorectal Continence and Defecation .669

Sexual Function.670

Parturition.671

PELVIC MUSCLE DYSFUNCTION.671

Pelvic Organ Prolapse.671

Urinary Incontinence.672

Anorectal Incontinence.672

Role of the Therapist in Management of Pelvic Floor Dysfunction .672

SUMMARY .672

T he caudal end of the trunk in an upright animal is, of necessity, closed by several layers of pelvic and perineal
fascia and striated muscle. Although the fascial contributions to the floor of the pelvic cavity are equal in
importance to those made by the muscles from a clinical perspective, this chapter focuses on the muscles'
role. Three layers of muscle are positioned at the caudal end of the human funnel-shaped pelvic basin; from deep to
superficial, they are the pelvic diaphragm, deep perineal muscles, and superficial perineal muscles. Their various func¬
tions include closing the pelvic outlet, permitting transit of the pelvic effluents (urethra, anal canal, and vagina) and
controlling their apertures, supporting the pelvic organs, regulating intraabdominal pressure, and contributing to
bowel, bladder, and sexual function. Two other muscles contribute to the walls of the pelvic cavity, the piriformis and
obturator internus. Any contributions they make to the pelvic cavity, however, are purely ancillary.

654

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

655

Although the muscles of the pelvic floor and perineum are primarily striated and under the control of the somatic nerv¬
ous system, they differ from other axial or appendicular striated muscles in several ways:

■ They frequently contain or blend with structures containing smooth muscle, and subsequently, the com¬
bined fiber types receive a visceral (autonomic nervous system, ANS) innervation in addition to a somatic
(somatic nervous system, SNS) innervation.

■ They are concerned with visceral functions (micturition, defecation, sexual function, parturition, and sup¬
port of pelvic organs).

■ They are innervated by lower motor neurons (LMNs) that are controlled by a special contingent of brain¬
stem and hypothalamic fibers, allowing them to function somewhat independently of conscious cortical
control.

■ Their contraction does not result in movement at a joint.

The special nature of these multifunctional muscles and the problems associated with their dysfunction should become

obvious to the reader following their further description. The specific objectives of this chapter are

■ To discuss clinically relevant aspects of the development and anatomy of the pelvic floor

■ To describe details of the structure, innervation, and function of the muscles of the pelvic diaphragm

■ To describe details of the structure, innervation, and function of the muscles of the perineum

■ To discuss neurological aspects of pelvic floor and perineal function

■ To describe pertinent aspects of the structure and innervation of pelvic and perineal visceral structures,
as well as ways in which their functions are coordinated with those of striated muscle fibers of the pelvic
floor and perineum

■ To discuss the various dysfunctions of the muscles of the pelvis and perineum relative to aging, gender,
nervous degeneration, muscle atrophy, and vaginal delivery

DEVELOPMENTAL ANATOMY
OF THE PELVIC FLOOR

Two distinctly human characteristics necessitated changes in
the pelvis of Homo sapiens. One is that humans give birth to
fetuses with uniquely large heads that require wider bony
channels of passage to the exterior. The other is assumption
of the upright posture. Both characteristics are reflected in
changes in the bony pelvis and fibromuscular pelvic floor at
the pelvic outlet, that is, the tissues interposed between the
pelvic cavity and the perineum that form one of several
transverse diaphragms in the trunk. Traditionally, anatomists
have defined the pelvic floor as the fascial and muscular lay¬
ers of the pelvic diaphragm, consisting of the levator ani,
coccygeus, and their associated fasciae (see below). Some cli¬
nicians include the perineal musculature and fasciae (par¬
ticularly the urogenital diaphragm) in the pelvic floor
because they are intimately related, anatomically, neurologi-
cally, functionally, and clinically. For the sake of conceptual¬
ization, Moore [72] suggests that the pelvic diaphragm be
considered the main floor and the urogenital diaphragm the
subflooring, thereby suggesting morphological and func¬
tional similarity while maintaining autonomy. In the follow¬
ing discussions, pelvic and perineal musculature are
considered separate entities.

Little is known about the development of pelvic floor mus¬
cles in humans. Although they are thought to develop from
somites in a manner similar to those of the anterolateral trunk
[19], they bear little resemblance to trunk muscles. The argu¬
ment is offered that during human development, after all but
3 or 4 of the original 8 to 10 coccygeal segments degenerate
[63,91,105], the remaining segments are moved into a posi¬
tion to close the pelvic outlet. This notion is not accepted by
Wilson [120], however, who believes that the muscles of the
pelvic floor develop independently, with specific attachments
and functions.

Changes in the pelvic floor are necessitated by the upright
posture of humans. Specializations in the soft tissues of the
human pelvic floor are essential for both supporting the
weight of abdominopelvic viscera and regulating intraabdom¬
inal pressure. Some argue, in fact, that pelvic floor changes
are essential for assumption of the upright posture [26,28].
Alterations in the specific fascial and muscular elements were
developed to resist increases in intraabdominal pressure that
are necessary for a variety of activities: those that involve con¬
traction of the diaphragm during expiration (e.g., speaking,
singing, coughing, sneezing, laughing, whistling, and hiccup-
ing); those that involve contraction of the diaphragm and clo¬
sure of the glottis (Valsalva’s maneuver), (e.g., straining
during lifting, vomiting, urinating, defecating, and delivering

656

Part III I KINESIOLOGY OF THE HEAD AND SPINE

a fetus per vaginam); and those that involve contrac¬
tion of trunk muscles in different body positions and
during changes in body position (e.g., standing, walk¬
ing, leaning over, bending over, and moving from supine to
standing). The most striking change in the upright human
pelvic floor is loss of muscle and acquisition of tendon and fas¬
cia to compensate for that loss in an attempt to withstand con¬
stant stress without undue energy expenditure [28]. The
postural change and constant stress also necessitate special¬
ization in the metabolic and contractile properties of human
pelvic floor muscles. In humans, they consist of type I,
fatigue-resistant fibers predominately [24,60,101,111].

MUSCLES OF THE PELVIS
AND PERINEUM

From an anatomical perspective, striated muscle fibers
located at the caudal end of the trunk are separated tradi¬
tionally into those that form the posterolateral walls of the
pelvic cavity and move the femur, those of the pelvic
diaphragm, and those of the perineum. From a functional
viewpoint, however, muscles of the pelvic diaphragm and per¬
ineum can be divided in two different groups: (a) fibers that
form sphincters and tether the sphincters (urethral, vaginal,
and anal) to the pelvic effluents and (b) fibers that flank and
attach the pelvic effluents (along with their sphincters) to the
perimeter of the bony pelvis. The following descriptions
include elements of both perspectives.

Pelvic Muscles Associated
with Somatic Function

Two somatic muscles contribute to the formation of the pos¬
terior and lateral walls of the pelvic cavity, the piriformis and
obturator internus, respectively. Both are related functionally
to the lower limb and are primarily external rotators at the
hip. They are important muscles of the buttock that exit the
pelvic cavity through the greater and lesser sciatic foramina to
insert on the greater trochanter of the femur (Fig. 36.1). The
piriformis is innervated by twigs arising from the sacral plexus
inside the pelvic cavity, while the nerve to the obturator inter¬
nus (and superior gemellus) leaves the pelvic cavity through
the greater sciatic foramen and reenters it through the lesser
sciatic foramen. Both of these muscles are discussed in
greater detail in Chapter 39.

Pelvic Muscles Associated
with Visceral Function

The striated musculature of the two sides of the pelvis, along
with their associated fasciae, form the pelvic diaphragm; it
has two parts, the levator ani and coccygeus (Table 36.1). In a
frontal section through the pelvis, the pelvic diaphragm has the
appearance of an inverted tent as it is slung between the two
innominate bones. The pelvic diaphragm marks the caudal
limit of the pelvic cavity. The pelvic outlet is, therefore, below
and outside the pelvic cavity, inasmuch as the pelvic diaphragm
spans the interval between the walls of the bony pelvis and not
the perimeter of the inferior pelvic aperture [89].

Arcus tendineus

Symphyseal surface
of pubis

External anal
sphincter

lliococcygeus

Entrance to
obturator canal

For anal canal
Pubococcygeus

Arcus tendineus

Pubic symphysis

For urethra

Figure 36.1: Muscles of the pelvic floor of the male. A. Medial view. B. Superior view.

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

657

TABLE 36.1: Muscles of the Pelvis

Muscle

Origin

Insertion

Innervation

Action

Lower limb muscles in wall of pelvic cavity

Piriformis

Sacral bone lateral to and
between pelvic sacral
foramina

Tip of greater
trochanter

Twigs of ventral
primary rami of SI-2

External rotation and
abduction of the femur

Obturator internus

Obturator membrane, margins
of obturator foramen

Above trochanteric
fossa

Nerve to the obturator
internus from ventral
primary rami of L5
and SI-2

External rotation of the
femur

Pelvic floor muscles (pelvic diaphragm)

Levator ani

Pubococcygeus

Posterior surface of the body
of the pubis and the anterior
part of the arcus tendineus

Urethral walls, perineal
body, anococcygeal
ligament and coccyx

Above by twigs of
ventral primary rami
of S3-4, below by the
pudendal nerve from
ventral primary rami
of S2-4

Elevates pelvic floor; resists
increased intraabdominal
pressure; supports contents
of pelvic cavity; compresses
urethra to control micturition

Levator prostatae

In the male, these are anterior
fibers of the pubococcygeus

Fibers swing behind
the prostate gland
and end in the
perineal body

Supports the prostate

Pubovaginalis

In the female, these are
anterior fibers of the
pubococcygeus

Fibers swing behind
the vagina and end in
the walls of the vagina
and the perineal body;
some fibers contribute
to the sphincter
vaginae

Contributes to sphincteric
action around the vagina

Puborectalis

Posterior surface of the body
of the pubis

Fibers of both sides
meet in the midline at
the anorectal junction

Below by the pudendal
nerve from ventral
primary rami of S2-4

Responsible for the perineal
flexure; sphincteric action
functions to control anal
continence

llliococcygeus

Posterior part of the arcus
tendineus and the ischial
spine

Anococcygeal ligament
and coccyx

Above by twigs of
ventral primary rami
of S3-4

Elevates pelvic floor; resists
increased intraabdominal
pressure; supports contents
of pelvic cavity

Coccygeus

Ischial spine

Lateral borders of S4-5
and Col-2

Above by ventral
primary rami of S4-5

Elevates pelvic floor; resists
increased intraabdominal
pressure; supports contents
of pelvic cavity

The levator ani is a broad fibromuscular sheet of variable
thickness that forms the anterior, larger part of the pelvic
diaphragm. All of the pelvic effluents traverse this part of the
diaphragm: in the male, the urethra and anus, and in the
female, the urethra, vagina, and anus. Each of the parts of
the levator ani has the following anatomical features in com¬
mon: partial or complete origin from the pubic body, arcus
tendineus, or ischial spine, and midline union with its mate
from the opposite side. The levator ani is divided into three
parts: the pubococcygeus, puborectalis, and iliococ-
cygeus. The thinnest, weakest part of the levator ani [15], the
iliococcygeus, is related closely to the obturator internus
through part of its origin, represented by a thickened fascial
band of the obturator fascia, the arcus tendineus (tendinous
arc of the levator ani) [89]. This thin, posterior part of the lev¬
ator is reinforced by activation of the obturator internus during

straining activities when the hips are rotated externally (e.g.,
during defecation and parturition [childbirth]) [15].

Several subdivisions of the levator ani make significant
contributions to pelvic floor function. Each subdivision con¬
tributes muscle fibers to the sphincters and blends with
smooth muscle fibers of each of the midline pelvic effluents
[73,89,92]. Anterior fibers of the pubococcygeus take several
directions: into the walls of the urethra to contribute to the
sphincter urethrae (pubourethralis); behind the prostate
in the male (levator prostatae); and behind the vagina in the
female (pubovaginalis) to contribute to the sphincter vagi¬
nae of the perineum. Posterior fibers of the pubococcygeus
blend with fibers of the rectum and form the puboanalis
[92]. The puborectalis is a thick bundle of fibers located on
the inferior surface of the pubococcygeus. As it swings behind
the alimentary passageway at the anorectal junction, it is

658

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 36.2: Puborectalis is a part of the levator ani that forms a
sling around the lower bowel and contributes to the flexure
formed at the anorectal junction.

responsible for the perineal (anorectal) flexure (Fig. 36.2),
an important contributor to anorectal continence. It has been
suggested that the fibers of the pubococcygeus that attach to
the midline pelvic viscera (e.g., pubourethralis, levator
prostatae, pubovaginalis, puboanalis, and puborectalis) could
be named more accurately the pubovisceralis [65]. This
grouping might be helpful in setting apart functionally the
muscle fibers of the pelvic diaphragm that are considered to
be the most important in maintaining continence and sup¬
porting the pelvic organs [66].

The posterior part of the pelvic diaphragm is the coc-
cygeus, which could be called the ischiococcygeus if one
were consistent with the naming of the other parts of this
muscular layer (i.e., pubococcygeus, iliococcygeus, and
ischiococcygeus) [89]. The coccygeus differs from the levator
ani in several aspects: it is considerably thinner; it has bony
connections only; it has no connection with the anococcygeal
ligament; and it is innervated exclusively by fibers from the
ventral primary rami of Sl-2, instead of the pudendal nerve,
which innervates the levator ani [66,89].

Perineal Muscles

The osteoligamentous frame of the perineum consists of the
structures that form the pelvic outlet, that is, the pubic arch,
ischial rami and tuberosities, sacrotuberous ligaments, and
coccyx (Figs. 35.19-35.21). The perineum is the diamond¬
shaped area caudal to the pelvic diaphragm that is subdivided
into two triangles (Fig. 36.3). The area located anterior to a
line drawn between the ischial tuberosities is the urogenital
triangle, while the anal triangle is situated posterior to the
same line. Superficial-to-deep dissections of the layers of the
perineum of the female and male are shown in Figure 36.4.

Striated musculature of the urogenital area is arranged in
two layers, deep and superficial (Table 36.2). Muscles in the
superficial perineal space include the superficial trans¬
verse perineus, bulbospongiosus, and ischiocavemosus;

Pubic

symphysis

Figure 36.3: Inferior view of the male perineum. The diamond¬
shaped perineum is bordered on each side by the bodies of the
pubes, ischiopubic rami, ischial tuberosities, sacrotuberous liga¬
ments, and coccyx. A line drawn transversely between the ischial
tuberosities divides the perineum into an anterior urogenital tri¬
angle and a posterior anal triangle.

those of the deep perineal space include the deep trans¬
verse perineus and sphincter urethrae (Fig. 36.5). Subdi¬
visions of the sphincter urethrae include circular fibers,
compressor urethrae, and the urethrovaginal sphincter

[73,92] (Fig. 36.6). The deep transverse perineus muscle,
along with its associated fasciae, is known as the urogenital
diaphragm (Figs. 36.7, 36.8). The perineal musculature of
the urogenital area makes significant contributions to a variety
of visceral functions: the sphincter urethrae aids in the volun¬
tary control of micturition; the male bulbospongiosus aids in
expelling semen or urine from the urethra and contributes to
penile erection; the female bulbospongiosus functions as the
sphincter vaginae (with fibers of the pubovaginalis and ure¬
throvaginal sphincter); and the ischiocavemosus contributes to
penile/clitoral erection.

One muscle is located in the anal triangle, the sphincter
ani extemus (Fig. 36.9). Surrounding the entire anal canal,
it is subdivided traditionally into subcutaneous, superficial,
and deep parts (Fig. 36.10). Its deep fibers blend with those
of the puborectalis and puboanalis, each making a significant
contribution to anorectal continence.

Somatic (External) Sphincters

Several bundles of pelvic and/or perineal striated muscle
fibers contribute to the sphincters that surround and guard
the pelvic effluents. The external urethral sphincter

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

659

Penile urethra

Pubic symphysis

Body of penis

Coccyx

Superior pubic
ramus

Obturator

foramen

Ischiopubic

ramus

Ischial

tuberosity

Sacrotuberous

ligament

Arcuate pubic ligament

Anococcygeal /
ligament

Figure 36.4: The perineum, shown as successively deeper layers of dissection. The male perineum is indicated on the left and the female
on the right. A. Superficial. B. Deep.

660

Part III I KINESIOLOGY OF THE HEAD AND SPINE

TABLE 36.2: Muscles of the Perineum

Muscle

Origin

Insertion

Innervation

Action

Deep perineal muscles

Deep transverse perineus
(with superior and inferior
fascial layers, forms the
urogenital diaphragm)

Ischiopubic ramus

Two muscles meet in
midline, fibers pass to
the perineal body; in the
female, surrounds vagina

Perineal branch of the
pudendal nerve from ventral
primary rami of S2-4

Supports and fixes perineal body
to aid in supporting pelvic viscera;
resists increased intraabdominal
pressure

Sphincter urethrae

Circular part

Compressor urethrae

Sphincter

urethrovaginalis

Encircles urethra
Passes anterior
to urethra

Female: encircles
urethra and vagina

Ischial ramus

Perineal branch of the
pudendal nerve from ventral
primary rami of S2-4

Compresses urethra to control
micturition; in female, the
urethrovaginal sphincter
compresses the vagina

Superficial perineal muscles

Superficial transverse
perineus

Ischial tuberosity

Two muscles meet in the
midline, fibers pass to
the perineal body

Perineal branch of the
pudendal nerve from ventral
primary rami of S2-4

Supports and fixes perineal body
to aid in supporting pelvic
viscera; resists increased
intraabdominal pressure

Ischiocavernosus

Ischiopubic ramus

Male: fascia of the
corpora cavernosa

Female: fascia of the
crura of the clitoris

Perineal branch of the
pudendal nerve from ventral
primary rami of S2-4

Contributes to erection by
forcing blood into body of
the penis/clitoris and preventing
venous return

Bulbospongiosus

Male: penile raphe,
perineal body

Female: perineal
body

Male: fascia of the
corpus spongiosum and
corpora cavernosa

Female: fascia of the
bulb of the vestibule

Perineal branch of the
pudendal nerve from ventral
primary rami of S2-4

Male: compresses penile bulb
to expel urine at the end of
micturition and semen during
ejaculation; contributes to
erection by forcing blood into
penile body and preventing
venous return

Female: contributes to clitoral
erection and functions as the
sphincter vaginae (with fibers
of the pubovaginalis)

Anal muscle

Sphincter ani externus
Subcutaneous part
Superficial part

Deep part

Circular fibers
surround entire
anal canal; parallel
fibers from the
perineal body

Parallel fibers to the
anococcygeal ligament;
deepest fibers
continuous with the
puborectalis

Inferior rectal branch of the
pudendal nerve from ventral
primary rami of S2-4

With the sphincter ani internus
and puborectalis, compresses
anus to maintain anal
continence

(EUS) is formed by the circular fibers of the pubococcygeus
and the sphincter urethrae of the deep perineal space. In the
female, the vaginal sphincter (VS) receives contributions
from the pubovaginalis, urethrovaginal sphincter portion of
the sphincter urethrae, and bulbospongiosus. And finally, the
external anal sphincter (EAS) is formed by subcutaneous,
superficial, and deep fibers of the sphincter ani externus. The
puborectalis, part of the levator ani portion of the pelvic
diaphragm, makes a special contribution to anorectal conti¬
nence by maintaining the perineal flexure; it contributes
sphincteric function to the EAS by maintaining approxi¬
mately an 80° posterior flexure at the anorectal junction at all
times except during defecation [5,26,73,118]. These muscle
fibers constitute somatic sphincters, innervated by somatic
efferent and somatic afferent nerve fibers that travel in twigs
from the ventral primary rami of S3-4 (from above) or the
pudendal nerve containing fibers from the ventral primary
rami of S2-4 (from below) to reach their destinations.

Functional and Metabolic Properties
of Pelvic and Perineal Muscle Fibers

Striated muscle fibers of the pelvic diaphragm and perineum
are predominately slow-twitch, type I fibers with electrophys-
iological characteristics that differ from those of other striated
muscles. Since these muscles are active electrophysiologically
at all times except during micturition and defecation [20,80],
they need to be resistant to fatigue. Histochemical assessment
of muscle fibers from the pubococcygeus, iliococcygeus, and
coccygeus from female cadaveric specimens reveal two thirds
of them to be type I, slow-twitch, tonic fibers [24]. Density of
type II, fast-twitch, phasic fibers is greater in the regions
immediately surrounding the urethral and anal orifices (i.e.,
in the sphincters) [20,33,35]. Constant tonic activity of type I
fibers provides support for pelvic organs and keeps the uro¬
genital orifices closed; this aids in unloading connective tissue
elements in the pelvic cavity during everyday, routine postural

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

661

urogenital diaphragm

Figure 36.5: Muscles of the perineum. A. Male. B. Female. Muscles of the anterior half of the perineum (urogenital triangle) are con¬
cerned with urogenital function; those of the posterior half contribute to anorectal continence. Note that the bulbospongiosus, super¬
ficial and deep transverse perineal muscles, and sphincter ani externus attach to the central tendon of the perineum.

Figure 36.6: Medial view of the muscles that com¬
press the urethra and vagina, including the ure¬
throvaginal sphincter, compressor urethrae, and
sphincter urethrae.

662

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 36.7: Coronal section through the anterior part of the perineum. A. Male. B. Female. Note the three structures that compose the
urogenital diaphragm, the deep transverse perineus muscle along with its inferior and superior fascial layers. Two other perineal muscles
are shown, the paired ischiocavernosus and bulbospongiosus. The anterior portion of the pelvic diaphragm (levator ani) is also shown.

Figure 36.8: Inferior view of the urogenital diaphragm. A. Male; on the left, the inferior fascia of the deep transversus perineus (per¬
ineal membrane or inferior fascia of the urogenital diaphragm) is intact but has been removed on the right to expose the musculature.
B. Female; the fascia of the deep transverse perineus has been removed on both sides to expose the muscle fibers.

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

663

Figure 36.9: Coronal section through the
posterior half of the perineum. Note the
sphincter ani externus medial to each
ischiorectal fossa and the pudendal canal
and its contents (pudendal nerve and inter¬
nal pudendal vessels) on the inferolateral
walls of the fossae.

alterations [80,107]. With sudden increases in intraabdominal
pressure, type II fibers immediately surrounding the urethral
and anal orifices are recruited to maintain their closure. The
periurethral and perianal muscle fibers are further character¬
ized as having an extremely slow discharge rate (3—4 cps) dur¬
ing waking and sleeping cycles [47], and studies of their
passive length-tension relationships reveal that they are
stiffer or develop greater tensions in response to passive ten¬
sile force than other striated muscle fibers [61]. This stiffness
is attributed to changes in the passive properties of the con¬
nective tissue elements surrounding the muscle fibers.

Figure 36.10: Midsagittal section through the lower rectum and
anal canal shows the sphincter ani externus and internus. Note
the position of the puborectalis and its contribution to the for¬
mation of the anorectal angle.

Clinical Relevance

PELVIC AND PERINEAL MUSCLE FIBER TYPES: A

clinician's ability to effect changes in the muscles of the
pelvis and perineum depends, in part upon understanding
the metabolic and contractile properties of these very differ¬
ent voluntary muscles and how the principles of exercise
physiology affect their training. This knowledge affects
directly the choices we make when prescribing exercises, spe¬
cific activities; and other types of therapeutic interventions for
treatment of patients with some types of pelvic floor dysfunc¬
tion. For maximal effectiveness, clinicians must be aware of
several characteristics of these multifunctional muscles: they
are composed of voluntary; striated muscle fibers that can be
affected by exercise; most of them are type I fibers that dis¬
charge tonically and should respond best to endurance exer¬
cises (i.e., multiple submaximal contractions); and the
predominately type II, phasic fibers that control the sphinc¬
ters should respond best to high-intensity exercises of short
duration [9,10,29,37,46,57,88,90,93,97,98,104].

Central Tendon of the Perineum

The central tendon of the perineum (perineal body) is

an important obstetric and gynecological structure of the
perineum (Fig. 36.11). Located at the central point of the
diamond-shaped perineum, it is located at the juncture
between the urogenital and anal triangles. In the female, this
dense node of tissue is located between the vaginal and anal
orifices; in the male, it is in the space between the root of the
penis and the anal orifice. The perineal body consists of a tri¬
angular fibromuscular condensation that is larger and more

664

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 36.11: Midsagittal section through the female central ten¬
don of the perineum. The central tendon of the perineum (per¬
ineal body) is the juncture between the urogenital and anal
triangles. In the female it is located between the vaginal and
anal orifices.

important clinically in the female than the male [89]. Firmly
anchored to it are somatic and visceral fibers from a number
of muscles of both the pelvis and perineum, including the
pubococcygeus, pubovaginalis, levator prostatae, sphincter
urethrae, superficial and deep transverse perinei, sphincter
ani externus, bulbospongiosus, sphincter ani internus, and
longitudinal (smooth) muscle of the rectum. These muscular
connections anchor the perineal body to the pubes, ischia,
and coccyx, thereby maintaining the midline position of the
urethra, vagina, and anus in the pelvic outlet [92]. Contrac¬
tion of muscles of the pelvic diaphragm and perineum,
through their connection to the perineal body, result in ele¬
vation of the pelvic floor [89,92], lending support to pelvic
viscera and resisting increases in intraabdominal pressure.
Their relaxation allows the pelvic floor to descend, an impor¬
tant component of micturition and defecation.

Clinical Relevance

CENTRAL TENDON OF THE PERINEUM: Being the low¬
est and final level of support of pelvic viscera; the perineal
body is particularly important in women. Its tearing and
separation from the muscles that attach to it during child¬
birth can lead to disorders collectively referred to as pelvic
floor dysfunction. Some obstetricians perform an episiotomy

(a surgical incision in the perineum) in an attempt to control
prophylactically the location and amount of tearing that
occurs during parturition [119]. A median episiotomy is an
incision through the perineal body, while a mediolateral epi¬
siotomy starts in the midline but curves posterolaterally [73].
Routine use of episiotomies is highly debated [77,119]. Some
recent evidence indicates that they may actually cause
more, rather than less, trauma to the muscles of the pelvic
diaphragm and perineum [4,38,64].

NERVOUS CONTROL OF MUSCLES
OF THE PELVIS AND PERINEUM

A lengthy discussion of the nervous control of the striated
musculature ordinarily is not included in a kinesiology text.
However, the muscles of the pelvic diaphragm and perineum
are not ordinary. This discussion, therefore, departs from con¬
vention for the following reasons:

• To understand how these muscles are structurally and
functionally different, it is necessary to discuss their neu¬
rological differences.

• Understanding the many functions of these muscles
necessitates some information regarding the visceral struc¬
tures with which they work in concert.

• To describe the ways in which these muscles work in con¬
cert with visceral structures requires information regard¬
ing their neurological control.

• To establish the need for different clinical interventions
between males and females with pelvic dysfunction, it is
necessary to substantiate the sexual dimorphism of these
muscles, as well as the neurons that control them.

• Clinicians, including physicians, nurses, and therapists, are
witnessing a great resurgence of interest in function and
dysfunction of pelvic floor and perineal structures. Infor¬
mation regarding the neuromuscular structures in this area
is minimized or completely omitted from standard gross
anatomy, neuroscience, functional anatomy, and clinical
textbooks. Their discussion is included here to ensure expo¬
sure to an extremely important area of clinical practice.

Spinal Centers

A century ago, Onufrowicz (who called himself Onuf)
described a cluster of small anterior horn cells that are
responsible for the somatic innervation of pelvic and perineal
somatic (skeletal, striated) musculature [82]. The cell bodies
of OnuPs nucleus (ON) are located in the ventral horn of
spinal cord segments S2-4. Axons of these cells travel pre¬
dominately in the pudendal nerve to reach their destina¬
tions (Figs. 36.12, 36.13); somatic afferent fibers from
receptors in these muscles, as well as perineal skin, also travel
in the pudendal nerve and terminate in the nucleus pro-
prius (NP) of spinal cord segments S2-4, predominately.

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

665

Figure 36.12: Contribution of nerve fibers to the innervation of
the pelvic floor from above. Note twigs from the ventral primary
rami of S3-4 entering the pelvic diaphragm, as well as the puden¬
dal nerve passing behind and below the coccygeus.

By most accounts, in a wide variety of mammals, neu-
roanatomical and biochemical differences exist between ONs
of males and females [56]. This sexual dimorphism is char¬
acteristic not only of neurons of ON, but also of the muscles
they innervate [18]. Hence, spinal neurons of ON are more
numerous, and the musculature of the pelvis and perineum
are developed to a greater extent in males than in females
[31]. Evidence of sexual dimorphism in the LMNs that inner¬
vate the pelvic and perineal musculature, as well as in the
muscles themselves, appears early in development and is sex
hormone dependent [18]. Furthermore, survival of greater
numbers of motor neurons in ON in the presence of andro¬
gens may favor the survival of striated muscle fibers in pelvic
and perineal musculature of the male; in females, however,
greatly reduced androgen secretion may result in attrition of
motor neurons by programmed cell death (apoptosis), result¬
ing in fewer pelvic and perineal muscle fibers [18]. Males
generally are engaged in activities that require lifting heavier
loads and therefore need to be able to resist greater increases
in intraabdominal pressures, while females, in their parturi¬
ent (child-bearing) capacity, acquire increased fascial and lig¬
amentous pelvic floor elements that are necessary to resist the
static loads encountered during a lengthy gestational period.
In addition, the species-survival roles played by the bul-
bospongiosus and ischiocavernosus muscles in erection and
ejaculation might further explain the male need for more
muscle fibers in these perineal muscles, along with a larger
motor pool to support their function.

Preganglionic parasympathetic neuronal cell bodies are
located in the sacral autonomic nucleus (SAN) of spinal
cord segments S2-4, while second-order cell bodies for vis¬
ceral afferent information are located in the intermediome-
dial (IMM) nucleus of the same segments [49]. This
arrangement conveniently places the somatic efferent (ON),
somatic afferent (NP), parasympathetic visceral efferent
(SAN), and visceral afferent (IMM) cell columns for all of the
striated and the parasympathetically innervated smooth mus¬
cle fibers in the pelvic floor and perineum in the same seg¬
ments of the spinal cord. Preganglionic neurons responsible
for the sympathetically innervated smooth muscle fibers of
this region are located in the intermediolateral (IML)
nucleus of spinal cord segments T11-L2, while second-order
cell bodies for visceral afferent information traveling with the
sympathetic fibers are located in the IMM nucleus of the
same segments (T11-L2) [73].

Supraspinal Centers

Several higher central nervous system (CNS) centers con¬
tribute to the control of somatic neurons (LMNs) and vis¬
ceral neurons (preganglionic autonomic) involved in pelvic
and perineal muscular function. Motor neurons located in
Brodmann s area 4 of the frontal lobe of the cerebral cortex
constitute a direct corticospinal projection to ON, the motor
pool for the pudendal nerve [71,75]. Other subcortical cen¬
ters also make significant contributions to the suprasegmental
control of ON, most notably the hypothalamus and brain¬
stem reticular formation. Micturition appears to be the
best-defined visceral function of the pelvis and is used as the
example in this discussion; presumably, different or similar
centers in the same regions of the cerebral cortex, dien¬
cephalon, and brainstem are involved in the control of defe¬
cation, parturition, and sexual function [26].

The central organization and coordination of micturition
depends on two reticular formation centers located in the
dorsolateral pontine tegmentum [8,40,41]. A medial region
(M -region or Barringtons area) functions as the pontine
micturition center (PMC), and a lateral region (L-region)
functions as the pontine urinary storage center (PUSC).
In addition to these pontine centers, there are numerous pro¬
jections from the hypothalamus to the PMC and PUSC and
to spinal neurons in the SAN and ON [54].

Degenerative Diseases

Since the neurons of ON innervate striated muscles that are
under voluntary control, they are classified as somatic neu¬
rons; however, they share features common to visceral effer¬
ent neurons. Like the neurons of the phrenic nucleus that
innervate striated fibers of the respiratory diaphragm, their
constant function is controlled by brainstem centers even in
the absence of wakefulness and consciousness [85]. Unlike
the LMNs that innervate the vast majority of striated muscles
of the body, they receive direct hypothalamic afferents, and
their function as well as the muscles they innervate must be

Upper motor neuron
in the anterior part of
the paracentral lobule-
Brodmann's area 4

Figure 36.13: Diagram of the descending pathways and peripheral nerve fibers that control micturition and continence. Cortical upper
motor neurons as well as those of the L-region excite urethral sphincter motoneurons in ON. The M-region excites GABAergic interneu¬
rons that inhibit motoneurons of ON. Preganglionic visceromotor neurons of the IML nucleus and SAN, influenced by the hypothala¬
mus (not shown), are either excitatory or inhibitory to smooth muscle of the bladder (detrusor) and internal urethral sphincter. Visceral
afferent fibers that transmit pain travel with the sympathetic division of the autonomic nervous system; those that transmit stretch
travel with the parasympathetic division. L-region, pontine urinary storage center (PUSC); M-region, pontine micturition center (PMC);
IML nucleus, intermediolateral nucleus (preganglionic sympathetics); SAN, sacral autonomic nucleus (preganglionic parasympathetics);
GVA, general visceral afferent; +, excitation; -, inhibition.

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Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

667

coordinated closely with activities of visceral neurons and vis¬
ceral muscles [95].

Although distinctly somatic [42,86], the cells of ON may be
morphologically, biochemically, and functionally different
from other somatic motor neurons [32,42,56] and intermedi¬
ate between a somatomotor and a visceromotor classification
[26,52,74,80]. Evidence in support of this position comes from
pathological changes observed in neurons in several neurolog¬
ical degenerative diseases. For instance, in the motoneuron
diseases poliomyelitis [55], Werdnig-Hoffman disease [110],
and amyotrophic lateral sclerosis [3,17,50,52,68], somatic
motor neurons are progressively lost, while visceral motor
neurons and neurons of ON are spared. In contrast, neurons
of ON are selectively vulnerable in visceromotor neuronal dis¬
orders such as Shy-Drager syndrome [26,67,69], Hurler’s syn¬
drome [110], and Fabry’s disease [26]. Other evidence in
support of the intermediate classification of cells of ON is that
these neurons share several biochemical characteristics with
CNS autonomic nuclei [3,11,12,70,74].

Clinical Relevance

STRIATED MUSCLE FIBERS OF THE PELVIC
DIAPHRAGM AND PERINEUM: A variety of data indi¬
cate that, physiologically and neurologically, the striated
muscles of the pelvic floor and perineum differ from other
voluntary, somatic musculature. For these reasons, treat¬
ment of their dysfunction by therapists may necessitate a
novel approach. To wit some females; even nulliparous;
continent ones are unable to contract their pelvic and per¬
ineal muscles voluntarily; others can do so only in concert
with other voluntary muscle groups; such as the abdominal
and gluteal muscles [84,85]. The decreased voluntary acces¬
sibility of these muscles; then, suggests that traditional exer¬
cise protocols may not be effective with this population of
patients and that clinicians must become more innovative in
the ways we approach their rehabilitation.

The intermediate classification of neurons of ON, between
those of somatic and visceral motor neurons, also has sig¬
nificant clinical application. For instance, in certain
motoneuron diseases (e.g., amyotrophic lateral sclerosis),
incontinence (urinary as well as anorectal) develops as a
late sequela of the disease.

SPECIFIC FUNCTIONS OF PELVIC
AND PERINEAL MUSCULATURE

Urinary Continence and Micturition

Essential to an understanding of the role of the voluntary
muscle fibers of the pelvic floor and perineum in urinary
continence is a brief description of the visceral structures
that are involved, specifically, the bladder, bladder neck, and
proximal urethra. Each contains smooth muscle fibers that

receive visceral efferent and visceral afferent innervations
that are responsible for visceromotor activity and for trans¬
mitting stretch.

The urinary bladder serves a dual function; it passively
stores urine and actively discharges its contents into the
urethra. Its wall is composed of a single unit of interlacing
bundles of smooth muscle, the detrusor, innervated by
both divisions of the ANS [14,21,53]. Preganglionic
parasympathetic neurons are located in the SAN at spinal
cord segments S2-4 and their axons travel in the pelvic
splanchnic nerves (nervi erigentes), while preganglionic
sympathetic neurons are located in the I ML nucleus of
spinal cord segments T11-L2 and their axons travel in tho¬
racic and lumbar splanchnic nerves (Fig. 36.14). Post¬
ganglionic neurons of both divisions of the ANS are located
in the inferior hypogastric (pelvic) plexus or one of its
subdivisions, the vesical plexus (Fig. 36.14). The dense
population of parasympathetic fibers to the detrusor is exci¬
tatory, while the sparse sympathetics are vasomotor and
inhibitory to the detrusor [14,21,53] (Table 36.3). Visceral
afferents travel with pelvic splanchnic nerves and transmit
stretch and pain, while those that accompany sympathetics
carry pain only [27,73].

Smooth muscle in the bladder neck and initial part of the
urethra is histologically, histochemically, and pharmacologi¬
cally distinct from the cells of the detrusor; furthermore, the
area is sexually dimorphic [53]. In the male, circular or truly
sphincteric fibers in this region make up an internal urethral
sphincter (IUS); these fibers receive a dense sympathetic
innervation that is excitatory and a sparse parasympathetic
innervation that is inhibitory. In the female, however, true
sphincteric muscle fibers and the sympathetic nerve fibers are
absent. This may be one of the many factors that contribute to
a higher incidence of urinary incontinence in females than in
males [87,116]. Although the existence of a morphological
muscular entity corresponding to an IUS is somewhat contro¬
versial [89,121], in both sexes, a bundle of muscle fibers inter¬
mingled with collagen and elastic fibers functions as a
physiological sphincter to control the passage of urine from
the bladder into the proximal urethra [27,89].

Supraspinal, spinal, and peripheral nervous elements, the
bladder, urethra, and sphincteric musculature all function in
concert to maintain urinary continence and allow micturi¬
tion in the following manner:

• The bladder fills passively between periods of voiding,
gradually increasing vesical pressure.

• Urine is retained in the bladder by direct activation of neu¬
rons in ON by the PUSC, resulting in tonic activity in the
EUS and compression of the urethra.

• Retention of urine by the bladder is facilitated also by acti¬
vation of neurons in the SAN of S2-4 and IML nucleus of
T11-L2 by cells of the paraventricular nucleus of the
hypothalamus, resulting in internal urethral sphincter exci¬
tation and closure, and detrusor inhibition.

• Urinary continence is maintained as long as vesical pres¬
sure does not exceed urethral pressure.

668

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Aortic plexus
from above

Figure 36.14: Anterior view of the visceral nerves of the male pelvis and perineum. The pelvic plexus (inferior hypogastric plexus) is
formed by contributions from parasympathetic pelvic splanchnic nerves and sympathetic fibers that descend through the superior
hypogastric plexus (the caudal continuation of the aortic plexus from above). Fibers from both divisions of the autonomic nervous sys¬
tem blend in the pelvic plexus and are distributed to visceral structures in the pelvis and perineum.

TABLE 36.3: Functional Coordination of Visceral and Somatic Musculature of the Pelvis and Perineum

Function

Visceral Efferents,
Parasympathetic

Visceral Efferents,
Sympathetic

Somatic Efferents
via Pudendal Nerve

Visceral Afferents

Urinary continence
and micturition

+ to detrusor, - to IUS

- to detrusor, + to IUS

± to EUS

Sense stretch in wall of
urinary bladder as it fills

Anal continence
and defecation

+ to rectal musculature,

- to IAS

- to rectal musculature,

+ to IAS

± to EAS

Sense stretch in wall of rectum
as it fills

Parturition

+ to bulk of uterine
musculature

- to musculature of
cervix and vagina

+ pelvic and urogenital
diaphragms

Sense stretch in walls of uterus
and vagina

Sexual

Vasodilation of helicine
arteries and penile/clitoral
erection

Male: ejaculation with -
to detrusor and + to IUS
Male & female:
detumescence (remission
from erection)

Female: + to VS

Male and female: sense degree
of vasodilation of helicine
arteries

Female: sense stretch
in walls of vagina

+ , excitatory; inhibitory; ±, excitation (voluntary contraction) and inhibition (voluntary relaxation); IUS, internal urethral sphincter; EUS, external urethral
sphincter, consisting of the sphincter urethrae and fibers of the pubococcygeus surrounding the urethra; IAS, internal anal sphincter; EAS, external anal sphincter
with some fibers from the puborectalis; VS, vaginal sphincter, consisting of pubovaginalis, urethrovaginal sphincter portion of the sphincter urethrae and bul-
bospongiosus.

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

669

• When the bladder fills to 400-500 mL (i.e., when vesical
pressure exceeds urethral pressure), the micturition reflex
is activated.

• The micturition reflex consists of the following cascade
of events:

Stretch receptors in the bladder wall are stimulated, and
these, in turn, activate visceral afferent fibers that enter
the spinal cord via dorsal roots at S2-4 to terminate on
second-order neurons in the IMM.

Axons of neurons in the IMM ascend in the spinoretic¬
ular tract to reach the brainstem reticular formation.
Spinoreticular fibers (carrying stretch sensations) proj¬
ect to the PMC.

By way of direct excitation of neurons in the SAN and
IML nucleus, along with inhibition of neurons in ON via
an intemeuron, activity in cells of the PMC results in con¬
traction of the bladder and relaxation of the IUS and EUS.

• Axons of neurons in the IMM nucleus also ascend in the
spinothalamic tract to the ventral posterior nucleus of the
thalamus; a cortical projection from the thalamus provides
a feeling of fullness in the bladder and a conscious desire
to urinate.

• The micturition reflex and the cortical projection together
culminate in micturition.

• If the micturition reflex occurs when it is inconvenient to
urinate, corticospinal fibers can override the reflex for a
finite period by directly exciting neurons of ON, thereby
increasing the force of contraction of the EUS sufficiently
to maintain closure of the urethra until appropriate facili¬
ties are available. If however, vesical pressure exceeds the
enhanced urethral pressure, micturition occurs, even with
maximal voluntary contraction of the EUS.

• Micturition can be facilitated by contraction of the tho¬
racic diaphragm and abdominal muscles to increase
intraabdominal pressure (Valsalva’s maneuver).

Anorectal Continence and Defecation

Anorectal continence and defecation are controlled and
coordinated by the same or similar structures that are
involved in urinary continence and micturition, with some
major differences [26,73,118]. The sigmoid colon, rectum,
and anal canal contain smooth muscle fibers that are inner¬
vated by visceral efferent nerve fibers as well as visceral affer¬
ent fibers that transmit stretch.

The rectum is the middle visceral structure at the caudal
end of the alimentary canal, in continuity above with the sig¬
moid colon and below with the anal canal. Each is located
in a different region; the sigmoid colon is situated in the
abdominal cavity, the rectum in the pelvic cavity, and the anal
canal in the perineum [121]. The sigmoid colon is the reser¬
voir for fecal material, and the rectum remains empty except
when feces enters it to excite defecation [5,23,36,43] or in
chronic constipation [5]. The walls of each of these alimentary
passageways are composed, in part, of thin inner circular and
thick outer longitudinal smooth muscle fibers [5]. Contraction

of the circular fibers results in the pulsatile constrictions of the
gut as seen in peristalsis, while longitudinal fiber activation
produces a shortened segment of bowel. Smooth muscle
fibers of the lower part of the alimentary canal are innervated
by both divisions of the ANS [5,89,121]. Preganglionic sympa¬
thetic neurons are located in the IML nucleus of Ll-2. Most
postganglionic sympathetic neurons, embedded in ganglia of
the superior and inferior mesenteric plexuses of the abdomi¬
nal cavity, travel in the superior and inferior hypogastric
plexuses to reach the caudal end of the bowel (Fig. 36.14).
Preganglionic parasympathetic neurons are located in the
SAN at spinal cord segments S2-4; their axons travel in the
pelvic splanchnic nerves to reach postganglionic neurons
located in the wall of the bowel segment or the inferior
hypogastric plexus [89]. Parasympathetic fibers are excitatory,
and sympathetics are inhibitory, to smooth muscle of the
bowel. Visceral afferents that travel with pelvic splanchnic
nerves (S2-4) transmit stretch as well as pain; those that
accompany sympathetic fibers carry pain only [5,73,89].

In the upper two thirds of the anal canal, the circular layer
of the muscularis externa (smooth muscle) is thickened and
forms the internal anal sphincter (IAS). Sympathetic stim¬
ulation results in excitation of the IAS, while parasympathetic
fibers inhibit it [73]. Under most circumstances, approxi¬
mately 80-85% of the resting pressure on the anal canal is
afforded by the IAS, while the EAS, innervated by somatic
fibers of the inferior rectal branch of the pudendal nerve, pro¬
vides 15-20% [13]. Voluntary recruitment of additional fibers
of the EAS and the puborectalis is an effective mechanism for
increasing anal pressure beyond the resting state when there
are sudden increases in intraabdominal pressure or when
defecation must be deferred [62,78,115].

The supraspinal control of anorectal continence and
defecation is less well understood than that of urinary conti¬
nence and micturition. Although there exists an element of
supraspinal (cortical) control, most researchers agree that the
greatest contribution to the control of defecation comes from
reflex mechanisms [5,26,36,73,89]. Neurons of the spinal cord
and autonomic ganglia, smooth muscle of the sigmoid colon,
rectum, anal canal and IAS, and striated muscle of the EAS and
puborectalis all play a role in the sensorimotor integration of
anorectal continence and defecation in the following manner:

• By most accounts, the sigmoid colon serves as the reservoir
for fecal material between periods of defecation.

• Activation of neurons in ON and the IML nucleus of the
spinal cord results in tonic activity in both the IAS and
EAS, as well as the puborectalis, maintaining closure of the
anal canal and orifice, except during defecation.

• Excitation of neurons in ON and the IML nucleus, and
muscular contractions of the IAS, EAS, and puborectalis
is increased during activities in which intraabdominal
pressure is increased (e.g., straining to lift a heavy object,
forced expiration, coughing, sneezing, parturition); in
contrast, the amplitude of the muscular response is
decreased when intraabdominal pressure is increased
during voluntary straining to defecate.

670

Part III I KINESIOLOGY OF THE HEAD AND SPINE

• Prior to defecation, fecal material moves from the sigmoid
colon (and as high as the left colic flexure) into the rectum
via peristalsis of the colon [5].

• The stimulus for the initiation of defecation is usually the
intrinsic defecation reflex [36], stimulated by distention
of the rectum after arrival of the feces and activation of vis¬
ceral afferent fibers by stretch [5,73,89]; an additional
mechanism for triggering the arrival of stool in the rectum
is the extrinsic defecation reflex [105], stimulated by
activation of proprioceptors in pelvic floor and perineal
muscle fibers, particularly the puborectalis and other
fibers of the levator ani [34,39,51,94].

• Completion of the defecation reflex results in relaxation of
the IAS; however, voluntary contraction of the EAS pre¬
vents further passage of the feces if the time is not con¬
venient for defecation.

• Facilitation of the defecation reflex is afforded by vis¬
ceral afferent stimulation of the SAN, which, in turn,
increases the peristaltic activity in the smooth muscle of
the rectum [36].

• If the desire to defecate is strong and the time and place
are socially acceptable, defecation commences; ignoring
this urge can gradually lead to chronic constipation [36].

• Relaxation of the puborectalis allows some straightening of
the perineal flexure and passage of the feces into the anal
canal; further relaxation of the puborectalis and reduction
of the perineal flexure occurs when one sits [5,26].

• Passage of the feces through the anal canal is allowed by
relaxation of the EAS and facilitated by contraction of the
thoracic diaphragm and abdominal muscles to increase
intraabdominal pressure (Valsalva’s maneuver).

• Prior to and during passage of fecal material through the
anal canal, the pelvic floor relaxes and descends, and at the
same time, longitudinal muscle of the anal canal contracts
to shorten it, thus assisting in expulsion of the stool [5].

• The closing reflex occurs with contraction of the IAS,
EAS, and puborectalis, resulting in closure of the anal ori¬
fice and restoration of the perineal flexure.

Clinical Relevance

DEFECATION REFLEXES AND DIGITAL STIMULATION:

Using digital stimulation, clinicians take advantage of the
internal and external defecation reflexes in bowel manage¬
ment of patients with spinal cord injuries and intact reflex
cord mechanisms [58]. A gloved , lubricated digit can be
inserted into the patient's anorectal passageway and moved
in a circular motion for 30-60 seconds to stretch the
mucosa and surrounding muscle. This technique stimulates
stretch receptors and activates visceral and somatic afferent
nerve fibers in and around the lower bowel passageway ,
resulting in reflex contraction of the smooth muscle of the
rectum and bowel emptying.

Sexual Function

Aspects of sexual function that deal with the function and
neural control of smooth muscle fibers in the internal and
external genitalia and striated muscles of the pelvic floor and
perineum are considered here. The prototype for our dis¬
cussion is the male; differences in sexual function in the
female are also addressed. There are four stages of sexual
function: excitation, erection, ejaculation (orgasm), and
remission. Function during these stages results predomi¬
nately from autonomic phenomena; two contain a somatic
component.

Excitation occurs with the passage of erotogenic thoughts
or with cutaneous stimulation, especially of the erogenous
zones, and transmission to the spinal cord via somatic afferents
and on to supraspinal centers, particularly the hypothalamus,
limbic system, and cerebral cortex. Excitation causes erection
of the penis. This is followed by ejaculation of semen, which
coincides with the orgasmic phase. In the final stage, remis¬
sion, the penis returns to the flaccid state (detumescence).
The following is a description of the neuroanatomical basis of
each of these stages of sexual function:

• Erection is a parasympathetic phenomenon; it results from
activity in neurons of the SAN, which results in dilation of
the helicine arteries of the penis and engorgement of the
cavernous spaces of the corpus spongiosum and corpora
cavernosa [73,89].

• Activation of striated muscle fibers contributes to erection;
turgidity of the erection is increased by pulsatile contrac¬
tions of the bulbospongiosus and ischiocavernosus muscles
secondary to stimulation of neurons in ON.

• Ejaculation results in expulsion of the semen (ejaculate;
sperm plus seminal fluid) from the external urethral ori¬
fice; it has two phases and is controlled by three types of
neurons.

The first phase is passage of the ejaculate into the ure¬
thra, and the second phase is passage of the ejaculate
out of the urethra through the external urethral orifice.
During emission, stimulation of the ejaculatory ducts
and seminal vesicles by sympathetic neurons in the
I ML nucleus of LI-2 results in delivery of the ejacu¬
late to the prostatic urethra [73] and prostatic fluid to
the ejaculate [6,89].

During ejaculation, activity in neurons of the IML
nucleus of LI-2 maintains closure of the internal ure¬
thral sphincter to prevent reflux of the ejaculate into the
bladder and leakage of urine [6,73], and neurons of ON
activate the EAS to prevent leakage of feces or gas [96].
Activation of parasympathetic neurons in the SAN
results in contraction of smooth muscle of the urethra to
expel the ejaculate at the external urethral orifice.
Expulsion of the ejaculate is facilitated by activation of
neurons of ON, resulting in pulsatile contractions of the
ischiocavernosus and bulbospongiosus [73,96].

• Following ejaculation, the penis returns to a flaccid state
(detumescence, remission).

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

671

Sympathetic fibers cause constriction of the helicine
arteries.

Striated fibers of the bulbospongiosus and ischiocaver-
nosus are relaxed to allow venous return of blood from
the cavernous spaces of the penis.

Neuroanatomically, sexual function in the female follows
the same pattern as in the male, with one obvious exception.
Although the female genital tract secretes fluids similar to
those of the male, there is no true ejaculate from the urethra
containing fluid or semen; genital tract fluids are deposited in
the vestibule. As in the male ejaculatory phase, however,
there are pulsatile contractions of the bulbospongiosus and
VS during the orgasmic phase in the female.

Parturition

During gestation, smooth muscle fibers of the uterus
become hypertrophied and stretched. In addition, pelvic
floor and perineal structures, including the musculature,
increase in bulk and strength to compensate for the erect
posture of the expectant mother. Specifically, sphincteric
fibers of the EUS, EAS, and VS undergo hypertrophy to
maintain urinary and anorectal continence in the presence
of the superincumbent fetus, which steadily increases in
bulk, and to block egress of the fetus through the cervical
canal [1]. The soft tissues that increase in bulk during preg¬
nancy for purposes of support will, of necessity, be stretched
or torn during labor and delivery and may even obstruct pas¬
sage of the fetus [6].

The weak, slow, rhythmic contractions of the uterus that
are present throughout most of pregnancy (Braxton Hicks
contractions) intensify toward the end of pregnancy and,
commencing with labor, become sufficiently strong to stretch
the cervix and move the fetus through the birth canal [106].
An increase in estrogen relative to progesterone and oxy¬
tocin, secreted by the neurohypophysis, are probably respon¬
sible for the uterine contractions [6,36]. Afferent nerve fibers
from the cervical canal and pelvic floor are involved in the
facilitation of neurogenic reflexes that contribute to a reflex
urgency in the expectant mother to bear down and expel the
fetus (399).

With progression of the labor, a true positive feedback
is established, one that is thought to be the prime mecha¬
nism for the onset and intensification of labor [36]. Specif¬
ically, descent of the fetus stretches the uterine cervix, and
visceral afferents stimulate uterine smooth muscle contrac¬
tions to push the fetus further into the cervical canal. At the
same time, pressure on the pelvic diaphragm and rectum
activates somatic afferent fibers that cause contraction of
the thoracic diaphragm and abdominal wall muscles, thus
increasing intraabdominal pressure and stimulating contrac¬
tion of the striated muscles of the pelvic diaphragm and per¬
ineum to resist the increase in intraabdominal pressure. As
labor proceeds, the major impediments to fetal passage are
offered by the cervix, pelvic diaphragm, and perineum [6].
As uterine contractions intensify in force and frequency, the

reflexly facilitated muscles at the pelvic outlet are torn to
allow egress of the fetus. At this time, periurethral and
perianal muscle fibers, as well as the perineal body, can rup¬
ture spontaneously.

PELVIC MUSCLE DYSFUNCTION

Pelvic floor dysfunction describes a wide variety of clinical
conditions involving impairment, separately or in combina¬
tion, of the nervous, muscular, and fascial elements of the
pelvic floor and perineum. These include disorders of mic¬
turition, defecation, and sexual function as well as organ pro¬
lapse and pelvic discomfort. Although female gender, parity,
and advanced age are recognized risk factors for pelvic floor
dysfunction, other factors may put men as well as women at
risk at nearly all ages. Interest in this area has escalated
sharply in the past 10-20 years, reflected by the number of
publications in scientific and clinical journals and the fre¬
quency with which this topic is addressed at professional
meetings. Data from many of the studies, particularly epi¬
demiological, are difficult to interpret and compare because
of discrepancies in the basic definition of the specific disor¬
der, errors, and inconsistencies in study design, and source of
subjects [116]. Clearly, more well-designed, longitudinal
studies need to be performed to answer the multitude of
questions related to pelvic floor dysfunction. The discussion
here emphasizes pelvic organ prolapse in women and both
varieties of incontinence (urinary and anorectal) because they
share etiological features and they involve the neuromuscular
elements that are the subject of this chapter. Comprehensive
reviews provide more information regarding the causes [87]
and epidemiology [116] of pelvic floor dysfunction.

Pelvic Organ Prolapse

Insofar as muscles of the urogenital and pelvic diaphragms
along with the pelvic fascia and visceral ligaments support the
organs of the pelvic cavity, weakness or rupture can lead to
pelvic organ prolapse, defined as protrusion of one or more
pelvic organs into the vaginal canal [89]. Uterine prolapse
occurs when the uterine cervix descends into the vagina. Rec-
tocele, cystocele, and urethrocele refer, respectively, to
bulging of the rectum, bladder, or urethra into the posterior
or anterior wall of the vagina.

The exact cause of pelvic organ prolapse and the number of
women who develop it are unknown [87,116,117]. Weakening
or loss of the noncontractile elements (visceral ligaments, fas¬
cia) of the pelvic floor and perineum and concomitant loss of
pelvic organ support is the traditional explanation [7,89]; how¬
ever, injury to the pudendal nerve and denervation of muscles
of the pelvic diaphragm and perineum have also been impli¬
cated in the etiology of prolapse [64,100,109,113]. An esti¬
mated 50% of parous women have pelvic organ prolapse to
some degree [7]. Conservative treatment of this disorder
involves exercises and other techniques to strengthen muscles

672

Part III I KINESIOLOGY OF THE HEAD AND SPINE

of the pelvic diaphragm and perineum, and surgery may be
indicated in some individuals [87]. The National Center for
Health Statistics reports that approximately 400,000 women
receive surgical intervention for genitourinary prolapse per
annum [79,81].

Urinary Incontinence

In 1988, the National Institutes of Health (NIH) Consensus
Conference [76] defined urinary incontinence as “the
involuntary loss of urine so severe as to have social and/or
hygienic consequences.” A similar definition has been
adopted by the International Continence Society [45]. This
disorder is a huge problem, socially, psychologically, and eco¬
nomically. Although the true incidence is unknown, the esti¬
mate is 13 million Americans in 1996 at a cost of $15 billion
[87], and 10 million in the United Kingdom in 1999 at a cost
of £1.4 billion [45]. Urinary incontinence is more common in
women than men, in older than younger women, and in mul¬
tiparous than nulliparous women [30,76,87,116]. Prevalence
of urinary incontinence is estimated to be 10-30% in females
and 1.5-5% in males, aged 15 to 64 years [30]. Its prevalence
in institutionalized individuals can be as high as 50% [30].

Factors contributing to the development of urinary incon¬
tinence are numerous. They include prostatectomy in males,
changes in hormone status and vaginal delivery in females,
and supraspinal neurological lesions, advanced age, func¬
tional impairment, and drugs [118] in both sexes. In males,
earlier detection of prostate cancer and its surgical treatment
has resulted in an increased incidence of postprostatectomy
incontinence [25,59,83]. Since parts of the female urinary
tract contain estrogen receptors, alterations in levels of circu¬
lating estrogen and progesterone during menstruation or with
menopause can affect continence [44]. Vaginal delivery can
damage the nerves to the pelvic floor as well as the muscles,
particularly the EUS [2,64,102,108,109,112]. The resultant
urinary incontinence, transient or long term, has been
observed in as many as 20-30% of women after their first
pregnancy and delivery [99]. Furthermore, compared with
males, a smaller motor pool controlling fewer muscle fibers in
this area and the absence of an IUS may all contribute to the
higher prevalence of urinary incontinence in females. With
advancing age, there is a significant loss of striated muscle
cells in the pelvic diaphragm, an increase in connective tissue
elements, and a decrease in vascularity [16,24]. Neurological
lesions of the spinal cord or below the pons can result in
detrusor-sphincter dyssynergia, characterized by lack of
coordination between detrusor contraction and EUS relax¬
ation; lesions above the pons may cause detrusor hyper-
reflexia, characterized by loss of inhibition to the detrusor
[118]. Taken together, the loss of estrogen, one or more vagi¬
nal births, the loss of muscle mass and replacement by con¬
nective tissue, and the decreasing functional level of many
individuals in an ever-enlarging aged population that is
increasingly dependent upon medications, one can readily
understand the scope of the urinary incontinence problem.

Anorectal Incontinence

Although not considered as widespread a problem as urinary
incontinence, anorectal incontinence is nonetheless a seri¬
ous problem. More than any other type of pelvic floor dys¬
function, anorectal incontinence causes social withdrawal and
mental anguish, and it may be (along with urinary inconti¬
nence) the single most important factor in the decision to
place an individual in an institution [45,87]. Since patients are
extremely embarrassed and reluctant to admit to anorectal
incontinence, even to their physicians, accurate epidemiolog¬
ical data regarding the “pelvic floor closet issue of the 1990s”
[116] are difficult to come by. In addition, definitions of
anorectal incontinence vary from involuntary loss of flatus
(gas) to liquid or solid fecal material [87,116]. Prevalence
reports range from 1 [114] to 18% [87].

Urinary and anorectal incontinence have some factors
common to their development—vaginal delivery, supraspinal
neurological lesions, advanced age, functional impairment,
and drugs. Iatrogenic or parturient damage to the levator ani,
particularly the puborectalis, the EAS, and their nerve fibers,
may be causative in many cases of anorectal incontinence
[2,22,103,108,109,112].

Role of the Therapist in Management
of Pelvic Floor Dysfunction

Conservative management of all types of pelvic dysfunction
that involve weakened pelvic and perineal musculature as
well as pelvic pain should include a therapist. As recently as
two decades ago, therapists regularly saw patients for prena¬
tal and postpartum pelvic floor (Kegel) exercises [48]. That
practice generally fell by the wayside when time constraints,
reimbursement issues, and flagging interest began to limit
evaluation and treatment of this population of patients. The
past few years, however, have witnessed a resurgence of clin¬
ical and scientific interest in individuals with pelvic floor dys¬
function. Increasingly, therapists are making this an
important emphasis in their practice and research activities.
This trend should continue and escalate as therapists, who are
eminently qualified to care for these patients, learn more
about the efficacy of therapeutic interventions in this pre¬
dominately female population.

SUMMARY

This chapter details the structure, function, and innervation of
the striated muscles of the pelvis and perineum of both sexes.
The three layers of muscle from deep to superficial are the
pelvic diaphragm, deep perineal muscles, and superficial per¬
ineal muscles. Function and metabolic properties of the fibers
that compose these muscles are discussed. The muscles of the
pelvic diaphragm and perineum are composed predominately
of fatigue-resistant type I fibers. Type II fibers predominate in
the regions immediately surrounding the urethral, vaginal, and
anal orifices. The roles of the specific fiber types in controlling

Chapter 36 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY IN THE PELVIS

673

the visceral functions of micturition, defecation, sexual func¬
tion, parturition, and pelvic organ support are discussed.

To facilitate an understanding of the special nature of the
pelvic and perineal muscles and the ways they are involved in
dysfunction, specifics of their neurological control and coordi¬
nation of their function with that of pelvic visceral structures
are presented. Function of the pelvic musculature exhibits a
finely regulated feedback system involving sympathetic and
parasympathetic influence for conscious and unconscious
control. Specific attention is given to sexual dimorphism of
these muscles as well as the neurons that control them. Males
exhibit stronger muscles providing better pelvic control dur¬
ing high loads. Females exhibit smaller muscles and more fas¬
cial tissue providing better static control, especially during
pregnancy. Pelvic dysfunction, more common in women than
in men, may involve dysfunction in the muscular, nervous, or
fascial components of the pelvis. Numerous clinically signifi¬
cant sequelae of pelvic and perineal musculature dysfunction
related to gender, age, vaginal delivery, muscular atrophy, and
nervous degeneration are presented.

References

1. Abitol MM: Quadrupedalism, bipedalism, and human preg¬
nancy. In: Vleeming A, Mooney V, Snijders CJ, et al. eds.
Movement, Stability and Low Back Pain. New York: Churchill
Livingstone, 1997; 395-404.

2. Allen RE, Hosker GL, Smith ARB, Warrell DW: Pelvic floor
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CHAPTER

Analysis of the Forces on the Pelvis
during Activity

CHAPTER CONTENTS

FORCES SUSTAINED AT THE LUMBOSACRAL JUNCTION.676

Two-Dimensional Example of the Analysis of Forces on the Pelvis.677

Loads at the Lumbosacral Junction.679

FORCES SUSTAINED AT THE SACROILIAC JOINTS.681

Overview of the Analytical Model of the Sacroiliac Joint.681

Sacroiliac Joint Forces from the Literature.683

MECHANICS OF PELVIC FRACTURES .683

SUMMARY .683

M any complaints of pelvic pain are mechanical and may be related to the forces sustained by the pelvis. This
chapter examines the forces exerted on the joints and in the bones of the pelvis during activity and the
loads sustained during injuries. The unique features of the lumbosacral junction described in Chapter 35
limit the generalizability of the findings in the lumbar spine to the lumbosacral junction, which exhibits its own
mechanical challenges. The sacroiliac joints apparently move and also are susceptible to mechanical dysfunctions. The
magnitude and direction of the forces across the sacroiliac joint may contribute to patients' complaints. High loads
associated with impacts such as those seen in motor vehicle accidents can produce pelvic fractures. An understanding
of the forces generated in such collisions may lead to better rehabilitation strategies to minimize the impairments fol¬
lowing such injuries. The purposes of this chapter are to examine the loads sustained by the pelvis and its associated
joints and to provide a simplified analysis of the forces applied to the region. The specific goals of this chapter are to

■ Provide examples of two-dimensional kinetic analysis of the pelvis
■ Examine the forces sustained by the lumbosacral junction
■ Analyze the forces across the sacroiliac joints
■ Investigate the mechanics of pelvic fractures

FORCES SUSTAINED AT THE
LUMBOSACRAL JUNCTION

The lumbosacral junction and the L4-L5 junction are the
most common sites of disc lesions in the low back [13]. In
addition, the lumbosacral junction is susceptible to anterior
slippage of L5 on SI, a phenomenon known as spondylolis¬
thesis (Fig. 37.1 ) [20]. An understanding of the forces gener¬
ated at this joint complex helps explain the pathomechanics

associated with these disorders. However, fewer studies
investigate the forces at the lumbosacral junction than inves¬
tigate the other segments of the lumbar spine. One analysis of
the region identifies 114 individual muscle units capable of
exerting unique forces on the lumbosacral junction [25].
Sophisticated analytical tools and many simplifying assump¬
tions beyond the scope of this book are required to solve for
the muscle and joint forces in this indeterminate system, a
system with more unknowns than equations to solve.

676

Chapter 37 I ANALYSIS OF THE FORCES ON THE PELVIS DURING ACTIVITY

677

Figure 37.1: Spondylolisthesis of L5 on SI. Radiograph shows that
L5 has slipped anteriorly on SI.

(Chapter 1 provides a brief overview of the approaches to
solving indeterminate systems.) The following is a greatly sim¬
plified analytic model to examine the forces on the pelvis.

Two-Dimensional Example of the
Analysis of Forces on the Pelvis

Examining the Forces Box 37.1 presents a two-dimensional
analysis of the forces on the lumbosacral junction. This
example uses the assumption that all of the muscular and
ligamentous forces can be lumped together into a single
muscle, M. Because this assumption undoubtedly is false,
the results derived from this analysis are, at best, rough esti¬
mates of the real loads sustained by the region and probably
underestimate the true reaction forces.

The simplified model in Examining the Forces Box 37.1
estimates loads in the single extensor muscle to be approxi¬
mately 1.12 times body weight, and compressive and shear
forces of 1,305 N and 379 N (293 and 85 lb), respectively
These loads are well below the loads to failure reported for
the lumbar spine in Chapter 34. However, these loads are
generated by lifting a small load (10% body weight) while
bending the knees.

Because the L5-S1 junction is so prone to disc lesions and
to spondylolisthesis, it is useful to consider what factors might

EXAMINING THE FORCES BOX 37.1

SIMPLIFIED TWO-DIMENSIONAL ANALYSIS
OF THE LOADS ON THE LUMBOSACRAL
JUNCTION

What are the loads on the lumbosacral junction for a
120-lb (534 N) woman who bends to pick up a 12-lb
load (10% of her body weight)? To solve this problem
using the basic mechanical tools described in Chapter 1 7
the muscles and ligaments supporting the lum¬
bosacral region are lumped into a single extensor
muscle [26], although Chapter 34 clearly demon¬
strates that many muscles and ligaments participate
together to support the low back during bending
activities.

The static equilibrium conditions used to solve this
question are

XM = 0

2F X = 0
2F y = 0

The following anthropometric quantities are found in
the literature [7 # 16]:

Weight of the head, arms, and trunk (HAT) is approxi¬
mately 69% of body weight = 320.4 N

Center of gravity of the HAT weight is approximately
60% of the length from hip joint to top of the
head = 0.46 m

Moment arm of the single equivalent extensor
muscle = 0.065 m

Angle of flexion of the trunk = 30°

Angle between the plane of the lumbosacral junction
and the transverse plane = 30°

Using the static equilibrium equations, first calculate
the equivalent extensor muscle force.

XM = 0

53.4 N X (0.48 m X sin 30°) + 320.4 N X (0.46 X sin 30°)
-(MX 0.065 m) = 0

12.8 Nm + 73.6 Nm = M X (0.065 m)

M = 1132 N, or 1.12 times body weight

Once the muscle force is known, the joint reaction
forces at the lumbosacral junction can be determined.

A coordinate system oriented in the fifth lumbar verte¬
bra allows direct calculation of the compression and
shear forces on L5. The shear forces lie parallel to the

(continued)

678

Part III I KINESIOLOGY OF THE HEAD AND SPINE

EXAMINING THE FORCES BOX 37.1 ( Continued)

x axis, and the compression forces lie parallel to the y
axis.

The shear force on L5 is determined by the following:

SF x = 0

j x -M x+ L x + W x = 0

J x - (M X sin 30°) + (L X sin 30°) + (W X sin 30°) = 0

J x - 566 N + 26.7 N + 160.2 N = 0

J x = 379.1 N or approximately 0.71 times body weight

The compression force on L5 is determined by the
following:

2F y = 0

J Y - M y - L y - W Y = 0

J Y - (M X cos 30°) - (LX cos 30°) - (W X cos 30°) = 0

J Y - 981 N - 46.3 N - 277.8 N = 0

J Y = 1305.1 N, or approximately 2.44 times body
weight

increase the compressive or shear forces to dangerous levels.
The compressive load is primarily a function of the muscle
force needed to support the junction (Fig. 37.2). Any increase
in the externally applied moment necessitates an increase in
muscle force. Picking up a larger load increases the external
moment, as does lifting a small load while holding it farther
from the body, which increases the moment arm of the load
(Fig. 37.3). Both cases require increased muscle force, lead¬
ing to larger compressive forces at the lumbosacral junction.

The orientation of the lumbosacral junction also affects the
magnitude of the compression and shear forces because the

compression force is approximately perpendicular to the bod¬
ies of L5 and SI, and the shear forces are parallel to the plane
between the bodies of L5 and SI. Shear forces appear to be
more dangerous than compressive loads on the spine. Because
the plane of the lumbosacral junction usually is oriented at a
larger angle from the horizontal than the rest of the lumbar
spine, the lumbosacral junction is particularly susceptible to
anterior shear forces [20] (Fig. 37.4). As the angle between the
plane of the vertebral bodies and the transverse plane increases,
the shear component of the weight of the head, arms, and
trunk (HAT weight) and any lifted weight also increases.

Chapter 37 I ANALYSIS OF THE FORCES ON THE PELVIS DURING ACTIVITY

679

Figure 37.2: Several muscles crossing the lumbosacral junction
contract simultaneously during activity, increasing the compres¬
sive force on the junction.

Slouched sitting posture also appears to increase the anterior
shear forces on the lumbosacral junction when the backrest
pushes the HAT weight anteriorly as the sacrum rotates pos¬
teriorly (Fig. 37.5) [21].

Clinical Relevance

SPONDYLOLISTHESIS: Spondylolisthesis frequently is
asymptomatic but may be painful especially in active individ¬
uals [2,20]. Individuals with spondylolisthesis frequently report
pain with increased activity, especially activities that use hyper¬
extension of the low back. In contrast, many other patients
with low back pain have increased pain with trunk flexion and
report that lumbar extension relieves the symptoms. Although
the cause of the back pain is frequently unclear, it is essential
that the clinician identify the movements that exacerbate the
symptoms and those that relieve the symptoms.

Loads at the Lumbosacral Junction

There are few studies that specifically examine the loads
on the lumbosacral joints during activity. Most of these
studies use the same basic approach to calculate the forces
in the muscles and ligaments and across the lumbosacral
junction as demonstrated in Examining the Forces Box
37.1. However, these studies apply more-sophisticated

Figure 37.3: A. The external moment (M EXT ) on the lum¬
bosacral junction is the sum of the moments due to the
weight of the head, arms, and trunk (W) and the moment
due to the load being lifted (F). An increase in the magni¬
tude of either W or F increases the external moment on
the lumbosacral junction. B. An increase in the moment
arm of the head, arms, and trunk weight (df or the
moment arm of the load (d 2 ) also increases the external
moment (M EXT ) on the lumbosacral junction.

680

Part III I KINESIOLOGY OF THE HEAD AND SPINE

Figure 37.4: A. The anterior shear force component of the
weight of the head, arms, and trunk (W) on the lumbosacral
junction is parallel to the plane of the L5-S1 junction. B. As the
inclination of the L5-S1 junction increases, the shear component
also increases.

Figure 37.5: Anterior shear force on the lumbosacral joint. The
backrest of the chair pushes the trunk anteriorly while the
sacrum rotates posteriorly.

mathematical tools to derive more-accurate solutions and
to estimate the loads in the individual muscles and liga¬
ments [1,6,17,26]. It is important, however, to recognize
that even these studies require simplifying assumptions,
and the outcomes from these calculations depend on the
accuracy of the assumptions [1,17,26]. Consequently, cur¬
rent studies yield only general approximations of the loads
actually generated in the region. Despite the limitations of
these studies, their results offer clinicians an insight into
the demands of some activities and help clinicians to iden¬
tify strategies to minimize a patients complaints.

LOADS IN THE LUMBOSACRAL REGION DURING
BENDING AND LIFTING

Peak joint moments between 200 and 250 Nm are reported
at the lumbosacral joint while lifting or lowering 10- to 15-kg
loads (approximately 22-33 lb) [6,17]. These data are consis¬
tent with loads in the lumbar spine reported in Chapter 34.
Models that report the joint reaction force on the joint center
of the lumbosacral junction show more variability. Estimates
of compressive loads on the disc range from 1,200 N (270 lb)
to more than 5,500 N (1,236 lb) [1,26,27]. Reported peak
anterior shear forces range from approximately 400 to 1200 N
(90-270 lb) [26]. Calculations of the joint moments and forces
depend on the assumptions made in the model, including the
size of the trunk and pelvis, the shape of the lumbar curve,
and the muscles and ligaments included in the model as well
as the movements of the spine that are studied [1,26].
Additional studies are needed to provide a more precise esti¬
mate of the loads sustained by the lumbosacral junction.

Peak compression loads on the lumbar spine reported in
Chapter 34 are over 7,000 N (1573 lb) when lifting a 27-kg
force (60 lb). However, in the lumbar spine, lifting with the
lumbar spine flexed greatly increases the anterior shear forces
by inhibiting the contraction of the extensor muscles (see
video from Chapter 34). Because spondylolisthesis is a com¬
mon occurrence at the L5-S1 junction and may produce
symptomatic back pain in some individuals, clinicians need
similar studies that examine the effects of posture and bend¬
ing technique on the shear forces on the lumbosacral joint to
guide intervention and prevention strategies.

LOADS ON THE LUMBOSACRAL JOINT
DURING WALKING

Average peak compression forces at the lumbosacral joint
range from 1.7 to 2.52 times body weight, and anterior shear
forces range from 0.22 to 0.33 times body weight during
preferred speed walking [3,13]. The resultant forces on the
lumbosacral facet joints, while smaller than the loads on the
disc, are approximately 1.5 times body weight [3]. The reac¬
tion forces on both the disc and facet joints peak during the
double limb support phases of gait, when the pelvis is tilted
anteriorly [15,24] (Fig. 37.6). Because anterior pelvic tilt is
associated with an increase in lumbar joint extension
(increased lumbar lordosis), trunk hyperextension appears

Chapter 37 I ANALYSIS OF THE FORCES ON THE PELVIS DURING ACTIVITY

681

Figure 37.6: During gait, the pelvis is tilted anteriorly more
at double limb support than during single limb support.

to increase loads on the L5-S1 junction, which may help
explain why some patients report increased low back pain
with walking.

FORCES SUSTAINED AT
THE SACROILIAC JOINTS

Forces on the sacroiliac joint are even less well studied than
those on the lumbosacral junction. Because the joint appears
to allow at least small movements, an appreciation of the
forces exerted across the joint may improve the clinicians
understanding of the pathomechanics of sacroiliac joint dys¬
function [11,22].

Overview of the Analytical Model
of the Sacroiliac Joint

Like all of the biomechanical analyses demonstrated
throughout this text, analysis of the forces at the sacroiliac
joint begins with a free-body diagram. The free-body dia¬
gram of the sacroiliac joint is complicated by the fact that so
many structures affecting the joint actually have no attach¬
ment on either the ilium or sacrum. To assist in identifying
the relevant loads during single limb stance, the sacrum is
considered a part of a rigid body including the head, arms,
trunk and non-weight-bearing lower extremity, and the ilium

Figure 37.7: To analyze the forces on the sacroiliac joint, it is use¬
ful to view the body as two segments, the ilium with the weight¬
bearing limb and the sacrum with the head, arms, trunk, and the
non-weight-bearing lower extremity.

as part of a rigid body including the pelvis and lower extrem¬
ity on the weight bearing side [8] (Fig. 37.7). Determination
of the forces on the pelvis in such a model requires the inclu¬
sion of hip and trunk muscles as well as pelvic ligaments and
the ground reaction force, leading to another indeterminate
system, with many more unknowns than equations to solve.
Models of the sacroiliac joint report up to approximately 100
unknowns [8,28]. Because the sacroiliac joint exhibits com¬
plex three-dimensional motion, a two-dimensional model is
insufficient to even approximate the mechanics of the joint
[11,22]. Examining the Forces Box 37.2 outlines the basic

682

Part III I KINESIOLOGY OF THE HEAD AND SPINE

EXAMINING THE FORCES BOX 37.2

EXAMINATION OF THE FORCES ON THE
SACROILIAC JOINT DURING STANCE
ON ONE LEG

To consider the forces on the innominate bone of the
sacroiliac joint, the lower extremity and innominate
bone are lumped together as a single rigid body. Forces
on this rigid body are depicted in the free body diagram.

Using the static equilibrium conditions to solve this
question yields the following equations:

2M X = 0 = 2F musi x ma mus , + 2M EXTx

where F musj is the force in each muscle and ligament,
ma musi' s the moment arm for that force (i.e., the per¬
pendicular distance between the force and the point
of rotation in the y-z plane), and M EXTx is the external
moments about the x axis applied by the segment
weights and ground reaction force.

SM Y = 0 = 2F musi x ma musi + 2M EXTy

where F musj is the force in each muscle and ligament,
ma musi' s the moment arm for that force (i.e., the per¬
pendicular distance between the force and the point
of rotation in the x-z plane), and M EXTy is the external
moments about the y axis applied by the segment
weights and ground reaction force.

2M Z = 0 = £F musi x ma musi + £M EXTz

where F musj is the force in each muscle and ligament,
ma musi' s the moment arm for that force (i.e., the per¬
pendicular distance between the force and the point
of rotation in the x-y plane) and M EXTz is the external
moments about the Z axis applied by the segment
weights and ground reaction force.

2F x = 0 = 2F musix + G x + J x

where F musjx is the force of each muscle and ligament
in the x direction, G x is the ground reaction force in
the x direction, and J x is the joint reaction force in the
x direction.

2F Y = 0 = 2F musiy + G Y + J Y

where F musjy is the force of each muscle and ligament
in the y direction, G Y is the ground reaction force in
the y direction, W is the weights of the segments that
all act in the y direction, and J Y is the joint reaction
force in the y direction.

2F Z = 0 = SF musiz + G z + J z ,

where F musjz is the force of each muscle and ligament
in the z direction, G z is the ground reaction force in
the z direction, and J z is the joint reaction force in the
z direction.

Knowing the anatomy of the structure through the use
of various imaging techniques allows measurement of
all of the relevant moment arms. Force plates in the
ground measure the ground reaction forces, and limb
segment weights are available. Therefore, the muscle
and ligament forces and the joint reaction forces are
the only unknowns in the equations. However, there
are still too many unknowns to be solved by these six
equations. Techniques briefly described in Chapter 1 are
needed to solve this statically indeterminate system.

Chapter 37 I ANALYSIS OF THE FORCES ON THE PELVIS DURING ACTIVITY

683

three-dimensional problem to solve for sacroiliac joint forces
and the forces in the surrounding muscles and ligaments, but
complete analysis is beyond the scope of this book.

Clinical Relevance

STANDING HIP FLEXION TEST FOR SACROILIAC
DYSFUNCTION: One test for dysfunction of the sacroiliac
joint complex requires the patient to stand on one leg and flex
the opposite hip, bringing the knee toward the chest
A normal response to the test is a posterior rotation
of the ilia on the sacrum. A positive response for pos¬
sible sacroiliac joint complex dysfunction is pain in the area of
the sacroiliac joint on the stance side. Motion analysis during
the test reveals that patients with pain associated with the
sacroiliac joint complex often demonstrate an anterior rotation
of the ilium on the sacrum on the weight-bearing side [10].

This test applies the principles of loading at the sacroiliac joint
to test the stability of the joint. Single limb stance applies enor¬
mous loads through the sacroiliac joint, requiring large stabi¬
lizing forces from surrounding ligaments and joints. An inabili¬
ty to maintain stability during single limb stance may be a
contributing factor to the patient's complaints.

Sacroiliac Joint Forces from the Literature

Loads on the sacroiliac joint between 0.85 and 1.1 times
body weight are reported for static single leg stance [8].
Forces on the sacroiliac joint over four times body weight
are reported at the end of single limb support during gait
[4]. Walking requires more muscle activity than static
single limb stance, and it is consistent that the calculations
show that the sacroiliac joint sustains larger loads during
ambulation. Authors report the need for muscular activity
and large muscle forces to stabilize the sacroiliac joint dur¬
ing function [14,18,19,22,23,28]. The extensor muscles
that attach close to the sacroiliac joint appear to help sup¬
port the low back and generate forces of more than
6,500 N (1,430 lb) [14]. Although only a few stud¬
ies examine forces at the sacroiliac joint, the joints
appear to sustain large loads, which may contribute to
sacroiliac joint dysfunction in some individuals.

MECHANICS OF PELVIC FRACTURES

Most pelvic fractures occur from motor vehicle accidents,
usually from side impacts [5]. Lateral impacts load the pelvis
through the acetabulum after the lateral aspect of the femur,
typically the greater trochanter, is hit. The site of the result¬
ing pelvic fracture(s) depends on the velocity of the impact
as well as the magnitude of the force applied. The impor¬
tance of impact velocity is consistent with the mechanical
properties of bone described in Chapter 3, which reports

that bone strength and elasticity depend on the rate at
which the bone is loaded. A force of approximately 8,600 N
(1,933 lb) applied at a rate consistent with a car traveling
about 25 miles per hour produces a fracture of the pubic
ramus on the side opposite the impact. More-extensive frac¬
tures and even dislocations of the sacroiliac joints and pubic
symphysis can result from higher loading rates or greater
impact forces. Automotive engineers can use these data to
design safety systems such as restraint systems and air bag
devices to reduce the injuries sustained in motor vehicle
accidents. These data also help practitioners appreciate the
extent of trauma that may be sustained by individuals in
motor vehicle accidents.

Although acute pelvic fractures are the most common
pelvic fracture, stress fractures of the pelvis also occur [9,12].
Fractures of the pubic ramus, usually the inferior ramus, are
reported in female military recruits and are associated with
the use of an unnaturally long stride, particularly by shorter
women and women who train with men [9,12]. Bone density,
fitness level, and menses do not appear to predict pelvic frac¬
tures, but African American women exhibit stress fractures
less frequently than Caucasian women do. Pelvic stress frac¬
tures occur most frequently in the narrowest part of the pubic
ramus and may be the culmination of repeated loads from the
adductor muscles during the gait cycle.

Clinical Relevance

STRESS FRACTURES OF THE PELVIS: Symptoms of
pelvic stress fractures include groin pain, which is also a
common symptom in individuals with chronic hip dysfunc¬
tion. Practitioners may need to consider the presence of
pelvic stress fractures in small women, particularly
Caucasian women, with complaints of groin pain and who
exhibit no direct signs of hip dysfunction. A history of
repeated loading, such as running, also is relevant.

SUMMARY

This chapter examines the loads sustained by the pelvis during
single limb stance and during impact loading. An accurate
analysis of the loads on the pelvis requires a more sophisticated
analysis than that presented in this text, but this chapter
reviews the basic application of static equilibrium equations to
determine the loads at the lumbosacral junction and at the
sacroiliac joints. Estimates of the compressive loads on the
lumbosacral junction are as high as 5,500 N (1,236 lb), with
estimated shear forces up to 1,200 N (270 lb). Normal walking
also produces loads of more than two times body weight at the
lumbosacral junction. Forces greater than four times body
weight are reported at the sacroiliac joint during gait. High-
impact loads such as those incurred during motor vehicle acci¬
dents can produce pelvic fractures as well as dislocations of the

684

Part III I KINESIOLOGY OF THE HEAD AND SPINE

joints of the pelvis. The pelvis also sustains stress fractures,
particularly in Caucasian women with low body mass.

This three-chapter unit on the mechanics of the pelvis
completes the discussion of the spine. This unit has repeat¬
edly noted that the pelvis transmits the weight of the head,
arms, and trunk to the lower extremities. The following unit
on the hip begins the discussion of the lower extremity.

References

1. Anderson CK, Chaffin DB, Herrin GD, Matthews LS: A bio¬
mechanical model of the lumbosacral joint during lifting activi¬
ties. J Biomech 1985; 18: 571-584.

2. Canale ST: Campbells operative orthopaedics. Philadelphia:
Mosby, 1998.

3. Cheng CK, Chen HH, Chen CS, Lee SJ: Influences of walking
speed change on the lumbosacral joint force distribution.
Biomed Mater Eng 1998; 8: 155-165.

4. Dalstra M, Huiskes R: Load transfer across the pelvic bone.
J Biomech 1995; 28: 715-724.

5. Dawson JM, Khmelniker BV, McAndrew MP: Analysis of the
structural behavior of the pelvis during lateral impact using the
finite element method. Accid Anal Prev 1999; 31: 109-119.

6. de Looze MP, Toussaint HM, van Dieen JH, Kemper HCG:
Joint moments and muscle activity in the lower extremities and
lower back in lifting; and lowering tasks. J Biomech 1993; 26:
1067-1076.

7. Dempster WT: Space requirements of the seated operator, geo¬
metrical, kinematic, and mechanical aspects of the body with
special reference to the limbs. In: Krogman WM, Johnston FE,
eds. Human Mechanics. Philadelphia: Aerospace Medical
Division, 1963; 215-340.

8. Goel VK, Svensson NL: Forces on the pelvis. J Biomech 1977;
10: 195-200.

9. Hill PF, Chatterji S, Chambers D, Keeling JD: Stress fracture of
the pubic ramus in female recruits. J Bone Joint Surg [Br] 1996;
78—B: 383-386.

10. Hungerford B, Gilleard W, Lee D: Altered patterns of pelvic
bone motion determined in subjects with posterior pelvic pain
using skin markers. Clin Biomech 2004; 19: 456-464.

11. Jacob HAC, Kissling RO: The mobility of the sacroiliac joints in
healthy volunteers between 20 and 50 years of age. Clin
Biomech 1995; 10: 352-361.

12. Kelly EW, Jonson SR, Cohen ME, Shaffer R: Stress fracture of
the pelvis in female navy recruits: an analysis of possible mech¬
anisms of injury. Milit Med 2000; 165: 142-146.

13. Khoo BCC, Goh JCH, Bose K: A biomedical model to deter¬
mine lumbosacral loads during single stance phase in normal
gait. Med Eng Phys 1995; 17: 27-35.

14. McGill SM: A biomechanical perspective of sacro-iliac pain.
Clin Biomech 1987; 2: 145-151.

15. Murray MP: Gait as a total pattern of movement. Am J Phys
Med 1967; 48: 290-333.

16. Nemeth G, Ohlsen H: Moment arm lengths of trunk muscles to
the lumbosacral joint obtained in vivo with compute tomogra¬
phy. Spine 1986; 11: 158-160.

17. Plamondon A, Gagnon M, Gravel D: Moments at the L5/S1 joint
during asymmetrical lifting: effects of different load trajectories
and initial load positions. Clin Biomech 1995; 10: 128-136.

18. Pool-Goudzwaard A, Hoek van Dijke G, van Gurp M, et al.:
Contribution of pelvic floor muscles to stiffness of the pelvic
ring. Clin Biomech 2004; 19: 564-571.

19. Richardson CA, Snijders CJ, Hides JA, et al.: The relation
between the transversus abdominis muscles, sacroiliac joint
mechanics, and low back pain. Spine 2002; 27: 399-405.

20. Salter RB: Textbook of Disorders and Injuries of the
Musculoskeletal System. 3rd ed. Baltimore: Williams & Wilkins,
1999.

21. Snijders CJ, Hermans PFG, Niesing R, et al.: The influence of
slouching and lumbar support on iliolumbar ligaments, interver¬
tebral discs and sacroiliac joints. Clin Biomech 2004; 19: 323-329.

22. Snijders CJ, Ribbers MTLM, de Bakker HV, et al.: EMG
recordings of abdominal and back muscles in various standing
postures: validation of a biomechanical model on sacroiliac joint
stability. J Electromyogr Kinesiol 1998; 8: 205-214.

23. Snijders CJ, Vleeming A, Stoeckart R: Transfer of lumbosacral
load to iliac bones and legs part 2: loading of the sacroiliac joints
when lifting in a stooped posture. Clin Biomech 1993; 8: 295-301.

24. Sutherland DH, Kaufman KR, Moitoza JR: Kinematics of nor¬
mal human walking. In: Rose J, Gamble JG, eds. Human
Walking. Philadelphia: Williams & Wilkins, 1981; 23-44.

25. van Dieen JH: Are recruitment patterns of the trunk muscula¬
ture compatible with a synergy based on the maximization of
endurance? J Biomech 2001; 30: 1095-1100.

26. van Dieen JH, de Looze MP: Sensitivity of single-equivalent
extensor muscle models to anatomical and functional assump¬
tions. J Biomech 1999; 32: 195-198.

27. van Dieen JH, Kingma I: Total trunk muscle force and spinal
compression are lower in asymmetric moments as compared to
pure extension moments. J Biomech 1999; 32: 681-687.

28. Van Dijke GAH, Snijders CJ, Stoeckart R, Stam HJ: A biomed¬
ical model on muscle forces in the transfer of spinal load to the
pelvis and legs. J Biomech 1999; 32: 927-933.

PART

Kinesiology of the
Lower Extremity

UNIT 6: HIP UNIT

Chapter 38: Structure and Function of the Bones and Noncontractile Elements of the Hip
Chapter 39: Mechanics and Pathomechanics of Muscle Activity at the Hip
Chapter 40: Analysis of the Forces on the Hip during Activity

UNIT 7: KNEE UNIT

Chapter 41: Structure and Function of the Bones and Noncontractile Elements of the Knee
Chapter 42: Mechanics and Pathomechanics of Muscle Activity at the Knee
Chapter 43: Analysis of the Forces on the Knee during Activity

UNIT 8: ANKLE AND FOOT UNIT

Chapter 44: Structure and Function of the Bones and Noncontractile Elements of the
Ankle and Foot Complex

Chapter 45: Mechanics and Pathomechanics of Muscle Activity at the Ankle and Foot
Chapter 46: Analysis of the Forces on the Ankle and Foot during Activity

685

UNIT 6

HIP UNIT

T he hip marks the proximal end of the lower extremity, whose primary functions are weight bearing and loco¬
motion. Consequently, all of the joints of the lower extremity, including the hip joint, commonly function
with the foot in contact with the ground, participating in a closed chain. In a closed chain, movement of any
segment of the chain produces movement in other links of the chain. Hip position and muscular control at the hip
often depend on the location and movement of the trunk on the lower extremity rather than on the movement of the
femur on the pelvis.

The functional requirements of the hip itself are quite varied. It is the most mobile joint of the lower extremity, allow¬
ing such extreme positions as standing and squatting. In addition to mobility, however, the hip must also possess suffi¬
cient stability to support the weight of the head, arms, trunk, and opposite lower extremity during single limb stance
and dynamic activities such as walking and jumping. The hip joint successfully combines these seemingly conflicting
functions of mobility and stability by its unique bony structure and surrounding soft tissue.

The purposes of the three chapters on the hip are to

■ Demonstrate how the structures of the hip promote both mobility and stability

■ Discuss how the muscles of the hip move it and also stabilize the weight of the head, arms, and trunk on the hip
joint

■ Examine how alterations in these structures can lead to impaired function and deleterious loads on the hip and
neighboring structures

686

Structure and Function of the
Bones and Noncontractile
Elements of the Hip

CHAPTER CONTENTS

CHAPTER

STRUCTURE OF THE BONES OF THE HIP.688

Innominate Bone.688

Femur.689

STRUCTURE OF THE HIP JOINT .691

Joint Capsule.691

Iliofemoral, Pubofemoral, and Ischiofemoral Ligaments.692

Additional Ligaments.693

Hip Joint Stability .693

ALIGNMENT OF THE ARTICULATING SURFACES.694

NORMAL MOTION OF THE HIP.698

Normal Range of Motion.698

Normal Limiting Structures of Hip ROM.699

Contribution of the Pelvis to Hip Motion.699

Interaction of the Hip Joint and Lumbar Spine in Hip Motion.699

Hip Motion in Activities of Daily Living .701

COMPARISON OF THE HIP JOINT TO THE GLENOHUMERAL JOINT.702

SUMMARY .702

T his chapter examines the bony structure of the hip and the connective tissues that stabilize it and protect
it during movement and weight-bearing activities. The purposes of this chapter are to

■ Investigate details of the hip's bony structure to understand how specific characteristics contribute to the
stability and mobility of the hip joint

■ Study the noncontractile supporting structures of the hip to understand their effects on its stability and
mobility

■ Examine the normal ranges of motion (ROMs) available at the hip

■ Examine the relative alignment of the pelvis and femur and consider its contributions to normal and
abnormal mechanics of the hip

■ Compare the structure and function of the hip and the glenohumeral joint, its counterpart in the upper
extremity

687

688

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

STRUCTURE OF THE BONES OF THE HIP

The hip is composed of two large bones, the innominate bone
of the pelvis and the femur. Each of these bones is discussed
individually below.

Innominate Bone

The innominate bone contributes the proximal articular sur¬
face of the hip. The two innominate bones together form the
bony pelvis. The details of the bony pelvis are presented in
Chapter 35. For the purposes of the present chapter, the dis¬
cussion of the innominate bone is limited to those factors that
directly apply to the hip. Thus the acetabulum, providing the
proximal articular surface of the hip, is discussed in detail.

Located on the lateral aspect of the innominate bone, the
acetabulum comprises the Y-shaped junction of the ilium,
ischium, and pubis, which forms a deep, spherical socket that
holds the head of the femur (Fig. 38.1). The orientation of the
acetabulum influences the mobility of the hip and the location
of weight-bearing forces on the femoral head. An anterior
view of the pelvis reveals that the acetabulum faces laterally
and slightly inferiorly (Fig. 38.2). A superior view of the pelvis
demonstrates that the acetabulum also faces anteriorly.

The superior aspect, or roof, of the acetabulum is formed
by the ilium, the anterior aspect by the pubis, and the poste¬
rior wall by the ischium. The deepest portion of the acetabu¬
lum, known as the floor, or acetabular fossa, is rough and
nonarticular. The articular, or lunate, surface of the acetabu¬
lum consists of a horseshoe-shaped rim ringing the acetabu¬
lar fossa on its anterior, superior, and posterior aspects [66].
The rim is incomplete inferiorly, leaving a gap between the

Figure 38.1: The acetabulum is formed by the three bones of the
innominate bone, the ilium, ischium, and pubis.

Figure 38.2: Orientation of the acetabulum. A. Anterior view of
the pelvis shows that the acetabulum faces laterally and inferiorly.
B. Superior view of the pelvis shows that the acetabulum faces
anteriorly.

anterior and posterior segments. A transverse acetabular liga¬
ment spans the gap, completing the acetabular rim.

The floor of the acetabulum consists of a thin shelf of bone
that may be no more than 2-4 mm thick [21]. The density of
the subchondral bone increases in the periphery of the
acetabulum and appears to peak in the acetabular roof and in
the anterior and posterior extremes of the articular surface
[47,68]. These variations in bony thickness reflect Wolffs law,
which states that a bones structure responds to the loads
placed on it [6]. Weight bearing at the hip joint involves the
thicker superior and peripheral aspects of the acetabulum,
while the thin, central, deepest part of the socket is unsuited
for weight bearing [6,27,68]. Well-organized arrays of trabec¬
ular bone surrrounding the acetabulum, but particularly
superior to it, reinforce the weight-bearing capacity of the
socket [44].

A fibrocartilaginous ring, or labrum, deepens the acetabu¬
lum, which helps to stabilize the hip joint, increase contact

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

689

area, and decrease joint stress [8]. These functions are ful¬
filled while avoiding a loss of mobility since the increased sur¬
face area is a compressible ring. Additionally the acetabular
labrum appears to seal a pressurized layer of synovial fluid that
may protect the articular surfaces from damage [14]. Although
magnetic resonance imaging (MRI) of individuals without
known hip pathology reveals considerable variability in labral
shape and length, and some individuals without hip pain
appear to lack some portions of the labrum, labral tears are a
recognized source of hip pain [8,37,43]. Labral tears may not
only contribute directly to joint pain but also may destabilize
the joint and allow increased stress on the articular surfaces,
eventually leading to degenerative joint changes [14,31,37].

Clinical Relevance

ACETABULAR LABRAL TEARS: Labral tears in the hip are
a suspected cause of pain in many individuals with chronic hip
pain (Fig. 38.3). Frank trauma or repetitive microtrauma from
repeated twisting or pivoting motions are likely mechanisms of
labral tears. Athletes participating in sports such as soccer or
golf are particularly susceptible to labral tears. However ; clinical
diagnosis is difficult. Suggestive findings include pain with
active or passive hip flexion, medial rotation and adduction,
and clicking in the hip with these motions. Magnetic resonance
arthrography (MRA) has better sensitivity (66-95%) and speci¬
ficity (71-88%) than clinical tests , but arthroscopic surgery
remains the most reliable diagnostic procedure [37,43].

The relative depth of the acetabulum with its labrum
changes during fetal development and early childhood [55].
The ratio of depth to diameter of the acetabulum is greatest
in utero and least at or around the time of birth; it gradually
increases again throughout childhood. The shallow acetabu¬
lum at birth is an important risk factor for congenital hip dis¬
location. By adulthood, the acetabulum without the labrum is
slightly less than a hemisphere [29].

Femur

The femur, normally the largest bone of the body, is com¬
posed of a head, neck, and shaft, or body, which ends dis-
tally in the femoral condyles. This chapter discusses only
those attributes of the femur that apply to the hip, specifi¬
cally, the head, neck, and proximal end of the body of the
femur. The remainder of the femur is discussed in Chapter 41
with the knee.

The head of the femur provides the distal articular surface
of the hip joint (Fig. 38.4). The head of the femur in the adult
forms approximately two thirds of a sphere, although its sur¬
face is not actually perfectly spherical. The articular cartilage
on the femoral head provides a more spherical shape to the
articular surface. Even the healthy femoral head looks slightly
flattened on x-ray since the articular cartilage is not visualized
by standard x-ray [21]. The femoral head is covered with
articular cartilage throughout its surface, with the exception
of a small pit (fovea of the head of the femur) on its postero¬
medial aspect where the ligamentum teres attaches. The
articular cartilage of the femoral head is thickest centrally and
thins at the periphery of the head [21,29,33].

Figure 38.3: A magnetic resonance arthrogram shows a labral tear at the hip. (Reprinted from "Arthropscopy, vol. 21, Kelly BT, Weiland
DE, Schenker ML, Philippon MJ: Arthroscopic labral repair in the hip: surgical technique and review of the literature, 1496-1504, 2005,
with permission from Arthroscopy Association of North America.)

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Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 38.4: The head of the femur forms approximately two
thirds of a sphere, although its surface is not perfectly spherical.

The articular cartilage of the femoral head and of the
acetabulum is among the thickest in the body. Reported
thicknesses range from 0.7 to 3.6 mm, with the greatest thick¬
nesses usually found in the anterosuperior aspect of the
acetabulum [12,30]. The acetabular and femoral articular car¬
tilage surfaces exhibit small incongruities in shape, thickness,
and stiffness, which may facilitate cartilage lubrication and
chondrogenesis. They may also contribute to degenerative
changes of the articular cartilage [2,12].

Despite the small incongruities between the femoral head
and acetabulum, the bones of the hip joint are generally con¬
gruent with each other, and the congruency is improved even
more by the articular cartilage. This congruency provides two
important benefits. First, the congruency allows larger areas
of the joint to articulate with one another throughout the nat¬
ural ROM of the hip. This means that the loads sustained dur¬
ing weight bearing can be spread across larger surface areas,

thus reducing the stress (force/area) the joint must with¬
stand. Additionally, the congruency facilitates stability of the
joint throughout the ROM.

The femoral neck extends laterally and posteriorly from the
head of the femur and is almost entirely enclosed by the hip
joint capsule. The orientation of the head and neck of the
femur, like that of the acetabulum, influences hip excursion
and weight bearing. An anterior view of the femur reveals that
the femoral head faces medially and superiorly in the acetab¬
ulum (Fig. 38.5). In the frontal plane, the angle of inclina¬
tion refers to the approximately 125° angle between the neck
of the femur and the shaft of the femur. A transverse plane
view demonstrates that the head of the femur projects anteri¬
orly. The neck forms an angle of approximately 15° with the
plane of the femoral condyles.

The femoral neck sustains large bending moments as well as
tensile and compressive forces during weight bearing and is
reinforced by thickened cortical bone and organized arrays of
cancellous, or trabecular, bone [53]. Cancellous bone extends
from the shaft of the femur to the neck and head of the femur
in organized arrays within the intertrochanteric region and
along the superior and inferior aspects of the neck. A medial
array of cancellous bone extends from the medial cortex of the
femoral shaft to the weight-bearing surface of the femoral
head. Another bundle runs on the lateral aspect of the shaft
from the base of the greater trochanter and femoral neck to the
inferior aspect of the head of the femur (Fig. 38.6). The
arrangement of cancellous bone in the femur also provides
another graphic example of Wolff s law [6].

There are several landmarks on the femur, distal to the hip
joint proper, that are relevant to the function of the hip

Femoral head

Figure 38.5: Orientation of the femoral head. A. Anterior view
of the femur reveals that it faces medially and superiorly.

B. Superior view of the femur reveals that it faces anteriorly.

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

691

Figure 38.6: The trabecular bone in the proximal femur is highly
organized to resist the loads on the femoral head and neck.

(Fig. 38.7). Several serve as attachments for muscles of the
hip, and some are palpable, providing important landmarks
during a physical examination. The base of the neck is distin¬
guished from the shaft of the femur anteriorly by a roughened
intertrochanteric line running distally and medially from
greater to lesser trochanter. It continues as a spiral line, distal

and medial to the lesser trochanter, and proceeds posteriorly
to form the medial lip of the linea aspera.

The greater trochanter is a large prominence on the prox¬
imal end of the femoral shaft. It has anterior, lateral, posterior,
and superior surfaces and is readily palpable about a hands
length distance distal to the iliac crest. This significant pro¬
trusion of bone gives rise to several muscles, including the
large gluteal muscles. The location of the greater trochanter
distal to the femoral neck lengthens the moment arms of the
attached muscles, improving their mechanical advantage to
generate joint moments [25,51,60]. The lesser trochanter is
posteromedial on the proximal femoral shaft and provides
distal attachment for the iliopsoas tendon. Separating the two
trochanters posteriorly is the intertrochanteric crest. At the
proximal aspect of the crest is the quadrate tubercle. Distal to
the crest and greater trochanter is the gluteal tuberosity,
which continues distally to form the lateral lip of the linea
aspera. Distal to the lesser trochanter, directed toward the
medial lip of the linea aspera, is the pectineal line.

A clear image of each of the bones composing the hip
is essential to understanding their relationship to each
other as well as to developing the skills to perform a
thorough and valid physical examination of the hip. The relevant
palpable landmarks surrounding the hip are listed below:

• Anterior superior iliac crest

• Posterior superior iliac crest

• Ischial tuberosity

• Iliac crest

• Greater trochanter

• Greater sciatic notch

Figure 38.7: A. Anterior view of the proximal femur reveals
important landmarks including the head, neck, and greater and
lesser trochanters. B. Posterior view of the proximal femur
reveals the lesser and greater trochanters, intertrochanteric crest,
gluteal tuberosity, pectineal line, and linea aspera.

STRUCTURE OF THE HIP JOINT

The hip joint is a synovial, ball-and-socket, or triaxial, joint. To
meet its antagonistic functions of stability and mobility, the
hip has its own unique articular structures including its liga¬
ments and fibrocartilaginous expansion, the labrum. The rel¬
ative orientation of the proximal femur and acetabulum also
influences the mobility and stability available at the hip joint.
This section reviews the supporting structures of the hip and
their effects on hip motion.

Joint Capsule

As a synovial joint, the hip is supported by a synovial capsule that
is attached to the bony rim of the acetabulum proximally and to
the intertrochanteric crest and line of the femur distally (Fig.
38.8). The capsule of the hip joint is composed primarily of
fibers running parallel to its length, the longitudinal fibers. It
also possesses a band of fibers oriented circumferentially around
the center of the femoral neck [62,71]. This bundle is known as
the zona orbicularis, or femoral arcuate ligament [23].

The capsule encloses most of the femoral neck and the
entire femoral head. The blood supply to synovial joints
is generally provided by a network of blood vessels, or
anastomoses, at the attachment of the capsule and bone. The
primary blood supply to the femoral head and neck arises from

692

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 38.8: The hip joint capsule attaches to the acetabulum
proximally and to the intertrochanteric crest and line distally.

the medial and lateral circumflex femoral arteries at the base
of the femoral neck that then travel proximally within synovial
folds of the capsule reflected onto the femoral neck [71]. Thus
most of the vessels supplying the head of the femur must travel
the length of the femoral neck to reach the femoral head. The
femoral head does receive an artery within the ligament to the
head of the femur that attaches to the floor of acetabulum and
the pit of the head of the femur. However, anatomists believe
that the essential blood supply to the femoral head originates
at the base of the femoral neck [20,64].

Clinical Relevance

FRACTURES OF THE FEMORAL NECK: Disruption of the
hip joint capsule at the base of the femoral neck or injury to
the neck itself may disrupt the blood supply of the femoral
head and endanger the integrity of the head itself A serious
potential sequela of a femoral neck fracture is avascular
necrosis of the femoral head ' which can result when the
femoral head is separated from its blood supply in the
femoral neck. When the displacement of the femoral neck is
severe or when the time between injury and intervention is
several hours or more\ the risk of avascular necrosis increases.
In such cases , the orthopaedic surgeon may choose to
perform a partial or total joint replacement (arthroplasty)
rather than try to repair the fracture with pins or screws
[62]. Arthroplasty is particularly advantageous when the
fracture cannot be reduced readily or when it occurs in a
frail patient. In contrast intertrochanteric and sub¬
trochanteric fractures present considerably less risk to the
vascular supply because the capsule and femoral neck and '
consequently , the blood supply to the femoral head are usu¬
ally spared [6163]. Therefore\ these fractures are more
amenable to treatment by internal fixation.

Iliofemoral, Pubofemoral,
and Ischiofemoral Ligaments

The hip joint capsule is reinforced anteriorly by three longitu¬
dinal bundles of fibers, the iliofemoral, ischiofemoral, and
pubofemoral ligaments, the first two being the most consistent
and strongest [22,59,71] (Fig. 38.9). The three ligaments orig¬
inate on their respective bony parts of the acetabular rim and

Figure 38.9: The iliofemoral, ischiofemoral,
and pubofemoral ligaments reinforce the hip
joint capsule anteriorly. A. Anterior view.

B. Posterior view.

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

693

attach distally on the femur. The iliofemoral ligament arises
not only from the iliac portion of the acetabulum but also from
the anterior inferior iliac spine (AIIS). It proceeds in two parts
along the anterior and superior aspects of the joint, creating
the image of a Y, with its base directed toward the AIIS, and
its top directed inferolaterally toward the intertrochanteric
line. This ligament prevents excessive extension and lateral
rotation ROM of the hip joint. In addition, the superior por¬
tion limits adduction ROM. The iliofemoral ligament appears
to be the strongest ligament of the hip joint, sustaining larger
tensile forces before rupturing [22].

The ischiofemoral ligament attaches to the ischial portion of
the rim of the acetabulum. A portion of the ligament runs hor¬
izontally, reinforcing the capsule posteriorly. Another portion
projects superiorly, spiraling over the superior aspect of the
femoral neck to attach to the superior and medial aspects of the
greater trochanter. These spiral fibers, like the iliofemoral and
pubofemoral ligaments, limit excessive hyperextension. The
posterior fibers limit medial rotation of the hip [22]. The
ischiofemoral ligament also limits adduction ROM when
the hip is flexed. The pubofemoral ligament originates from
the pubic portion of the acetabular rim and from the superi¬
or pubic ramus. It extends along the inferior aspect of the
capsule. It, too, limits excessive extension ROM. Additionally,
it helps to prevent too much abduction ROM.

Additional Ligaments

The hip also contains an intraarticular ligament known as the
ligament to the head of the femur, or ligamentum teres
(Fig. 38.10). This ligament lies deep within the joint and runs
from the acetabular fovea to the pit of the head of the femur.

Figure 38.10: The ligament to the head of the femur arises from
the floor of the acetabulum and attaches to the fovea on the
head of the femur.

The ligament carries a small artery from the acetabulum to
the femoral head, but although the artery within this ligament
may provide some blood supply to the femoral head, it is
unlikely to be an adequate blood supply in the absence of
arteries from the femoral neck. The ligament itself is believed
to provide little mechanical support to the hip, especially in
adults [48,59]. However, adaptive changes are reported in this
ligament in individuals with avascular necrosis of the femoral
head, suggesting that the ligament may bear more load and
perhaps provide some support in such individuals [7].

In addition to the ligaments that span the hip joint, the
transverse acetabular ligament (TAL) appears to provide some
support during weight bearing. Lohe et al. note that the acetab¬
ular notch widens during weight bearing [39]. These authors
report that the TAL sustains tensile forces as the notch widens.
The functional significance of this finding is not known, but this
ligament may provide increased shock absorption at the hip
during weight bearing.

Hip Joint Stability

The hip is stabilized by its bony configuration and then by its
strong capsular and reinforcing ligaments. These ligaments
consist of longitudinal and circumferential fibers criss-crossing
one another. This fiber arrangement allows the capsule to
function much like a Chinese finger puzzle that, when
stretched, clamps down on the structures within. As the hip is
extended, the fibers of the capsule clamp down on the bony
contents within, firmly holding the femoral head in the acetab¬
ulum (Fig. 38.11). In contrast, hip flexion slackens the joint
capsule. Like the glenohumeral joint, the hip may gain addi¬
tional stability from a negative intraarticular pressure [31].

Figure 38.11: The fibers of the capsule and surrounding liga¬
ments function like a Chinese finger puzzle, clamping down
on the joint as the joint surfaces are distracted.

694

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Clinical Relevance

HIP JOINT CONTRACTURES: Inflammation of the hip can
occur for many reasons; including rheumatoid arthritis and
infection. Whatever the reason, joint inflammation produces
pain by causing swelling that stretches the joint capsule. To
relieve pain, the patient often assumes a position of hip flex¬
ion, thereby putting the joint capsule on slack and reducing
the stretch that produces pain. Yet prolonged hip flexion, par¬
ticularly in the presence of inflammation, can result in a hip
flexion contracture. Flexion contractures at the hip are a com¬
mon finding in patients with arthritis. Instructing patients to
stretch regularly into hip extension through exercise or static
positioning can help prevent hip flexion contractures.

ALIGNMENT OF THE ARTICULATING
SURFACES

The individual orientation of the acetabular and femoral com¬
ponents has already been described. It is now necessary to
understand the relationship of these articulated structures in
normal upright stance. This understanding allows investigation
of the effects of common hip malalignments. In the normal
erect posture, the acetabulum and femoral head are aligned so
that the head of the femur is directed slightly anteriorly and
superiorly in the acetabulum. This orientation exposes the
anterior aspect of the femoral head, leaving a large articular
surface available for movement toward flexion (Fig. 38.12). The

Figure 38.12: Alignment of the articulated femur and acetabu¬
lum in the anatomical position. Because the femur faces the
anterior-superior aspect of the acetabulum and the acetabulum
also faces anteriorly, the anterior surface of the head of the
femur is exposed in the anatomical position.

orientation of the femur and acetabulum facilitates advance¬
ment of the thigh in front of the trunk (flexion), while limiting
the potential for backward movement of the thigh beyond the
trunk. Flexion and abduction of the hip move the femoral head
toward the deepest part of the acetabulum.

Clinical Relevance

TREATMENT OF DEVELOPMENTAL DISPLACEMENT

OF THE HIP (DDH): At birth the acetabulum is shallow,
and if the hip joint exhibits excessive laxity, the femoral head
can easily slide out of the acetabulum, subluxing or dislocat¬
ing, especially when the hip is extended [55,61]. The practice
of swaddling infants or wrapping the child tightly in a blan¬
ket increases the risk of DDH by maintaining the hip in
extension. Cultural differences among various societies
including differences in bundling or swaddling and patterns
of carrying newborns and young children have been shown to
be associated with the incidence of congenital hip dislocation
[4,34]. The goal of treatment in care of DDH is to position
and maintain the femoral head deep in the acetabulum to
allow the supporting structures to tighten and to promote
normal growth of the femoral head and acetabulum. Splints
or casts position the infant's hips in hip flexion beyond 90°
and some abduction to obtain maximum joint contact and a
stable hip position (Fig. 38.13) [55,61].

Bony alignment of the femur and acetabulum also affects
the loads applied to the hip joint and the rest of the lower
extremity. The joint reaction force on the normal proximal
femur during upright standing is more vertically aligned than
the femoral neck, creating a bending moment on the head and
neck of the femur [42,53,60]. The bending moment produces
tensile forces on the superior aspect of the femoral neck and
compressive forces on the inferior aspect of the neck [1,53]
(Fig. 38.14). Femoral necks with a wider superior to inferior
diameter are better able to withstand the bending moments
sustained during weight bearing. Men have wider femoral
necks than women, which may help explain why the incidence
of femoral neck fractures is much higher in women [11,45].

The medial and lateral trabecular arrays of bone found in
the proximal femur appear well aligned to resist these com¬
pressive and tensile forces, respectively, protecting the
femoral neck from the bending moment that could sever the
femoral head from the neck [42]. As bone density decreases in
osteoporosis, the risk of femoral neck fracture rises [40].
Clinicians must appreciate the role of joint alignment on the
mechanics and pathomechanics of joint function to intervene
effectively in the treatment and prevention of joint injuries.

Intrinsic alignment of the femur is an important element in
the relationship between the femur and acetabulum. The
femoral head is directed toward the superior and anterior aspect
of the acetabulum, resulting from the angle of inclination
between the femoral neck and shaft in the frontal plane and the

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

695

Figure 38.13: The Pavlik splint is one of a variety of splints designed to position the infant's hips in flexion and abduction to facilitate
normal development of the femoral head and acetabulum. (From Tecklin JS: Pediatric Physical Therapy. Baltimore: Lippincott Williams
& Wilkins, 1999)

transverse plane orientation of the femoral neck. As stated ear¬
lier, the angle of inclination is typically reported to be 125°.
Yoshioka et al. report an average angle of 131° in a sample of 32
cadaver specimens [72]. A hip with an excessive frontal plane

angle is said to have a coxa valga deformity, or valgus defor¬
mity of the hip (Fig. 38.15). This deformity directs the femoral
head more superiorly in the acetabulum. Many biomechanical
alterations appear to result from a coxa valga [34,42,53]. The

bending moment (M) on the neck of the femur, creating tensile
forces on the superior surface of the femoral neck and compres¬
sive forces on its inferior surface.

Figure 38.15: Frontal plane alignment of the hip. A. The angle
of inclination in normal alignment is approximately 125-130°.

B. In coxa valga, the angle of inclination is greater than normal.

696

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

joint reaction force on the femur is more parallel to the femoral
neck in coxa valga. This alignment subjects the femoral neck to
more compressive forces and less of a bending moment, which
may explain why, in coxa valga, cancellous bone in the femoral
neck appears to be arranged in columns parallel to the neck
rather than in the medial and lateral intersecting bundles seen
in well-aligned femora. The perpendicular distance between
the hip joint center and the trochanter is decreased in coxa
valga, putting the hip abductor muscles at a disadvantage by
reducing their moment arm. With decreased moment arms, the
hip abductor muscles must generate larger contractile forces to
support the hip joint, resulting in increased joint reaction forces
[25,60]. In addition, the joint reaction force is displaced laterally
in the acetabulum and is applied over a smaller joint surface,
leading to increased joint stress. In other words, coxa valga
deformities are likely to increase the risk of degenerative joint
disease within the hip by increasing the joint reaction force as
well as the stress sustained by the femoral head.

In contrast, a coxa vara deformity is a decrease in the
angle between the shaft and neck of the femur, increasing the
bending moment applied to the femoral neck [29,42] (Fig.
38.16). The increased bending moment increases the com¬
pressive forces on the medial aspect of the femoral neck and
the tensile forces laterally, leading to an increase in the medial
and lateral trabecular arrays. In addition, a coxa vara defor¬
mity moves the trochanter farther from the joint center, effec¬
tively lengthening the moment arm of the hip abductors. This
puts the hip abductors at a mechanical advantage and may
actually reduce the force they are required to exert during
stance, thus reducing the joint reaction force. Orthopaedic
surgeons use the positive effect of altering the femoral neck
alignment and improving the mechanical advantage of the
abductor muscles in surgical osteotomies to reduce the loads
on the hip for treatment of osteoarthritis and aseptic necrosis
[17,25]. However, coxa vara tends to increase the medial pull

Figure 38.16: In a coxa vara deformity, the angle of inclination
is less than normal.

on the femur into the acetabulum, which may contribute to
erosion of the acetabulum [35,42]. Additionally, an increased
advantage for the abductor muscles may be accompanied by
fatigue in the antagonist muscles [5]. The moment arm of the
joint reaction force may also be increased with a net result
of an increased bending moment on the femoral neck.
Carpintero et al. suggest that coxa vara is a risk factor for
stress fractures of the femoral neck [5]. All things considered,
normal frontal plane alignment of approximately 125° appears
to minimize the negative consequences of weight bearing on
the healthy hip joint.

Clinical Relevance

SLIPPED CAPITAL FEMORAL EPIPHYSIS: A slipped

capital femoral epiphysis is a gradual or sudden inferior
and posterior displacement of the epiphysis , or growth plate
at the base of the femoral head [61]. The mechanisms pro¬
ducing a slipped capital epiphysis help to illustrate the
changes in femoral loading with coxa valga and coxa vara
(Fig. 38.17). Unlike the adult , the newborn possesses a
femoral neck-shaft angle that is significantly larger than
125°. In other words , coxa valga is the "normal" alignment
of the hip at birth. This valgus alignment gradually decreas¬
es to normal adult values throughout growth. During early
development when the femoral neck has a maximum

(continued)

Figure 38.17: Mechanics of a slipped capital epiphysis. A. In the
young child, the femur exhibits a coxa valga alignment normally,
and the capital epiphysis is approximately perpendicular to the
joint reaction force (F). B. As the child grows, the coxa valga
decreases and the epiphysis is no longer perpendicular to the
joint reaction force. In this case, the joint reaction force consists
of both a compressive force (F c ) and a shear (F s ) force.

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

697

(Continued)

valgus alignment the epiphyseal plate of the femoral head
(capital femoral epiphysis) lies approximately perpendicular to
the joint reaction force on the head of the femur. In this posi¬
tion the joint reaction force applies a compressive force on
the epiphysis. As the valgus decreases; the growth plate lies
more oblique to the joint reaction force. Consequently , the
joint reaction force exerts both compressive and shear forces
on the epiphyseal plate. As the obliquity of the epiphysis
increases; the shear force on it also increases. The shear force
tends to slide the head of the femur off the epiphysis. If the
shear force exceeds the strength of the growth plate, a slipped
capital epiphysis results [53]. This disorder is seen most often
in adolescent males. Although hormonal imbalances have
been implicated in the development of slipped capital epiph¬
ysis; other factors including obesity and sudden growth spurts
are significant contributors as well', since these increase the
joint reaction force and its shear component [56,61].

Transverse plane alignment of the proximal femur also
contributes to the function and dysfunction of the hip joint. In
the adult, the femoral neck and head face anteriorly with
respect to the plane of the femoral condyles in approximately
15° of anteversion (Fig. 38.18). However, like frontal plane
alignment, transverse plane alignment changes throughout
development. Means of 32° and 40° of anteversion at birth
are reported [41,54]. Anteversion gradually decreases during
growth until adult values of approximately 15° are present
later in adolescence, that is, by about 16 years of age. Jenkins
et al. report average anteversion of 12° (±3°) in 5 adults using
a clinical examination, but 17° (±7°) using MRI [28].

Excessive femoral anteversion places the head of the femur
farther anteriorly in the acetabulum than normal (Fig. 38.19).
Medial rotation of the hip compensates for excessive femoral
anteversion by putting the femoral head in a more normal loca¬
tion within the acetabulum. In standing, such compensatory

Figure 38.18: Transverse plane orientation of the hip joint. The
hip normally exhibits approximately 15° of anteversion.

Figure 38.19: Uncompensated excessive femoral anteversion. If
there is no compensation for excessive anteversion, the femoral
head projects too far anteriorly or even outside the acetabulum.

medial rotation of the hip results in an in-toed posture if accom¬
panied by no other compensation [32] (Fig. 38.20). Because
individuals with excessive femoral anteversion compensate for it
with medial rotation of the hip, subjects with excessive femoral
anteversion typically display increased medial rotation ROM
and a concomitant decrease in lateral rotation ROM [65].
Children with excessive anteversion frequently choose the “frog
sitting” posture over other sitting posture alternatives.

Over time, many individuals with continued excessive
femoral anteversion develop a secondary compensation in the

Figure 38.20: To compensate for excessive femoral anteversion,
locating the femoral head appropriately in the acetabulum,
young children typically rotate the hip medially, producing a
pigeon-toed posture.

698

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 38.21: To compensate for excessive femoral anteversion,
locating the femoral head appropriately in the acetabulum, an
adult continues to rotate the hip medially, so that in standing,
the knees face medially. However, the tibia also undergoes adap¬
tation by developing lateral torsion, so that in standing, the feet
are directed straight ahead and the subject no longer exhibits a
pigeon-toed posture.

tibia, lateral tibial torsion, which turns the foot laterally with
respect to the knee [32]. As a result, the in-toed standing pos¬
ture disappears. However, close examination of the femoral
condyles reveals that the standing posture continues to be char¬
acterized by medial rotation of the hip (Fig. 38.21). In
other words, despite the disappearance of an in-toed
posture, the original deformity remains.

Clinical Relevance

TREATMENT FOR EXCESSIVE FEMORAL
ANTEVERSION: Investigations into common conservative
treatments of excessive femoral anteversion reveal no
change in anteversion following standard clinical treatments
such as shoe modifications, twister cables, and splints [54].
These authors also find no change in alignment with no
intervention at all. Conservative treatments are similar to
one another in that they apply mechanical forces to the foot
and leg to influence the hip. The result of such treatments
apparently is an increase in lateral tibial torsion with no
change in the femoral anteversion deformity. Many , perhaps
most, individuals with excessive femoral anteversion develop
secondary tibial torsional deformities spontaneously.

Investigators recommend that treatment of femoral antever¬
sion be reserved for only those who display functional
difficulties associated with the anteversion deformity and
suggest that tibial osteotomy be considered for those who
show little or no tibial torsion by the age of 7 and have little
lateral rotation ROM of the hip.

Retroversion is a transverse plane deformity in which the
femoral neck is rotated posterior to the frontal plane, although
lower than normal anteversion is also sometimes described as
retroversion. Retroversion or less than normal anterversion
typically results in increased lateral rotation ROM of the
hip and concomitant diminished medial rotation ROM.
Excessive out-toeing can be a postural manifestation of retro¬
version [65]. Retroversion also appears to increase the risk of
slipped capital femoral ephiphysis in adolescents [15].

NORMAL MOTION OF THE HIP

Motion of the hip is certainly influenced by the supporting
structures detailed in the preceding section. In addition, hip
motion is almost inextricably linked to the motion of the low
back and pelvis. In this section, the values of normal hip joint
motion reported in the literature are presented with a discus¬
sion of the structures that are the normal limiters of motion.
This is followed by discussions of the contributions by the
pelvis and low back to apparent hip motion.

Normal Range of Motion

With the exception of hip flexion ROM, values of “normal”
ROM of the hip vary widely in the literature, as demonstrated
in Table 38.1. Many of the values cited do not include a
description of the population from which the values were
determined or details of the methodology used to obtain the
values. The clinician then is left to determine whether a
patients ROM values are actually “normal.” This lack of
a clear description of the normal variation of ROM values in
a population of individuals without hip pathology, in combi¬
nation with the absence of a consistent measurement proce¬
dure, limits the usefulness of such “normal” ROM values to
the clinician. At best, these numbers can offer a general per¬
spective by which to judge the adequacy of a patients hip
ROM. Despite the lack of normative data, the research pro¬
vides some useful perspectives. Gender has a slight effect on
hip ROM [13,69]. Medial rotation ROM appears greater in
women and adduction appears greater in men, but other
motions appear similar. Aging appears to produce a clini¬
cally insignificant reduction in hip ROM in all directions,
at least until the age of 80 [13,36,57,69]. Consequently, sig¬
nificant decreases in ROM suggest the existence of a joint
impairment.

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

699

TABLE 38.1: Hip ROM (°) in Healthy Individuals Reported in the Literature

Reference

Flexion

Extension

Abd

Add

Roass and Andersson [58] a

120 ± 8.3

9 ± 5.2

39 ± 7.2

31 ± 7.3

33 ± 8.2

34 ± 6.8

Roach and Miles [57p

121 ± 13

19 ± 8

42 ± 11

—

32 ± 8

32 ± 9

Departments of the Army

120

and the Air Force [9]

Boone and Azen [3] c

122 ± 6.1

9.8 ± 6.8

45.9 ± 9.3

26.9 ± 4.1

47.3 ± 6.0

47.2 ± 6.3

Hislop and Montgomery [24]

120

—

15-20

Gerhardt and Rippstein [16]

125

Escalante [13] d

123

—

Van Dillen et al. [67] e

—

-2.5

—

3 Data from 108 men, aged 30-40 years.

fa Data from 821 males and 862 females, aged 25-74 years.

c Data from 109 males, aged 18-54 years.

d Data from 687 individuals, aged 65-79 years.

e Data from 25 women and 10 men, aged 31 ±11 years.

Normal Limiting Structures of Hip ROM

One aid in making clinical judgments about ROM measures
is an understanding of the structures that normally limit hip
movement. Sources state that hip flexion ROM is limited pri¬
marily by soft tissue approximation between the thigh and
trunk [52]. However, a review of Table 38.1 reveals that most
sources also say that the normal hip flexion ROM is no more
than 125°. Few subjects exhibit contact between the thigh
and abdomen with 120° of hip flexion ROM. Therefore, other
structures appear to contribute to the end of ROM in flexion.
These limiting structures are most likely the posterior joint
capsule and the gluteus maximus. Soft tissue contact limits
hip flexion ROM with greater excursion and in overweight
individuals. Obesity is significantly related to decreased hip
flexion range [13].

Extension flexibility is limited by the anterior joint capsule
with its three reinforcing ligaments. The one-joint hip flexors
also provide some limits to extension ROM. The hip adduc¬
tor muscles and the pubofemoral ligament limit hip abduc¬
tion excursion; the superior part of the iliofemoral ligament
and the hip abductor muscles restrict hip adduction ROM.
Finally, lateral rotation flexibility is limited primarily by the
anterior capsule and by the iliofemoral and pubofemoral lig¬
aments; medial rotation excursion is checked by the lateral
rotator muscles, the posterior capsule, and perhaps by a por¬
tion of the ischiofemoral ligament [52,64].

Contribution of the Pelvis to Hip Motion

ROM measurements of the hip are usually taken with the
lower extremity functioning in an open chain, in which the
femur is moved with respect to the pelvis. Flexion is defined
as the femur moving toward the anterior aspect of the pelvis
and trunk; extension is the reverse. Abduction is the femur
moving toward the lateral aspect of the pelvis, and adduction

is the reverse. Lateral rotation occurs when the femoral
head rotates anteriorly in the acetabulum, and medial rota¬
tion is the femoral head rotating posteriorly. However, in
daily life, the lower extremity functions frequently in a closed
chain, so that the pelvis moves on the femur. In upright stand¬
ing with the femur fixed, an anterior pelvic tilt flexes the hip,
since the pelvic motion brings the anterior aspect of the pelvis
closer to the femur; a posterior pelvic tilt on a fixed femur
extends the hip (Fig. 38.22). When the pelvis is elevated on
one side in the frontal plane and the lower extremity remains
fixed, the lateral aspect of the pelvis on the opposite side
moves closer to its respective femur (Fig. 38.23). This pelvic
position results in abduction of the hip on the side opposite
the elevation. The hip on the elevated side is adducted.
Finally, in erect standing with both femurs fixed, forward
rotation of the pelvis on one side in the transverse plane
results in lateral rotation of the hip on the forward side and
medial rotation of the opposite hip (Fig. 38.24).
Understanding the pelvic contribution to hip position
is essential to understanding hip movement in activi¬
ties such as walking, climbing, and dancing. For example, in
gait at heel strike, the pelvis is rotated forward in the trans¬
verse plane on the side of heel contact, contributing to lateral
rotation of the hip on the side with heel contact and to medi¬
al rotation of the hip on the opposite side.

Interaction of the Hip Joint and Lumbar
Spine in Hip Motion

One of the likely sources of difference in the normal ROMs
reported in the literature is the difficulty in separating
the contributions of pure hip joint motion from lumbar
spine motion. Hip joint motion is measured by determining
the position of the thigh relative to the trunk. Unless care is
taken to control the movement of the pelvis and thus the

700

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Posterior

Figure 38.22: Effect of pelvic position on sagittal plane hip position. A. An anterior pelvic tilt flexes the hip. B. A posterior tilt produces
hip extension.

lumbar spine, the measured movements may actually reflect
both hip and spine motion. In hip flexion ROM, when the
femur has reached the end of its excursion in the acetabu¬
lum, posterior tilting of the pelvis continues to move the
femur toward the trunk, apparently contributing to addi¬
tional hip flexion while flattening the lumbar spine
(Fig. 38.25) [10]. Conversely, an anterior pelvic tilt in the
absence of movement at the hip joint contributes to apparent

hip extension while extending the lumbar spine. Trunk side¬
bending and pelvic movement in the frontal plane can appear
to be hip abduction or adduction motion (Fig. 38.26). It is
important to recognize the pelvic and femoral contributions
to hip motion may occur simultaneously, not just sequentially.
Therefore, considerable care is needed to distin¬
guish true hip motion from apparent hip motion
coming from the pelvis and low back.

Figure 38.23: When standing with the feet together and the
pelvis elevated on one side, the hip on the elevated side is in
adduction, and the opposite hip is in abduction.

Figure 38.24: When the pelvis rotates over the femur in the
transverse plane, the hip on the forward side is laterally rotated,
and the hip on the opposite side is medially rotated.

Chapter 38 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE HIP

701

Figure 38.25: A. An anterior pelvic tilt can substitute for hip
extension. B. A posterior pelvic tilt can substitute for hip flexion.

Clinical Relevance

COMPENSATIONS FOR DECREASED HIP MOTION:

Pelvic and lumbar spine movement can provide additional
movement between the trunk and thigh in individuals with

limited or absent hip motion. Patients with a fused or
painful hip use an anterior pelvic tilt to substitute for hip
extension to advance the body over the limb during stance
and use a posterior pelvic tilt to assist in advancing the limb
during swing [19,70]. The interaction between the hip joint
and lumbar spine may also help explain the source of low
back pain in some patients [18]. Limited hip mobility may
lead to overuse and hypermobility in the low back. Repeat
use of lumbar motion to compensate for limited hip motion
can produce injury or pain. Thereforeassessment of hip
mobility is an important component of the evaluation of an
individual with low back pain. Activities such as rising from
a chair and squatting require up to 130° of hip flexion
[26,49]. Individuals who lack such excursion may use lum¬
bar flexion to get out of a chair or squat to tie a shoe or
retrieve an object from the floor.

Hip Motion in Activities of Daily Living

Hip motion is essential to many daily activities, including ris¬
ing from a chair or toilet, picking up something from the
floor, walking, and climbing stairs. Normal walking utilizes
approximately 20-30° of flexion, reaching a maximum at
about initial contact. Stair climbing utilizes more, approxi¬
mately 45-65° and slightly less for stair descent [38,46].
Rising from a chair typically requires more than 100° of hip
flexion, usually less than the amount of flexion used
when bending to tie a shoe or squatting to pick up
something from the floor [50].

Clinical Relevance

PRECAUTIONS AFTER TOTAL HIP REPLACEMENT:

Dislocation is one of the most common complications of
total hip replacement (THR). A primary cause of dislocation
is impingement between the femoral and acetabular com¬
ponents which causes the femoral head to be "pried" out
of the acetabulum. Impingement typically occurs with
excessive hip flexion, adduction, or medial rotation.
Surgeons and therapists carefully instruct the THR patient
to avoid these motions, specifically avoiding flexion
beyond 90° and any hip adduction or medial rotation.
However, such instructions are difficult to follow since ris¬
ing from a chair typically uses at least 100° of flexion and
tying a shoe uses even more. Special adaptations such as
chairs with extra cushions or leg extenders, raised toilet
seats, extended shoe horns, and elastic shoe laces can be
very helpful to a patient who must avoid excessive hip
flexion or other motions (Fig. 38.27).

Figure 38.26: A lateral tilt of the trunk and pelvis can substitute
for hip abduction.

702

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 38.27: Rising from a chair with leg extensions (chair on the
left) requires less hip flexion than rising from a standard chair.

COMPARISON OF THE HIP JOINT
TO THE GLENOHUMERAL JOINT

Although the hip and glenohumeral joints are the two most
notable ball-and-socket, or triaxial, joints of the human body,
they possess very different architectures. This helps explain
their considerably different functional capabilities. First, the
shapes of the articular surfaces are quite different in the hip
and glenohumeral joint. While both the head of the femur and
head of the humerus are spherical, the femoral head completes
almost two thirds of a sphere, while the humerus is only hemi¬
spherical. The proximal articular surfaces, the acetabulum and
the glenoid fossa, are even more different from one another.
The acetabulum is a deep receptacle for the femoral head and
has a curvature similar to that of the femoral head itself; with
the labrum it covers more than half the femoral head in the
articulated joint. In contrast, the glenoid fossa is very shallow,
articulating with only a small portion of the humeral head at
any instant. These differences help to explain why the gleno¬
humeral joint is more mobile than the hip joint and why the hip
joint is more stable than its upper extremity counterpart.

The soft tissue supporting structures of the hip and gleno¬
humeral joints also contribute to the different functional
capabilities of these two joints. The articular capsule and
reinforcing ligaments of the hip provide substantial passive
support to the hip joint, while the capsule and reinforcing lig¬
aments of the glenohumeral joint provide only a portion of
the support needed for the integrity of the joint. Recognizing
the structural differences that contribute to functional dif¬
ferences between joints can help the clinician understand
the underlying pathology in a region and identify a success¬
ful intervention strategy.

Clinical Relevance

INSTABILITY IN THE HIP COMPARED TO THE
GLENOHUMERAL JOINT: Instability at the hip is rarely a
problem in adults , but glenohumeral joint instability is a rela¬
tively common problem in adults. Instability in the hip is
more commonly the problem of a young developing hip.
However ; although strengthening exercises for the gleno¬
humeral joint may be a useful approach to increasing stability ;
such a strategy is inadequate to restore stability at the hip.

The hip's stability depends more on bony architecture and the
integrity of noncontractUe tissue than on muscular support.

SUMMARY

This chapter examines how the bony architecture as well as
the noncontractile supporting structures of the hip con¬
tributes to its functional requirements of stability and mobility.
The deep socket formed by the acetabulum and labrum pro¬
vides inherent stability to the hip joint. A strong joint capsule
with reinforcing ligaments offers additional support. The rel¬
ative alignment of the femur and innominate bone allows
considerable joint mobility, especially in flexion, and increases
the mechanical advantage of muscles at the hip. Malalign¬
ments of the hip alter the mechanics of the hip and may con¬
tribute to increased loads and stresses on the joint, leading to
joint degeneration.

Hip joint ROM is reported and demonstrates little effect
from age. Loss of hip mobility suggests frank joint impair¬
ments and may result in excessive motion at the spine and
pelvis. Since increased pelvic and lumbar spine motion may
result from decreased hip mobility, assessment of hip motion
is an essential part of the examination of an individual with
low back pain. A grasp of these morphological and function¬
al relationships is essential to understanding normal function
of the hip as well as to recognizing and treating dysfunctions
of the hip or spine successfully. The following chapter pres¬
ents the structures that provide further support and, most
importantly, active motion at the hip—the muscles.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity at the Hip

CHAPTER CONTENTS

HIP FLEXORS.707

Psoas Major.707

lliacus.709

Psoas Minor.710

EXTENSORS OF THE HIP.711

Gluteus Maximus.712

ABDUCTORS OF THE HIP.714

Gluteus Medius.715

Gluteus Minimus.715

Functional Role of the Hip Abductors.715

Effects of Weakness of the Abductor Muscles .716

Effects of Tightness of the Abductor Muscles.717

ADDUCTORS OF THE HIP.717

Pectineus .718

Adductor Brevis.719

Adductor Longus.719

Adductor Magnus.719

Functional Role of the Adductors of the Hip.720

Effects of Weakness .720

Effects of Tightness.720

LATERAL ROTATORS OF THE HIP.721

Group Actions.722

Effects of Weakness and Tightness .722

MEDIAL ROTATORS OF THE HIP .723

COMPARISONS OF MUSCLE GROUP STRENGTHS.723

SUMMARY .724

he preceding chapter provides an understanding of the roles that the bones and supporting structures play
in the function of the hip. This chapter discusses the effects that the surrounding musculature has on the hip
joint under normal and pathological conditions.

Muscles that move the hip can be grouped into one-joint and two-joint muscles that (a) flex, (b) extend, (c) abduct,
(d) adduct, and (e) rotate the hip (Fig. 39.1). This chapter focuses on the one-joint muscles of the hip. The two-joint

705

706

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 39.1: Muscles of the hip. A. Anterior view of the hip shows the hip flexors and the adductor muscles. B. Posterior view of the hip
shows the gluteus maximus, medius, and other adductors.

muscles are mentioned briefly and are presented more thoroughly in Chapter 42 with the knee. This separation is, of
course, artificial, and the clinician must remember that the two-joint muscles, although obviously important muscles
of the knee, also provide important functions at the hip.

The muscles of the hip usually are classified by their actions, a useful and convenient classification. However, this can
lead to erroneous oversimplifications unless care is taken to recognize that virtually all hip muscles perform multiple
actions, and the "primary" action of some muscles is often unclear. In addition, the position of the hip has large effects
on the actions many muscles produce [13,15,23,33]. This chapter groups muscles by their purported primary action,
using the standard classification scheme, but also discusses the contribution of each muscle to other movements and
the influence of hip position on the muscles' actions. The purposes of this chapter are to

■ Describe the actions produced by the one-joint hip muscles and how those actions are influenced by hip
position

■ Examine the impact of muscle impairments at the hip

■ Begin to discuss the functional roles performed by the hip muscles during stance and gait

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

707

HIP FLEXORS

The one-joint hip flexors consist of the psoas major, iliacus,
and psoas minor, although the latter does not actually cross
the hip joint (Fig. 39.2). These muscles lie on the posterior
wall of the abdomen and inner surface of the greater pelvis.
Additional two-joint hip flexors include the rectus femoris,
tensor fasciae latae, and sartorius [9,75].

Figure 39.2: One-joint muscles include the psoas major and minor
and iliacus. Two-joint hip flexors include the sartorius, tensor fas¬
ciae latae, and rectus femoris.

MUSCLE ATTACHMENT BOX 39.1

ATTACHMENTS AND INNERVATION
OF THE PSOAS MAJOR

Proximal attachment: On the lateral aspects of the
vertebral bodies from T12 to L5 and the intervening
intervertebral discs. The muscle also attaches to the
bases of the respective transverse processes.

Distal attachment: The lesser trochanter. The tendon
of the psoas major, which combines with the ten¬
don of the iliacus, crosses over the superior ramus
of the pubis. It is separated from the pelvis and the
hip joint by the psoas bursa.

Innervation: Ventral roots of spinal nerves L1-L3(4).

Palpation: Some clinicians report an ability to pal¬
pate the muscle belly of the psoas major through a
relaxed abdomen [60,68], but in many individuals,
this muscle is not palpable.

Psoas Major

Because the psoas major lies deep in the abdomen it is less
well studied than some muscles of the hip (Muscle
Attachment Box 39.1).

ACTIONS

MUSCLE ACTION: PSOAS MAJOR

Action

Evidence

Hip flexion

Supporting

Hip lateral rotation

Supporting

Hip medial rotation

Refuting

Lumbar spine side-bending

Supporting

Lumbar spine flexion

Conflicting

Lumbar spine hyperextension

Conflicting

Lumbar spine stabilization

Supporting

Some of the actions of the psoas major are universally
accepted, while others remain controversial. Some of the
actions of the psoas major appear clearly contradictory; the
following discussion presents the current evidence for each
action.

The role of the psoas major as a hip flexor is clear from its
location anterior to the mediolateral axis of hip flexion that
passes approximately through the center of the femoral head
[14]. Although it has a smaller moment arm for flexion than
some other hip flexors such as the rectus femoris, sartorius,
and tensor fasciae latae, its large physiological cross-sectional
area (PCSA) makes it a strong hip flexor [8,15,23,28].

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Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

According to one study of eight healthy young adults, resisted
hip flexion while standing on the opposite limb recruits the
psoas major more vigorously than other exercises and activi¬
ties designed to elicit maximal contraction [28].

The psoas major is described as a medial rotator of the hip
[55] and as a lateral rotator [30]. Analysis of its moment arm
reveals a negligible rotation moment arm with the hip in neu¬
tral and a slight lateral rotation moment arm with the hip
flexed to 90° [13,15]. Additionally, active lateral rotation elic¬
its greater electrical activity of the psoas major than medial
rotation, and neither generates more than approximately half
the level of activity that hip flexion produces [5,28]. These
reports suggest that the psoas major plays only a small role in
lateral rotation under normal conditions.

The role of the psoas major in controlling the hip during
upright posture remains debatable. The center of mass of the
head, arms, and trunk (HAT) lies posterior to the flexion and
extension axis of the hip joint, applying an extension moment to
the hip [12,51] (Fig. 39.3). Contraction of the psoas major is
able to produce a flexion moment to counteract the extension
moment [5], but more recent electromyographic EMG data
reveal minimal activity (2% of maximum voluntary contraction)
of the psoas major during quiet standing, which increases only
slightly when standing in trunk hyperextension [28].

Figure 39.3: In quiet standing, the center of mass (COM) of the
HAT creates an extension moment (M) on the hip that can be
resisted by contraction of the psoas major.

Considerable confusion exists regarding the role of the
psoas major on the lumbar spine. EMG analysis reveals slight
electrical activity of the psoas major during trunk curl-up
activities and more activity during sit-up exercises and leg
raises from the supine position. The trunk exercises that elicit
more activity of the psoas major also use more hip flexion,
providing additional evidence that the psoas major is prima¬
rily a flexor of the hip.

Analysis of its moment arm and EMG data reveal that the
psoas major is an effective lateral flexor of the trunk [28,57].
It contracts concentrically lifting the trunk from the side-lying
position and eccentrically when side-bending to the opposite
side from a standing position. In contrast, analysis of the
moment arms in the sagittal plane reveals small extension
moment arms in the upper lumbar region and slight flexion
moment arms in the lower lumbar region [57]. The muscle is
better aligned to apply significant compressive loads to the
lumbar spine than to flex or extend it. This compressive load
may be sufficient to assist in stabilization of the spine. Slight
activity of the psoas major (<15% maximum voluntary con¬
traction, or MVC) during standing lift activities and ipsilateral
side-bending supports its role as a stabilizer of the lumbar
spine [28]. The compressive loads applied by psoas major
contractions also may explain why individuals with low back
pain report pain with hip flexion activities.

Clinical Relevance

PSOAS MAJOR CONTRACTION INCREASES LOW
BACK PAIN: Patients with herniated iumhar discs frequently
complain of pain getting in and out of automobiles; espe¬
cially trying to lift the lower extremity on the painful side or
while driving and moving the limb back and forth between
accelerator and brake. These maneuvers require active hip
flexion and probably involve contraction of the psoas major.
Contraction applies compressive loads to the lumbar spine
and also pulls on the lumbar intervertebral discs; which
may increase the pain. During the acute phase of a low
back pain episode; attempts to avoid active hip flexion may
help reduce the pain.

EFFECTS OF WEAKNESS

Weakness of the psoas major decreases the strength of hip
flexion. Such weakness could produce difficulties in tasks
such as lifting a limb in and out of the bathtub and climbing
stairs. Although active hip flexion is an important element of
normal locomotion, the amount of force required of the hip
flexors during normal gait is relatively small [6,52,59].
Therefore, slight-to-moderate weakness of the psoas major
may have an imperceptible impact on locomotion.

A study of 210 women ranging in age from 20 to 79 years
reports a steady decline in the cross-sectional area of the psoas
major apparent from the fifth decade on [66]. Such loss in

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

709

muscle bulk, presumably accompanied by loss of strength, may
contribute to the functional declines documented with age,
such as diminished balance and difficulty in stair climbing.

EFFECTS OF TIGHTNESS

Tightness of the psoas major restricts hip extension range of
motion (ROM). It may also limit trunk side-bending flexibility.
In the upright posture, tightness of the psoas major is often
manifested by increased lumbar extension, that is, by an exces¬
sive lumbar lordosis. This posture results from the pull on the
lumbar vertebrae toward the femur and simultaneous
compensation of backward-bending elsewhere in the
spine for the individual to keep the eyes on the horizon.

Iliacus

The iliacus is a large muscle with a PCS A equal to or greater
than the PCSA of the psoas major [8] (Muscle Attachment
Box 39.2 ). It is regarded with the psoas major as the primary
hip flexor. Together they are known as the iliopsoas muscle.

ACTIONS

MUSCLE ACTION: ILIACUS

Action

Evidence

Hip flexion

Supporting

Hip lateral rotation

Supporting

Hip medial rotation

Refuting

MUSCLE ATTACHMENT BOX 39.2

ATTACHMENTS AND INNERVATION
OF THE ILIACUS

Proximal attachment: The floor of the iliac fossa but
also the sacrum and the ligaments of the lum¬
bosacral and sacroiliac joints anteriorly

Distal attachment: Together with the psoas major
on the lesser trochanter. Some additional fibers
run slightly distally beyond the lesser trochanter
and others attach to the anterior aspect of the
joint capsule. The expansive proximal attachment
indicates that the muscle has a large cross-sec¬
tional diameter suggesting that it is a very power¬
ful muscle.

Innervation: The L2 and L3 branches of the femoral
nerve

Palpation: The insertion of the iliacus may be pal¬
pable in the femoral triangle, medial to the proxi¬
mal portion of the sartorius.

The primary actions of the iliacus reported in the literature
are, like those of the psoas major, contradictory. The iliacus
moves the hip joint directly and is an essential flexor of that
joint. Because it attaches with the psoas major, the moment
arm analyses at the hip are the same for the psoas major and
iliacus. Moment arm analysis suggests that the iliacus has lit¬
tle capacity to rotate the hip from the extended position and
only a small advantage for lateral rotation once the hip is
flexed. Like the psoas major, the iliacus exhibits EMG activ¬
ity during sit-up and curl-up activities, presumably participat¬
ing in the hip flexion component of these exercises [18]. It
also may provide some support at the hip in upright standing
to prevent the HAT weight from hyperextending the hip [5].

EFFECTS OF WEAKNESS

Weakness of the iliacus decreases hip flexion strength. The
functional effects are similar to those with weakness of the
psoas major. Although these muscles frequently are weak
together, in some cases such as in a spinal cord lesion, it is
possible for some of the psoas major to be spared while the
iliacus is involved.

Although the iliacus may be slightly active to prevent hip
hyperextension in quiet standing, the hip contains structures
that can support the hip in quiet standing, even in the
absence of muscular support. The anterior capsule with its
three reinforcing ligaments provides passive limits to hip
hyperextension. The subject who lacks muscular control at
the hip can stand unsupported by assuming a position of hip
hyperextension so that the weight of the HAT generates an
extension moment at the hip. By resting in maximum hyper¬
extension, the individual can use the passive support of the
ligaments to prevent additional backward bending (Fig. 39.4).
This is known as hanging on the ligaments.

EFFECTS OF TIGHTNESS

Tightness of the iliacus reduces hip extension ROM. In the
standing position, tightness of the iliacus results in an anterior
pelvic tilt that is accompanied by hyperextension of the lum¬
bar spine, if available, for the individual to maintain vision of
the horizon. Therefore, as seen with a tight psoas major, a
tight iliacus frequently leads to an increased lumbar lordosis.
If, however, the subject lacks hyperextension flexibility in the
spine, tightness of the iliacus or psoas major can produce a
forward lean and a flattened lumbar spine in upright posture.
Van Dillen et al. report decreased hip extension ROM in peo¬
ple with low back pain compared with age- and gender-
matched people without back pain [69].

Clinical Relevance

HIP FLEXION CONTRACTURES: Bilateral hip flexion con¬
tractures are common and are an occupational hazard of
office workers , truck drivers, and students , that is , of those

(continued)

710

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 39.4: In the absence of muscle activity, the anterior hip
joint capsule and ilio-, ischio-, and pubofemoral ligaments pro¬
vide passive resistance to the hyperextension moment at the hip
joint, generated by the weight of the HAT.

(Continued)

who spend most of their day sitting. It is not surprising to
find hip flexion contractures in sedentary elders [70].

However ; individual variations in low back flexibility and
strength can markedly affect the resulting compensations.
Although stemming from the same musculoskeletal impair¬
ment hip flexor tightness, the varied compensations produce
different postural and functional presentations, often leading
to disparate musculoskeletal complaints. The individual with
a flexible lumbar spine exhibits an excessive lumbar lordosis
and may complain of pain from increased loads on the lum¬
bar facet joints (Fig. 39.5). The individual with diminished
low back flexibility exhibits a flattened lumbar lordosis and
forward lean that may lead to muscle strain and injury to
the intervertebral disc from excessive loading.

Unilateral hip flexion contractures also occur ; particularly
in an individual with an inflammatory process at the hip
such as arthritis or following trauma. When only one hip
has the contracture or when one hip has a larger contrac¬
ture than the other hip, the effects on posture may vary.

The important factor in understanding the manifestation of
a unilateral hip flexion contracture is to determine which
attachment site is more displaced. Has the pelvis been
pulled toward the femur or has the femur been pulled

toward the pelvis? In the former, the trunk moves in
response, and the result is similar to the postures described
above in bilateral contractures. In the latter, the lower
extremity is drawn toward the trunk, which effectively pro¬
duces a shortened lower extremity. The patient can respond
in a variety of ways to equalize the leg length. Compen¬
sations include dropping the pelvis ipsilaterally, plantarflex-
ing the foot on the ipsilateral side, and flexing the knee on
the ipsilateral or contralateral side (Fig. 39.6).

Psoas Minor

The psoas minor usually is grouped with the hip flexors but
has no attachment on the femur and, consequently, no direct
action on the hip (Muscle Attachment Box 39.3). It is more
accurately described as a trunk muscle. However, because it
is so intimately related to the psoas major, it is described here.
It is reportedly absent in about 40% of the population [55].
Even when present, its actions cannot be isolated from those
of other muscles of the trunk.

Figure 39.5: Postures associated with bilateral hip flexion con¬
tractures. A. An individual standing in an anterior pelvic tilt
demonstrates an increased lumbar lordosis if the lumbar spine
has adequate flexibility. B. If an individual lacks adequate lumbar
spine flexibility, an anterior pelvic tilt produces a forward lean.

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

711

Figure 39.6: Unilateral hip flexion contracture produces a func¬
tional leg length discrepancy. In a typical compensation, the indi¬
vidual stands with the ipsilateral knee flexed and foot plan-
tarflexed.

ACTIONS

MUSCLE ACTION: PSOAS MINOR

Action

Evidence

Lumbar spine flexion

Inadequate

Lumbar spine side-bending

Inadequate

MUSCLE ATTACHMENT BOX 39.3

ATTACHMENTS AND INNERVATION
OF THE PSOAS MINOR

Proximal attachment: Lateral aspects of the bodies
of T12 and LI and the disc in between

Distal attachment: The iliopubic eminence of the
innominate bone and the iliac fascia. Its muscle
belly, considerably smaller than that of the psoas
major, travels alongside the latter muscle.

Innervation: Ventral ramus of LI

Palpation: Not palpable.

The psoas minor is considerably smaller and weaker than the
psoas major. No known investigations exist that examine the
role of the psoas minor. Its size and frequent absence suggest
that its functions as well as the impairments from weakness or
tightness of the psoas minor are minimal.

EXTENSORS OF THE HIP

The gluteus maximus is the primary one-joint hip extensor,
although the two-joint hip extensors (the hamstrings) and
other one-joint hip muscles included in other muscle groups
(adductor magnus) are very important extensors of the hip as
well (Fig. 39.7).

Figure 39.7: The one-joint hip extensor is the gluteus maximus,
but other hip extensors include the hamstrings and the adductor
magnus.

712

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 39.4

ATTACHMENTS AND INNERVATION
OF THE GLUTEUS MAXIMUS

Proximal attachment: The posterior surfaces of the
sacrum and coccyx, the posterior aspect of the ilium
posterior to the posterior gluteal line, and the tho¬
racolumbar fascia

Distal attachment: The proximal end of the iliotibial
band. A deep portion also attaches to the gluteal
tuberosity. The large attachments of the gluteus
maximus reveal that the muscle has a very large
cross-sectional area and therefore is normally very
strong.

Innervation: The inferior gluteal nerve (L5, SI, and S2)

Palpation: The gluteus maximus is easily palpated
lateral to the second sacral vertebra along the iliac
crest and PSIS. Although the gluteus maximus forms
most of the buttock, it is usually covered by a sub¬
stantial layer of subcutaneous fat.

Gluteus Maximus

The gluteus maximus is a large muscle with a PCS A at least
30% greater that that of the iliopsoas [8] (Muscle Attachment
Box 39.4). It forms most of the contour of the buttocks.

ACTIONS

MUSCLE ACTION: GLUTEUS MAXIMUS

Action

Evidence

Hip extension

Supporting

Hip lateral rotation

Supporting

Hip abduction

Supporting

Hip adduction

Supporting

The gluteus maximus is a powerful extensor of the hip,
with both a large PCS A and a relatively large moment arm
[15,39]. Hislop and Montgomery suggest that manual resist¬
ance is unable to overcome or “break” the isometric contrac¬
tion of a gluteus maximus in an individual with normal
strength [21]. The function of the gluteus maximus as a hip
extensor depends on the position of the body in space as well
as the position of the hip joint itself. In the prone position, the
gluteus maximus lifts the weight of the lower extremity to
extend the hip with a concentric contraction. In quiet stand¬
ing, because the HAT weight tends to extend the hip, the glu¬
teus maximus is electrically silent [5]. The hip extensors,
including the gluteus maximus, contribute postural support
when a subject leans forward and the HAT weight creates a

flexion moment at the hip. Under these circumstances, the
hip extensors contract eccentrically to control the forward¬
bending or concentrically to return the individual to an
upright position. Yet EMG data from individual subjects
reveal little or no activity in the gluteus maximus during for¬
ward-bending activities such as bending to lift a 25-lb load
[17]. In contrast, the gluteus maximus is active during trunk
hyperextension from the prone position [11]. Ascending stairs
elicits activity in the hamstrings and adductor magnus along
with the gluteus maximus [4,37]. Single-stance wall squats
and mini-squats also elicit considerable electrical activity in
the gluteus maximus [4]. The gluteus maximus exhibits less
activity during active extension from the flexed position and
more activity during extension from the extended or hyperex-
tended position [37,77].

Consideration of the structure of the hip extensors helps
elucidate the effect of hip position on the extension role of the
gluteus maximus. Mechanical analyses and computed tomog¬
raphy (CT) scans to examine the length of the moment arms
of the hip extensors from a position of 0° of flexion to 90° of
flexion reveal that the moment arm of the gluteus maximus
appears greatest at 0° of flexion and decreases steadily to 90°
of flexion [15,39,46]. Other hip extensors, namely the ham¬
strings and adductor magnus, reach their maximum moment
arms when the hip is more flexed. The gluteus maximus also
consists of relatively long muscle fibers and, although it has a
large moment arm in comparison with the hamstrings, it still
attaches proximally on the femur [46]. These structural char¬
acteristics enhance the gluteus maximus s ability to produce a
large joint excursion [23]. The gluteus maximus appears par¬
ticularly suited to help fully extend or hyperextend the hip
joint. These studies suggest that hip extensor recruitment is,
at least in part, dictated by the mechanical advantage of the
muscles available.

The apparent contradiction in abduction and adduction
actions by the gluteus maximus can be explained by dividing
the muscle into superior and inferior segments [39]. The supe¬
rior portion lies superior to the axis of abduction and adduc¬
tion, while the inferior portion lies inferior to it (Fig. 39.8). As
a result, the superior portion of the gluteus maximus con¬
tributes to abduction of the hip, and its inferior portion con¬
tributes to adduction. The whole of the gluteus maximus lies
posterior to the axis of medial and lateral rotation, and there¬
fore, the muscle is a lateral rotator of the hip joint with the hip
extended [13,15,33]. As the hip flexes, the moment arm for
lateral rotation decreases, and by the time the hip reaches 90°
of flexion, the superior portion of the gluteus maximus actu¬
ally has a medial rotation moment arm [13]. EMG data sug¬
gest that the addition of active hip abduction or lateral rotation
to active hip extension from the extended position significantly
increases the electrical activity of the gluteus maximus [11].
Clinicians can enhance gluteus maximus recruitment during
exercise by combining hip hyperextension with abduction and
lateral rotation.

The role of the gluteus maximus in locomotion has been
studied extensively. The gluteus maximus is mildly active

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

713

Figure 39.8: The gluteus maximus lies superior and inferior
to the axis of abduction and adduction and consequently
can contribute to either motion.

with the hamstrings during ambulation at the end of swing
and the beginning of stance [2,16,37,52,58,76]. These mus¬
cles appear to work together to slow the flexion of the hip at
the end of swing and to initiate hip extension in early stance
by pulling the trunk forward over the stance limb
[16,34,36,56,58,76]. Gluteus maximus activity increases sub¬
stantially with walking up an incline and with running, in
which the muscle appears to play an essential role in stabi¬
lizing forward trunk lean [34].

WEAKNESS

Weakness of the gluteus maximus results in decreased
strength of hip extension and lateral rotation. A classic gait
pattern resulting from gluteus maximus weakness, known
as the gluteus maximus lurch, has been described anec¬
dotally [63] (Fig. 39.9). The lurch is a rapid hyperexten¬
sion of the trunk prior to, and continuing through, heel
contact on the side of the gluteus maximus weakness. It
has been suggested that the backward “lurch” moves the
center of mass of the HAT weight to a position posterior
to the hip joint, thus eliminating the need for the gluteus
maximus to extend the hip. However, it is also important
to recognize that such a significant gait deviation is more
likely a result of weakness of other hip extensors in addi¬
tion to the gluteus maximus.

Figure 39.9: The gluteus maximus lurch is characterized by a
backward lean of the trunk at heel strike to move the center of
mass of the HAT posterior to the hip joint to eliminate the need
for the hip extensor muscles.

Clinical Relevance

GLUTEUS MAXIMUS WEAKNESS AND GAIT:

Sutherland et al. report an excessive anterior pelvic tilt and
lumbar lordosis seen during ambulation in children with
early signs of Duchenne's muscular dystrophy [65]. These
authors suggest that weakness of the gluteus maximus
explains these gait deviations. The gait pattern documented
by Sutherland et al. is similar to, albeit more subtle than , the
gluteus maximus lurch described anecdotally and may
reflect discrete gluteus maximus weakness.

TIGHTNESS

Tightness of the gluteus maximus limits hip ROM in flexion
and medial rotation and, perhaps, adduction, although its
effects in the frontal plane are more difficult to ascertain since
it appears to be both an abductor and adductor. Athletes such
as runners who have strongly developed gluteus maximus
muscles can exhibit such tightness. Because hip movement is

714

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

intimately related to low back movement, tightness of the glu¬
teus maximus may produce excessive movement in the lum¬
bar spine.

Clinical Relevance

GLUTEUS MAXIMUS TIGHTNESS AND LOW BACK
PAIN: Restrictions in hip flexion ROM can require an indi¬
vidual to use excessive trunk flexion during such activities
as squatting to pick up an object from the floor or to tie a
shoelace. Tightness of the gluteus maximus therefore may
be a contributing factor to low back pain.

ABDUCTORS OF THE HIP

The gluteus medius and gluteus minimus are the primary
abductors of the hip, although, as noted in the previous dis¬
cussion, the gluteus maximus also abducts the hip (Fig. 39.10).

MUSCLE ATTACHMENT BOX 39.5

ATTACHMENTS AND INNERVATION
OF THE GLUTEUS MEDIUS

Proximal attachment: The lateral surface of the ala
of the ilium between the posterior and anterior
gluteal lines

Distal attachment: By tendon to the lateral aspect
of the greater trochanter.

Innervation: The superior gluteal nerve, L4,5 and SI

Palpation: Much of the muscle is covered by the
gluteus maximus. However, it can be palpated
along the posterior surface of the iliac crest at its
most superior aspect. It can also be palpated
along its length by placing a hand at the iliac crest
with the fingers pointing toward the greater
trochanter.

Figure 39.10: The primary hip abductor muscles are the gluteus
medius and minimus. The tensor fasciae latae and the sartorius
are two-joint muscles that also abduct the hip.

Additional two-joint abductor muscles are the tensor fasciae
latae and sartorius [9,75]. The gluteus medius and minimus
attach to the ala of the ilium and lie on the lateral aspect of
the hip and buttocks (Muscle Attachment Box 39.5). The
electrical activity of the gluteus medius during function is
well studied. The gluteus minimus lies deep to the medius
and to the tensor fasciae latae and is less well studied
(.Muscle Attachment Box 39.6). Its functional responsibility
is usually inferred from knowledge of the gluteus medius.
Because these two muscles appear to function together fre¬
quently, their functional roles and the effects of weakness
and tightness in these muscles are discussed together after
discussion of their individual actions.

MUSCLE ATTACHMENT BOX 39.6

ATTACHMENTS AND INNERVATION
OF THE GLUTEUS MINIMUS

Proximal attachment: The anterior aspect of the ala
of the ilium between the anterior and inferior
gluteal lines

Distal attachment: The superior and anterior
aspects of the greater trochanter. It lies deep to
the gluteus medius but is positioned more anteri¬
orly than the gluteus medius. It cannot be pal¬
pated directly.

Innervation: The same as that of the gluteus
medius, superior gluteal nerve, L4,5 and SI

Palpation: Not palpable

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

715

Gluteus Medius

ACTIONS

MUSCLE ACTION: GLUTEUS MEDIUS

Action

Evidence

Hip abduction

Supporting

Hip medial rotation

Supporting

Hip lateral rotation

Supporting

The gluteus medius undoubtedly is an abductor of the hip.
Some authors also report that it medially rotates the hip
[21,44] or that the anterior fibers medially rotate the hip and
the posterior fibers laterally rotate the hip [30]. Analysis of its
moment arms confirms that when the hip is extended, the
anterior and middle portions of the gluteus medius are medial
rotators of the hip, and the posterior segment laterally rotates
the hip [13,15,39]. When the hip is flexed, however, virtually
the whole muscle contributes to medial rotation and has little
or no capacity to abduct the hip. EMG data show decreased
recruitment of the gluteus medius during hip abduction with
the hip flexed to 20° [7].

Clinical Relevance

STRENGTHENING EXERCISES FOR THE GLUTEUS
MEDIUS: Weakness of the gluteus medius produces signifi¬
cant gait deviations and may be associated with hip and knee
complaints [19]. Exercises to strengthen the gluteus medius are
important and commonly prescribed. One exercise recom¬
mended to strengthen the gluteus medius is the "fire hydrant"
exercise in which the individual assumes a quadripedal posi¬
tion and lifts one lower extremity with the hip flexed and
abducted (Fig. 39.11). Another common exercise uses a weight
machine in which the subject sits and abducts the hip against
a resistance. Both of these exercises require the subject to
abduct the hip when the hip is flexed. However ; evidence
demonstrates that the gluteus medius is incapable of hip
abduction with the hip flexed. The subject is more likely using
the gluteus maximus and tensor fasciae latae to abduct the
hip. Individuals who want to strengthen the gluteus medius
specifically must abduct the hip with the hip extended to
ensure recruitment of the gluteus medius. These data demon¬
strate how a thorough understanding of a muscle's action is
essential to prescribe an appropriate exercise regimen.

Gluteus Minimus

ACTIONS

MUSCLE ACTION: GLUTEUS MINIMUS

Action

Evidence

Hip abduction

Supporting

Hip medial rotation

Supporting

Hip lateral rotation

Supporting

Figure 39.11: The "fire hydrant" exercise, a common exercise for
the hip abductors, combines hip abduction with flexion. Since
the gluteus medius and minimus muscles are ineffective abduc¬
tors with the hip flexed, they are unlikely to be strengthened
substantially by this exercise.

The gluteus minimus is another strong abductor of the hip,
although its PCS A is substantially smaller than that of the glu¬
teus medius [8]. Like the gluteus medius, with the hip
extended, its anterior portion is a medial rotator of the hip with
a larger rotation moment arm than that of the gluteus medius,
and its posterior portion is a lateral rotator [13,15]. Hip flexion
increases its medial rotation capacity and appears to reduce its
ability to abduct the hip. The gluteus minimus also is firmly
attached to the hip joint capsule [71]. This attachment may
help protect the capsule from impingement by pulling it out of
the way of the greater trochanter during active hip abduction.
The shoulder, elbow, knee, and ankle exhibit similar mecha¬
nisms to protect their sensitive joint capsules.

Functional Role of the Hip Abductors

The broad proximal attachments of both the gluteus medius
and gluteus minimus indicate that these muscles are quite
strong and are likely to participate in functional activities
that require considerable force. Although active abduction
of the hip in an open chain is used in activities such as get¬
ting on and off a bicycle, the essential role of the abductor
muscles occurs during closed chain activities such as walk¬
ing and running. These activities of bipedal ambulation are
characterized by intermittent periods of one-legged stance.
During the time of single limb support, the weight of the
opposite limb and that of the HAT (the HAT-L weight)
exerts an adduction moment on the stance hip, tending to
make the body fall onto the unsupported side and adducting
the hip on the stance side (Fig. 39.12). To hold the pelvis
and the weight above it stable, the abductor muscles on the
support side pull from their distal attachments on the femur
to their proximal pelvic attachments. This pull, if strong
enough, holds the pelvis level and prevents its dropping on
the unsupported side. Similarly, the hip abductors provide

716

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 39.12: During single-limb stance, the force of the weight
of the HAT-L (F w ) tends to adduct the stance hip, applying an
adduction moment (M AD ). The pull of the abductors (F A ) holds
the pelvis level, applying an abduction moment (M AB ).

proximal support to stabilize the femur and help maintain
frontal plane alignment of the knee and foot within the lower
extremity closed chain [10,22].

A magnetic resonance imaging (MRI) study examines the
relative activity of the gluteus medius and gluteus minimus
muscles during active abduction in different positions of
abduction and during single-limb standing [32]. The authors
report that the gluteus minimus demonstrates more activity
than the gluteus medius during abduction with the hip
abducted to 20° and also during single-limb stance. They also
note that while the abduction moment arm of the gluteus
medius is longer than that of the gluteus minimus with the
hip in the neutral or adducted position, the opposite is true in
the abducted position. This study confirms the importance of
the gluteus minimus as an abductor and as a support during
single-limb stance.

Clinical Relevance

THE ROLE OF THE HIP ABDUCTORS IN MAINTAINING
LOWER EXTREMITY ALIGNMENT IN WEIGHT
BEARING: Many upright activities such as stair ascent and
descent or stepping on and off a curb require complex stabi¬
lization of the hip, knee; and ankle in all three planes of
motion. The ability to stabilize the knee and foot in the
three planes appears to be in part the responsibility of the
hip abductor muscles. Stair descent consists of repeated sin¬
gle limb squats; as an individual lowers body weight onto
the lower step by flexing the hip and knee that still bear
weight on the upper step. With inadequate stability of the
weight-bearing hip, the weight-bearing knee tends to move
into more valgus and the foot tends to pronate [10,22,40].
These abnormal alignments may help explain why
people with anterior knee pain often display weak¬
ness in hip abduction [24].

Effects of Weakness of the Abductor
Muscles

Weakness of the gluteus medius and minimus results in a sig¬
nificant decrease in abduction strength, since they are the
primary abductors of the hip. The functional ramifications of
such weakness are most apparent in weight-bearing activi¬
ties, specifically in single-limb support. The functional prob¬
lem occurs during stance on the side of the weakness. As
single-limb support begins and the abductor muscles are too
weak to hold the pelvis level, the HAT-L weight tends to
cause the pelvis to drop on the unsupported side. Because
this is a very unstable phenomenon and puts the subject at
risk of falling, most subjects use a typical substitution. To
avoid the pelvic drop on the unsupported side, the subject
leans the trunk toward the supporting side (Fig. 39.13). This
lean moves the center of mass of the HAT-L weight to the
lateral aspect of the hip joint on the stance side. In this posi¬
tion, the HAT-L weight no longer tends to adduct the hip. In
fact, the weight creates a slight abduction moment, thus
eliminating the need for active abduction force. The result¬
ing gait pattern is so characteristic of hip abductor weakness
it is dubbed a gluteus medius limp, although it is likely to
involve both the gluteus medius and the gluteus minimus
[32,42]. Perhaps the functional deficits resulting from weak¬
ness of the gluteus medius and minimus would be described
better as an abductor limp. (See video in Chapter 28.)

Clinical Relevance

TRENDELENBURG TEST: A simple clinical screening pro¬
cedure for abductor weakness uses single-limb stance and

(continued)

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

717

Figure 39.13: Gluteus medius limp. During single-limb stance,
an individual with weakness of the hip abductors leans laterally
during single-limb stance on the weak side, moving the center
of mass of the HAT-L weight to the lateral side of the hip joint,
producing an abduction moment (M AB ) on the hip.

(Continued)

the postural compensation present with abductor weakness.
The test is known as the Trendelenburg test and uses
quiet single-limb standing. It is positive for abductor weak¬
ness on the stance side when the subject leans
excessively toward the stance limb or when the
pelvis drops on the unsupported side [42].

Hip abductor weakness is associated with anterior knee
pain and the presence of hip osteoarthritis [3,24]. It is not
clear whether abductor weakness is a risk factor for or
the consequence of these disorders. Additional research is
needed to determine if strengthening these muscles can pre¬
vent or decrease the pain and dysfunction associated with the
disorder. It is clear that clinicians must consider the role of

Figure 39.14: Standing posture with one hip abducted. An indi¬
vidual standing with one hip abducted typically lowers the pelvis
on the abducted side to allow floor contact.

the hip abductor muscles in treating individuals with lower
extremity dysfunction.

Effects of Tightness of the Abductor
Muscles

Abductor tightness, although not common, does exist.
Tightness of these muscles results in decreased ROM in
adduction and, perhaps, in lateral rotation. Such tightness is
found in individuals with arthritis whose position of comfort
frequently includes hip flexion and abduction. The functional
consequences of an abduction contracture are seen most
often in upright posture and may include changes in pelvic
alignment to maintain erect posture or in the positions of the
other joints of the lower extremity to optimize the base of
support (Fig. 39.14).

ADDUCTORS OF THE HIP

The primary one-joint adductors of the hip include the
pectineus, adductor brevis, adductor longus, and adductor
magnus (Fig. 39.15) (Muscle Attachment Boxes 39.7-39.9).
The gracilis is a two-joint adductor of the hip. The adductors of
the hip share two important characteristics: they all have some
attachment to the pubis, and they all receive some innervation

718

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 39.15: The one-joint hip adductor muscles include the
pectineus, the adductor brevis and longus, and the adductor
magnus. The two-joint adductor is the gracilis.

from the obturator nerve. They are only partially palpable
along the medial aspect of the thigh, covered in part by subcu¬
taneous fat and also by the large muscles of the thigh, the ham¬
strings, and the quadriceps. The role of the adductors in func¬
tion and the effects of weakness and tightness are discussed as
a group following the discussion of their individual actions.

Pectineus

ACTIONS

MUSCLE ACTION: PECTINEUS

Action

Evidence

Hip adduction

Supporting

Hip flexion

Supporting

Hip medial rotation

Supporting

MUSCLE ATTACHMENT BOX 39.7

ATTACHMENTS AND INNERVATION
OF THE PECTINEUS

Proximal attachment: The superior ramus of the
pubis between the pubic tubercle and the iliopubic
eminence

Distal attachment: The pectineal line on the poste¬
rior aspect of the femur between the lesser
trochanter and the linea aspera.

Innervation: The femoral nerve (L2 # 3) and the
obturator (L3). However, the nerve supply may be
variable [55].

Palpation: The pectineus lies between the psoas
major and the adductor longus and cannot be
palpated readily.

MUSCLE ATTACHMENT BOX 39.8

ATTACHMENTS AND INNERVATION
OF THE ADDUCTOR BREVIS

Proximal attachment: The body and inferior ramus
of the pubis

Distal attachment: The pectineal line and the proxi¬
mal half of the linea aspera.

Innervation: The obturator nerve (L2, 3, 4)

Palpation: The adductor brevis lies posterior to the
pectineus and adductor longus and anterior to the
adductor magnus and cannot be palpated.

MUSCLE ATTACHMENT BOX 39.9

ATTACHMENTS AND INNERVATION
OF THE ADDUCTOR LONGUS

Proximal attachment: The body of the pubis at the
intersection of the crest and symphysis

Distal attachment: The medial lip of the linea
aspera. The adductor longus attaches more anteri¬
orly on the pubis than any other adductor. It has a
prominent long tendon proximally, which gives the
muscle its name.

Innervation: The obturator nerve (L2, 3, 4)

Palpation: The proximal tendon is easily palpated in
the groin and serves as an important landmark for
fitting above-knee prostheses.

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

719

The actions of flexion and adduction by the pectineus are
consistent with its location at the hip and are supported by
analysis of its moment arms [15]. Analysis of the rotation
moment arm of the pectineus and EMG data suggest that the
muscle also is active in medial rotation with other adductors
[1,55,67].

Adductor Brevis

ACTIONS

MUSCLE ACTION: ADDUCTOR BREVIS

Action

Evidence

Hip adduction

Supporting

Hip medial rotation

Supporting

Hip lateral rotation

Refuting

Hip flexion

Supporting

Hip extension

Conflicting

The adductor brevis has one of the largest adduction
moment arms of the muscles of the thigh and appears capa¬
ble of hip adduction with the hip in any position of flexion
[15]. Like that of the pectineus, the role of the adductor bre¬
vis in rotation is controversial. Anatomy texts report that it
either medially rotates the hip [6] or laterally rotates it
[44,55]. However, EMG data reveal activity in the adductor
brevis only during medial rotation [5]. Moment arm analysis
also supports a role only in medial rotation [1]. Moment arm
analysis also supports the view that the adductor brevis
changes from a hip flexor to an extensor as the hip flexes [15].
Although no EMG studies of the adductor brevis during sim¬
ple active flexion and extension movements have been found,
Basmajian and DeLuca report that during walking, the brevis
is most active at toe-off [5]. Since the hip is flexing at toe-off,
these data indirectly support the adductor brevis s role as a
flexor when the hip is extended.

Adductor Longus

ACTIONS

MUSCLE ACTION: ADDUCTOR LONGUS

Action

Evidence

Hip adduction

Supporting

Hip flexion

Supporting

Hip extension

Refuting

Hip medial rotation

Conflicting

Hip lateral rotation

Conflicting

Mechanical analysis reveals that the muscle possesses an
adduction moment arm regardless of sagittal plane hip posi¬
tion [15]. Open chain adduction appears to elicit consistent
EMG activity of the adductor longus [5]. Few EMG studies
exist, but existing EMG and mechanical data also support the

role of the adductor longus as a flexor of the hip, and one study
suggests that its role as a flexor exceeds its role as an adductor
[5,20]. The adductor longus exhibits a consistent, albeit small,
medial rotation moment arm [1,15]. However, EMG data pro¬
vide contradictory and inconsistent evidence for a role in rota¬
tion [5,20,74]. The muscle appears to play a more consistent
role in hip flexion and adduction than in rotation.

Adductor Magnus

The adductor magnus rightfully bears the name “magnus,”
since it is substantially larger than any other adductor muscle,
being similar in size to the biceps femoris of the hamstring
muscle group [8,72] (Muscle Attachment Box 39.10).

ACTIONS

MUSCLE ACTION: ADDUCTOR MAGNUS

Action

Evidence

Hip adduction

Conflicting

Hip extension

Supporting

Hip medial rotation

Conflicting

Hip lateral rotation

Conflicting

Although named an adductor, the role of the adductor mag¬
nus in hip adduction remains unclear and probably varies.
Some of the confusion lies in the size of the muscle and
whether investigators study the muscle as a whole or in seg¬
ments. Assessment of the muscles moment arm reveals that
the muscle as a whole possesses an adduction moment arm
[33]. When segmented, however, the anterior portion exhibits
a significant adduction moment arm with the hip in any posi¬
tion of hip flexion. The middle and posterior segments have
smaller adduction moment arms and only in portions of the

MUSCLE ATTACHMENT BOX 39.10

ATTACHMENTS AND INNERVATION
OF THE ADDUCTOR MAGNUS

Proximal attachment: The inferior ramus of the
pubis, the ischial ramus, and the ischial tuberosity

Distal attachment: Along the length of the femur
from the quadrate tubercle, along the linea aspera
and medial supracondylar line to the adductor
tubercle. This broad attachment gives rise to the
largest of the adductors, whose size justifies its title
"magnus."

Innervation: A branch from the tibial portion of the
sciatic nerve, L4 and the obturator nerve, L2, 3, and 4

Palpation: This muscle is most easily palpated at its
distal attachment on the adductor tubercle.

720

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

hip flexion range. The limited EMG data available provide lit¬
tle additional insight, reporting contradictory results [5,20].
Such contradictions likely arise by recording from different
segments of the muscle. More EMG data recorded from all
three portions of the muscle may help clarify the role of the
adductor magnus in hip adduction.

In contrast, the role of the adductor magnus as a hip exten¬
sor is generally well accepted [20,33,39,46]. It is even
described as another hamstring muscle [55]. The posterior
segment of the adductor magnus has a larger extension
moment arm than the gluteus maximus or hamstrings when
the hip is extended [15].

EMG data describing the contribution of the adductor
magnus to hip rotation also are contradictory [5,20], and
again, analysis of the muscle s rotational moment arms helps
explain the controversy. Portions of the muscle possess slight
medial rotation moment arms, while other segments possess
lateral rotation moment arms [1,15]. However, these moment
arms are small, and the adductor magnus plays, at most, an
accessory role in hip rotation.

Functional Role of the Adductors
of the Hip

Despite the areas of disagreement surrounding the individual
actions of the adductor muscles, most investigators agree on
one important functional role of the adductors—to stabilize the
pelvis during weight shifting from one limb to the other. This
role is seen during gait as the adductors contract during the
transitions from stance to swing and swing to stance [16,52].

The adductors also help stabilize the hip during squatting
activities. In squatting to lift something from the floor, an indi¬
vidual typically has the hips slightly abducted (Fig. 39.16).
Inspection of the ground reaction force reveals that it produces
an abduction moment at the hip that must be countered by an
adduction moment produced by contraction of the adductor
muscles. Anyone who has gardened can probably verify the
adductor muscles’ role by recalling the muscle soreness in the
inner thighs after the spring garden cleanup!

Effects of Weakness

Adductor weakness is not common but may result from an
injury to the obturator nerve. Such injuries have been
reported following surgeries such as laparoscopic or endo¬
scopic prostatectomies and even rarely following vaginal
deliveries [49,62,64]. Symptoms include gait instability and
an abducted gait in which the affected limb contacts the
ground with the hip excessively abducted [49]. In most cases
symptoms resolve with conservative management, including
exercise and gait training.

Effects of Tightness

Tightness of the adductors is relatively common and may
result from adaptive changes in muscles that are not routinely
stretched. Such tightness is likely in sedentary individuals or

Figure 39.16: During a squat, the ground reaction force (GRF)
produces an abduction moment (M AB ) on the hip, requiring con¬
traction of the adductors to produce a balancing adductor
moment.

in individuals on bed rest who do not receive active or passive
exercises. In addition, adductor muscles are commonly
affected by central nervous system disorders resulting in spas¬
ticity. Examples of such disorders include cerebral vascular
accidents (strokes), multiple sclerosis, and cerebral palsy.

In an ambulatory individual, extreme tightness of the
adductors of the hip can create significant problems in gait,
leading to scissors gait. During swing, the limb with the
tightness may have difficulty passing the stance limb, caus¬
ing the individual to trip over the stance limb. The limb
with the tightness also may land in front of the opposite limb
at the beginning of double-limb support, again presenting a
threat of tripping.

Clinical Relevance

ADDUCTOR SPASTICITY IN CHILDREN: Spasticity of the
hip adductors is a common clinical finding in individuals
with cerebral palsy. In children , adductor spasticity is an
important contributing factor to hip dislocation and hip dys¬
plasia. At birth , the normal alignment of the femur is valgus ,

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

721

directing the femoral head toward the superior aspect of
the acetabulum when the hip is in the neutral position.
Adduction from the neutral position moves the femoral
head laterally in the acetabulum. Because the acetabulum
is shallowest at birth, prolonged positioning of the hip in
adduction can sublux or dislocate the hip joint [53]. The
presence of spasticity of the adductors presents significant
additional risk for dislocation, and surgical release of spastic
muscles around the hip, including the adductors; reduces
the dislocating forces, helping to minimize the incidence of
dislocation and hip dysplasia [43,54].

LATERAL ROTATORS OF THE HIP

The lateral rotators of the hip include the piriformis, obtura¬
tor internus, superior and inferior gemelli, quadratus femoris,
and obturator externus (Fig. 39.17) (Muscle Attachment
Boxes 39.11-39.15). These muscles make up the group of
short rotators of the hip lying deep to the gluteus maximus,
which itself is an important lateral rotator of the hip. None of
these muscles can be palpated directly, since they lie deep to
the large gluteus maximus. However, the horizontally aligned
muscles emerging from the greater sciatic notch may be pal¬
pable as a group in the notch through a relaxed gluteus max¬
imus. Their actions and the effects of impairments in these
muscle groups are discussed together.

Figure 39.17: The deep lateral rotators of the hip include the piri¬
formis, the superior and inferior gemelli, the obturator internus
and externus, and the quadratus femoris.

MUSCLE ATTACHMENT BOX 39.11

ATTACHMENTS AND INNERVATION
OF THE PIRIFORMIS

Proximal attachment: The anterior aspect of the
sacrum at the level of about S2 through S4. It also
has some attachment to the sacrotuberous ligament
and to the periphery of the greater sciatic notch as
it passes through it, exiting the pelvis. The sciatic
nerve usually exits the pelvis alongside the piri¬
formis and emerges at its inferior border.

Distal attachment: The superior and medial aspects
of the greater trochanter

Innervation: The ventral rami of L5 and SI,2

Palpation: The piriformis may be palpable indirectly
by palpating through the gluteus maximus into the
greater sciatic notch.

MUSCLE ATTACHMENT BOX 39.12

ATTACHMENTS AND INNERVATION
OF THE OBTURATOR INTERNUS

Proximal attachment: The obturator membrane and
the borders of the obturator notch. It exits the
pelvis through the lesser sciatic notch.

Distal attachment: The medial aspect of the greater
trochanter

Innervation: The nerve to the obturator internus,

L5 and SI,2

Palpation: Not directly palpable.

MUSCLE ATTACHMENT BOX 39.13

ATTACHMENTS AND INNERVATION
OF THE SUPERIOR AND INFERIOR GEMELLI

Proximal attachment: The inferior aspect of the
ischial spine and the superior aspect of the ischial
tuberosity, respectively. The two muscles are inti¬
mately associated with the obturator internus, one
superior and the other inferior to it.

Distal attachment: The medial aspect of the greater
trochanter with the obturator internus

Innervation: The nerve to the obturator internus,

L5, SI (superior gemellus) and nerve to the quadra¬
tus femoris, L5, SI (inferior gemellus)

Palpation: Not directly palpable.

722

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 39.14

ATTACHMENTS AND INNERVATION
OF THE QUADRATUS FEMORIS

Proximal attachment: The lateral border of the
ischial tuberosity

Distal attachment: The intertrochanteric crest and
quadrate tubercle of the femur. It is a flat, quadri-
laterally shaped muscle.

Innervation: The nerve to the quadratus femoris
(L4,5, SI)

Palpation: Not palpable.

Group Actions

MUSCLE ACTION: LATERAL ROTATORS

Action

Evidence

Hip lateral rotation

Supporting

Hip abduction

Conflicting

Hip adduction

Conflicting

These muscles are all lateral rotators of the hip, but the posi¬
tion of the hip in the sagittal plane significantly affects their
capacity to rotate the hip [13,15]. Moment arm analysis
reveals that the quadratus femoris exerts a lateral rotation
moment regardless of the hip flexion position [13,15]. With
the hip extended, the obturator internus and gemelli also gen¬
erate lateral rotation moments, but when the hip flexes, their
moment arms approach zero so that they generate little or no
rotation moment. The piriformis appears to change from a
lateral rotator with the hip extended to a medial rotator with
the hip flexed. In contrast, the lateral rotation moment arm of
the obturator externus increases as the hip flexes. These data

MUSCLE ATTACHMENT BOX 39.15

ATTACHMENTS AND INNERVATION
OF THE OBTURATOR EXTERNUS

Proximal attachment: The anterior aspect of the
pubic and ischial borders of the obturator foramen
and the obturator membrane. The muscle passes
posteriorly across the posterior aspect of the
femoral neck.

Distal attachment: The intertrochanteric fossa.
Innervation: A division of the obturator nerve (L3 # 4)
Palpation: Not directly palpable.

reveal that the roles of these deep muscles in rotation are
more complex than their title of “lateral rotators” suggests.
Clinicians must recognize the influence of hip position on
their contribution to rotation. Additionally, a valid assessment
of lateral rotation strength requires standard test positions to
ensure that the same muscles participate in each test.

Anatomy texts suggest a role for these muscles in abduc¬
tion and adduction of the hip [44,55]. Biomechanical studies
suggest that their contributions to these motions depend on
hip position [15]. The piriformis possesses an abduction
moment arm regardless of hip position, but the obturator
internus can abduct only when the hip is flexed [15,33]. In
contrast, the obturator externus and quadratus femoris are
capable of hip adduction, but only when the hip is extended
or slightly flexed. It is important to note that the PCS A of
these muscles is considerably smaller than those of the pri¬
mary hip abductor and adductor muscles, and consequently,
these lateral rotators are far less powerful than the primary
hip abductors and adductors. Because of their proximity to
the hip joint, virtually surrounding the proximal portion of the
joint, the small lateral rotator muscles also may serve as
dynamic stabilizers of the hip joint.

Effects of Weakness and Tightness

The gluteus maximus remains the strongest of the lateral
rotators, and therefore, discrete weakness of these small mus¬
cles may be hard to detect. Like weakness, isolated tightness
of the short lateral rotators may be hard to observe. However,
the proximity of the sciatic nerve to these muscles, particu¬
larly to the piriformis, which may even be pierced by the
nerve, makes tightness of these muscles clinically relevant
[55]. If tight, these muscles may exert pressure on the sciatic
nerve, producing pain radiating into the lower extremity.

Clinical Relevance

PIRIFORMIS SYNDROME: The piriformis syndrome

refers to pain associated with tightness or spasm of the piri¬
formis muscle that puts pressure on the underlying sciatic
nerve ’ causing radicular symptoms similar to signs of disc
pathology. The symptoms can be aggravated by stretching
or contracting the piriformis; that is; by medially rotating the
hip passively to stretch the muscle or by resisting lateral
rotation , causing the piriformis to contract [38]. The clinician
uses these passive or resisted movements to elicit the
patient's symptoms and to identify piriformis syndrome.

The classic stretch for the piriformis muscle is hip flexion
with adduction and medial rotation. Yet laterally rotating
the hip has little effect on the stretch of the piriformis. In
fact research suggests that sitting with the knees crossed
(Fig. 39.18) applies a significant stretch to piriformis of the
upper leg despite the fact that the hip is rotated laterally
almost 20° [61].

Chapter 39 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE HIP

723

Figure 39.18: Sitting with the legs crossed applies a stretch to the
piriformis particularly in the top leg by combining flexion and
adduction of the hip.

MEDIAL ROTATORS OF THE HIP

Unlike the other actions of the hip, there are no muscles at
the hip whose primary and consistent action is medial rotation
of the hip. From the data provided in this chapter, it is evident
that the gluteus medius and minimus are capable of medial
rotation of the hip, particularly when the hip is flexed. The
gluteus maximus and piriformis also may contribute to medial
rotation of the hip when the hip is flexed. Some of the hip
adductors may generate small medial rotation moments, but
their contributions are small and variable.

Another muscle described as a medial rotator of the hip is
the tensor fasciae latae, described in more detail with the
muscles of the knee [5,9] (Chapter 42). Although EMG activ¬
ity is reported in the tensor fasciae latae during medial rota¬
tion, mechanical analysis reveals no substantial moment arm
for hip rotation regardless of hip position [9,15]. The medial
hamstrings, the semimembranosus, and semitendinosus also
exhibit small medial rotation moment arms with the hip in
extension [1]. These data reveal that the muscles that medi¬
ally rotate the hip depend on hip position and are intimately
related to the function of the knee.

The most common functional use of active medial rotation
of the hip occurs during the stance phase of gait when the
pelvis rotates over the fixed femur and as the hip moves from
the flexed to the extended position. During this period, the
abductors contract to support the pelvis in the frontal plane,
and the hamstrings are supporting the hip and knee in the
sagittal plane. Their activity in medial rotation demonstrates
an efficient use of muscles to perform simultaneous tasks.

COMPARISONS OF MUSCLE
GROUP STRENGTHS

An understanding of the relative muscle strengths of various
muscle groups in the hip provides a helpful perspective for
clinicians making judgments about their patients’ strengths in
the muscles of the hip. This section reviews the data available
on the relative strengths of hip muscles in healthy individuals.

Several studies investigate the strength of whole muscle
groups of the hip and the effects of joint position on their
strength. Data consistently demonstrate that hip flexion
strength decreases as the hip flexes from 25 to 130° of flexion,
as the muscles go from a lengthened to a shortened position
[26,31,73]. Few studies examine hip flexion strength with the
hip fully extended, and those studies vary in measurement
procedures and results, some demonstrating a continued
increase in strength [31,73] and another showing a loss in
strength [26]. Any loss in hip flexion strength with the hip
extended must be verified by further research but may result
from a decrease in the muscles’ angle of application.

Most data demonstrate that hip abduction and adduction
strength also decreases as the muscles contract from length¬
ened to shortened positions [31,45,47,50]. The length-
tension relationship appears to dictate the force production in
hip rotation as well, so that medial and lateral rotation
strengths increase as the muscles are lengthened [23,31,41].
These studies emphasize the need to standardize the test
positions when assessing hip strength.

Comparisons of the strength of opposing muscle groups
also provide useful information to assist clinicians in deter¬
mining the clinical relevance of muscle strength tests. Hip
adduction appears to be stronger than abduction [29,45,48].
However, the position in which the muscles are tested affects
the comparisons. Murray and Sepic [45] compare the strength
of hip abductor and adductor strength in 80 healthy males and
females aged 18 to 55 years with the hip in neutral and in the
abducted position. The adductor muscles generate larger iso¬
metric torques than the abductor muscles in both hip posi¬
tions. However, because both the abductors and adductors
produce larger isometric moments as they are lengthened,
there is less disparity between the muscle groups with the hip
in neutral than when the hip is abducted, the position in which
the abductor muscles are shortened and the adductors are
lengthened. Peak adductor force is larger than peak abductor
force, at least partially because the total PCS A of the adductor
muscles is larger than that of the abductor muscles [45].

724

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Comparisons of hip rotation strength have also been carried
out. Several investigators report that with the hip and knee
flexed, the standard position for testing the strength of hip
rotation, the medial rotators generate more force than the lat¬
eral rotators under isometric and concentric conditions
[25,35,41]. Jarvis noted a reversal in this relationship when
testing with the hip extended and knee flexed in 50 women
from 21 to 50 years of age [25]. However, Lindsay et al. also
compared strengths with the hip extended and knee flexed and
reported that the medial rotators are stronger than the lateral
rotators in 60 men and women from 18 to 30 years of age [35].
These differences may be methodological or may represent
population differences in the effects of joint position. In either
case, the clinician is reminded by these studies that accurate
strength assessment at the hip requires careful attention to the
position of the joint throughout the test. In addition, it appears
that the present level of knowledge prevents any conclusion
regarding the relative strength of the hip rotators.

Although only limited data are available, hip strength is
greater in men than in women and is greater in younger
adults than in older adults [27,45]. Attempts to identify dif¬
ferences between the left and right sides are limited and have
produced contradictory results. Neumann et al. reported no
significant difference between left and right isometric hip
abductor strength overall in 40 healthy right-handed individ¬
uals [47]. However, they noted a significantly higher strength
in the right than in the left when the hip is positioned in neu¬
tral and in adduction. Jarvis found no difference in isometric
strength of rotation between left and right sides in 50 healthy
women [25]. May, however, reported significantly greater
strength on the left than on the right for medial rotation in all
positions tested in 25 healthy young men [41]. At the present
time there is insufficient data to identify a consistent effect of
the side tested on hip strength.

SUMMARY

This chapter details the function of the individual one-joint
muscles of the hip. The evidence presented in this chapter
reveals that although hip muscles are typically described as
flexors, extensors, abductors, adductors, and rotators, the role
of each muscle depends greatly upon joint position. A mus¬
cle s action depends on its moment arm, which changes with
varying joint position. An understanding of the effects of joint
position on muscle function allows the clinician to identify
optimal positions for exercise.

Muscle impairments at the hip produce significant func¬
tional challenges. Weakness in the muscles of the hip may
produce specific gait deviations as well as difficulties in activ¬
ities such as stair climbing and rising from a chair. Muscular
tightness at the hip limits hip ROM and may increase the
loads on the low back by requiring excessive compensatory
lumbar movement.

Strength comparisons reveal that hip position influences
the force of contraction of all the muscle groups of the hip.

The length-tension relationship appears to be the dominant
factor influencing force production by the muscle groups of
the hip, which generally develop greater force as they con¬
tract in a lengthened position. Age and gender also appear to
affect the strength of the hip muscles.

The muscles of the hip are large and capable of large con¬
tractile forces. In addition, the hip joint supports the weight
of the HAT-L during single-limb stance. Consequently, the
hip joint is subjected to large forces in weight-bearing and
even non-weight-bearing activities. The following chapter
examines the loads sustained by the hip during activities of
daily living and considers how those loads contribute to hip
joint dysfunction.

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CHAPTER

Analysis of the Forces on the Hip
during Activity

CHAPTER CONTENTS

KINETICS OF SINGLE-LIMB STANCE.727

ANALYSIS OF FORCES UNDER DYNAMIC CONDITIONS.733

PRACTICAL APPLICATIONS OF FORCE ANALYSIS.734

SUMMARY .735

T he previous two chapters presented the structural details of the bones and joints of the hip as well as a

functional analysis of its one-joint muscles. Both chapters also discussed relevant hip pathomechanics. The
magnitude, direction, and duration of loads sustained by the hip provide links among structure, function,
and pathology that affect the joint's activity. This chapter examines the loads that the hip sustains during static and
dynamic activities. The purposes of this chapter are to

■ Present two-dimensional analyses of the forces sustained by the hip joint during single leg stance
■ Investigate the factors that influence the magnitude of the forces on the hip joint
■ Examine the loads applied to the hip joint during dynamic activities
■ Discuss the stress sustained by the femoral head during activity
■ Consider the clinical relevance of force analysis at the hip joint

KINETICS OF SINGLE-LIMB STANCE

Examining the Forces Box 40.1 presents a simplified mathe¬
matical analysis of the forces generated during single-limb
stance. An understanding of the factors influencing single¬
limb stance is a prerequisite to understanding the effects on
the hip joint of dynamic activities such as walking and run¬
ning. The task of single-limb stance requires balancing the
weight of the head, arms, trunk, and opposite lower extremity
(HAT-L weight) over the supporting limb. As discussed in
Chapter 1, for an object to remain upright, a vertical line
through the objects center of mass must fall within the
objects base of support. In the upright human, this means
that the center of mass of the HAT-L weight must be verti¬
cally aligned over the stance foot. Consequently, the individ¬
ual shifts the pelvis laterally toward the stance foot, placing
the center of mass over the base of support and putting the
hip joint of the stance limb in adduction (Fig. 40.1). The

HAT-L weight generates an adduction moment on the stance
hip, tending to cause the pelvis to drop on the unsupported
side, and the abductor muscles pull on the pelvis to counter¬
act the adduction moment. To understand the challenge of
single-limb stance, the clinician must answer the following
two questions: (a) what is the force required of the abductors
to support the pelvis? and (h) what is the joint reaction force
on the head of the femur during this task?

The two-dimensional free-body diagram of the femur in
Examining the Forces Box 40.1 shows the primary forces
involved in this task. It is helpful at this point to identify the
rotation that each force causes. The ground reaction force,
equal to body weight, pushes vertically upward on the stance
foot and applies an adduction moment on the hip, tending to
rotate the stance limb in a counter-clockwise direction about
the hip, or to adduct the hip. The weight of the stance limb acts
downward vertically at the limb s center of mass and creates
an abduction moment at the hip, tending to rotate the hip

727

728

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

EXAMINING THE FORCES BOX 40.1

2-D ANALYSIS OF SINGLE-LIMB STANCE

The problems:

• What is the force required of the abductor muscles
to support single-limb stance?

• What is the force on the femoral head during single¬
limb stance?

The static equilibrium conditions needed to solve these
problems are

SM = 0

£F X = 0
£F y = 0

The following quantities can be defined:

d 1 = perpendicular distance from the point of rota¬
tion (hip joint center) to the line of pull of the
abductors

d 2 = perpendicular distance from the point of rota¬
tion (hip joint center) to the line of force of the
weight of the lower extremity
d 3 = perpendicular distance from the point of rota¬
tion (hip joint center) to the line of force of the
ground reaction force (GRF)

W L = weight of the lower extremity, approximately
one-seventh body weight (W)

GRF = ground reaction force pushing up on the
stance foot, equal to body weight (W)

F = force of the abductor muscles
F x , F y = the x and y components of the abductor
force

J = joint reaction force on the head of the femur
J x , J Y = the x and y components of the joint reac¬
tion force

Note that the distances defined above can all be
measured directly from radiographs and therefore
can be regarded as "known" quantities. Body weight
is also a "known" quantity. Therefore, the only
"unknowns" are the abductor and joint reaction
forces. Using these quantities, the static equilibrium
equations can be written for single-limb stance. The
moment equation is used to determine the abductor
force:

2M: (GRF X d 3 ) - (W L X d 2 ) 1 (F X d^ = 0
Replacing the known values for GRF and L:

(W x d 3 ) - (1/7 X W x d 2 ) - (F X d,) = 0
6/7 X W X (d 3 - d 2 ) = F X d 1
W X (d 3 - 1/7 d 2 ) X 1/d 1 = F

Note that d 2 is very small and d 3 is approximately twice
the size of d r These dimensions, available from radi¬
ographs, depend upon the size of the individual.
However d 1 and d 3 are approximately 1 to 3 inches.
Therefore, the magnitude of F ranges from approxi¬
mately 1.5 to 2 times body weight; that is, the force

Chapter 40 I ANALYSIS OF THE FORCES ON THE HIP DURING ACTIVITY

729

clockwise. The abductors acting at the greater trochanter apply
an abduction moment to the hip. The joint reaction force is
assumed to act directly at the joint axis and, therefore, has a
moment arm of zero, creating no moment. The HAT-L weight
is not included individually in the free body diagram, but is a

Figure 40.1: In single-limb stance, the individual shifts laterally to
keep the center of mass (COM) over the base of support.

part of the joint reaction force, which is affected not only by the
HAT-L weight but also by the muscle pull. Application of stat¬
ic equilibrium conditions allows calculation of the force of the
abductors needed to remain upright during quiet single-limb
stance.

Solutions to this problem reveal that the abductors exert a
pull with a force of about twice body weight to support the
HAT-L weight during single-limb stance and that the joint
reaction force on the head of the femur is approximately
2.5 times body weight. Similar loads during single-leg stance
are reported in the literature [2,13,25]. The magnitude of
these loads helps the clinician appreciate why the articular car¬
tilage on the femoral head is among the thickest in the body.

The explanation for these large loads lies in the compari¬
son of the moment arms of the ground reaction force and the
abductor muscles. The lateral shift used by the subject to
keep the center of mass of the HAT-L weight over the foot
serves also to move the hip joint (the point of rotation) later¬
ally, farther from the ground reaction force, increasing the
moment arm of the ground reaction force. In contrast, the
moment arm of the hip abductors remains almost constant
and is considerably smaller than that of the ground reaction
force, putting the abductors at a mechanical disadvantage and
requiring them to generate large contractile forces to balance
the effect of the ground reaction force.

Use of a similar analysis allows examination of the effect a
cane in the opposite hand has on reducing the load on the hip
joint (Examining the Forces Box 40.2). The benefit of the cane
rests on its effect on the moment arm of the ground reaction
force under the foot. The basic task is the same: the subject
must stand so that the center of mass falls within the base of
support. However, the cane in the opposite hand enlarges the
base of support, allowing the subject to stand more erectly
(Fig. 40.2). Consequently, the stance foot is aligned more
closely under the stance hip, and the moment arm of the
ground reaction force is smaller than when standing without a

730

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

EXAMINING THE FORCES BOX 40.2

2-D ANALYSIS OF SINGLE-LIMB STANCE
USING A CANE IN THE CONTRALATERAL
HAND

The problems:

• What is the force required of the abductor muscles
to support single-limb stance while using a cane in
the contralateral hand?

• What is the force on the femoral head during single¬
limb stance while using a cane in the contralateral
hand?

The static equilibrium conditions needed to solve these
problems are the same as those in Box 38.1:

SM = 0

XF x = 0

XF y = 0

Assume that the cane bears 15% of the body weight
so that the remaining 85% is borne by the stance limb.

The following quantities can be defined:

d 1 = perpendicular distance from the point of rota¬
tion (hip joint center) to the line of pull of the
abductors

W L = weight of the lower extremity, approximately
one-seventh body weight (W)

GRF = ground reaction force pushing up on the
stance foot, equal to 85% of body weight (W)

F = force of the abductor muscles
F x , F y = the x and y components of the abductor
force

J = joint reaction force on the head of the femur
J x , J Y = the x and y components of the joint reac¬
tion force

£M: (GRF x d 3 ) - (W L x d 2 ) - (F X d,) = 0

Replacing the known values for GRF and W L :

(0.85 X W x d 3 ) + (1/7 x W x d 2 ) - (F X d,) = 0
0.99 x W X (d 3 + d 2 ) = F X d 1
0.99W X (d 3 + d 2 ) X 1/d 1 = F

(continued)

Chapter 40 I ANALYSIS OF THE FORCES ON THE HIP DURING ACTIVITY

731

Figure 40.2: In single-limb stance with a cane in the contralateral
hand, the individual is able to stand more erectly while keeping
the center of mass (COM) over the widened base of support
(BOS).

cane. Static equilibrium calculations reveal that the use of a
cane in the opposite hand reduces the force required of the
abductor muscles to approximately 50% of body weight and
the joint reaction force to approximately 1.13 times body
weight, more than a 50% reduction in the joint reaction force.
These data provide concrete support to the admonition
offered by Dr. William Blount over a half century ago, “Don’t
throw away the cane” [7].

Individuals usually learn to use a cane in the hand opposite
the impaired side, although casual observation of individuals
walking with a cane or a single crutch suggests that many per¬
sons use the cane in the hand on the same side as the lower
limb problem. It is useful to examine the mechanical implica¬
tions of using the cane on the contralateral or ipsilateral side.
Figure 40.3 reveals that when the cane is used in the ipsilateral
hand, the ground reaction force on the cane actually increases
the adduction moment on the hip produced by the HAT-L
weight. Thus, for the patient to benefit from the cane on the
ipsilateral side, the individual must use other mechanisms to
lower the requirements of the abductor muscles and hence
reduce the joint reaction force. With the cane on the ipsilateral
side, the base of support is even farther lateral to the hip joint
than with no cane. Thus the subject must lean farther laterally
during ambulation with the cane in the ipsilateral hand than
when the cane is in the opposite hand, to put the center of
mass over the widened base of support (Fig. 40.4). This
increased lean of the trunk laterally over the stance foot and
cane reduces the HAT-L weights contribution to the adduc¬
tion moment and thus reduces the abductor requirement [28].
However, the increased lean requires more work for the rest
of the body and may increase the loads on neighboring joints
such as the lumbar spine or knee and ankle. Chan et al. report

732

Figure 40.3: In single-limb stance with a cane in the ipsilateral
hand, the ground reaction force of the cane exerts an adduction
moment (M AD ) on the trunk.

that 14 females with knee osteoarthritis actually generated
larger hip abduction muscle moments using the cane on the
ipsilateral side than when using the cane on the opposite side
or when using no cane at all [8].

The increased lateral lean may also cause the individual to
bear more weight on the cane. Increased weight on the cane
puts the subject at risk for upper extremity overuse syndromes
such as carpal tunnel syndrome. This analysis demonstrates
that there are clear and significant benefits to the individual
with hip pathology who uses the cane in the contralateral rather
than ipsilateral hand. Because the use of the cane in the ipsi¬
lateral hand seems so intuitive, this analysis also pro¬
vides real evidence to help the clinician justify patient
instruction in the most appropriate use of the cane.

Clinical Relevance

GAIT TRAINING TO USE A CANE: Mechanical analysis
of the loads on the hip and direct assessment of the electri¬
cal activity of the hip abductors during gait demonstrate the
benefits of a cane when used in the hand opposite the
impaired hip. Careful instruction in the proper use of a cane
optimizes the potential benefits of the cane while protecting
the patient from injuring other regions such as wrist and
hand or low back and knee.

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 40.4: Because the base of support is lateral to the hip joint
of the stance limb, to benefit from the cane the individual must
lean laterally as far as or farther than with no cane at all.

Mechanical analysis of standing with and without a cane
demonstrates that the joint reaction force on the femoral
head is largely a function of the muscle force required.
Electromyographic (EMG) analysis demonstrates a 30%
reduction in activity of the hip abductors during gait with the
cane in the contralateral hand compared with walking without
a cane [27]. Use of the cane decreases the external moment
created by the ground reaction force, thereby reducing the
force required of the hip abductors. Chapter 39 discusses the
effect of hip abductor weakness and describes the classic glu¬
teus medius limp. In this limp, during single-limb support
on the weak side, the subject leans laterally over the stance
foot, moving the center of mass so that the HAT-L weight cre¬
ates an abduction moment on the hip, thus eliminating the
need for the weakened abductors (Fig. 40.5) [42]. The glu¬
teus medius limp demonstrates, again, the benefit of reduc¬
ing the external moment. Individuals with a painful hip use
this same gait pattern to reduce the load on the painful hip by
reducing the abductor muscle force [18,19,26]. In this case,
the gait pattern is described as antalgic, indicating that it
results from pain.

In the preceding examples, the force of the abductors is
reduced by reducing the external moment. The force required
of the abductors also can be reduced by improving the muscles’
ability to generate a moment. Specifically, the mechanical
advantage of a muscle can be altered by changing its moment
arm. In Chapter 38, the effects of coxa vara and coxa valga

Chapter 40 I ANALYSIS OF THE FORCES ON THE HIP DURING ACTIVITY

733

Figure 40.5: In gluteus medius limp, the individual leans laterally
while standing on the weak side, moving the center of mass lat¬
eral to the hip joint and producing an abduction moment on the
stance hip.

deformities on the moment arm of the abductors are consid¬
ered. Coxa valga deformities reduce the moment arm of the
abductors, while coxa vara tends to increase the moment arm.
The mechanical analyses presented in the current chapter offer
a more complete explanation of the mechanisms at work in
these clinical situations. A mathematical model demonstrates
the result of surgical relocation of the greater trochanter on the
moment arm of the abductor muscles and thus on the joint
reaction force of the hip [20]. Moving the greater trochanter
laterally results in an increase in the abductors’ moment arm
and, consequently, a considerable increase in the moment gen¬
erated by a given contraction. This then reduces the amount of
muscle force needed to support the HAT-L weight in single¬
limb support and thus also reduces the joint reaction force.
Moving the trochanter medially appears to significantly
increase the joint reaction force for similar reasons.

Clinical Relevance

CHANGING MUSCLES MECHANICAL ADVANTAGE
THROUGH SURGERY: Surgeons apply the basic concepts
of mechanical analysis to improve a muscle's mechanical

advantage and thus a patient's function by reconstructive
hip surgery such as osteotomies and even total joint arthro¬
plasties. Joint implants can be designed that influence the
abductor mechanical advantage by altering the length of
the neck of the femoral component. Similarly , the alignment
of the prosthesis as it is implanted can alter the distance
from the joint center to the greater trochanter ; thereby alter¬
ing the moment arm of the abductors.

ANALYSIS OF FORCES UNDER DYNAMIC
CONDITIONS

The examples provided so far have used analysis of static
equilibrium conditions. However, normal walking results in
significant increases in forces as a result of the accelerations
present in locomotion. Although a more detailed discussion of
the principles of dynamic equilibrium used to determine the
forces involved in gait are presented in Chapter 48, the con¬
ceptual framework is similar to the static equilibrium condi¬
tions. The equations of motion for dynamic equilibrium take
on the more general form

2F = ma (Equation 40.1)

2M = la (Equation 40.2)

where F represents the external forces, M represents the
external moments, m is mass, I is moment of inertia, and a
and a represent linear and angular accelerations. Using this
approach, several investigators have calculated the loads on
the femoral head during normal ambulation. Based on the
application of these equations of motion, estimates of peak hip
joint reaction forces during gait range from approximately 2.5
to 7 times body weight [1,10,12,34,39]. Investigators also
report direct measurements of joint forces using instrumented
femoral head prostheses in individuals who undergo hip joint
arthroplasty [4-6,15,16,32]. Recognizing that these measure¬
ments occur in individuals with joint impairments, the direct
measurements suggest that the mathematical analyses may
overestimate hip joint forces [15,16]. Yet even the direct
measurements demonstrate that the hip sustains loads well in
excess of body weight (likely at least two to three times body
weight) during normal locomotion. These joint reaction
forces increase with fast walking and with running [4,5,34].
Healthy, young adult males and females take an average of
10,000-12,000 step-cycles per day, which can be annualized
to almost 2 million step-cycles/year [36,37]. It is no wonder
that a painful hip can lead to severe locomotor dysfunction
and disability.

An understanding of joint reaction forces provides a use¬
ful perspective on the mechanical requirements of daily
tasks. However, the concept of a joint reaction force is an
oversimplification of the physical situation. Joint reaction
forces are generally considered to be applied to a joint at a
single point. In reality, the contact forces at a joint are

734

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

applied over a distinct area. Thus the loads generated at the
hip as the result of weight bearing and of muscle contractions
actually are distributed across the joint surface. A complete
discussion of hip joint forces requires discussion of the
loads/unit area, or the stress, sustained by the hip joint sur¬
faces during activity. Comparative anatomy provides a per¬
spective on the importance of stress as a parameter by which
to assess the hip. The human hip is considerably larger than
that of apes when normalized for overall body size [22]. This
relative increase in joint size results in an improved ability to
spread the loads sustained over a larger surface area during
stance, thus reducing the stress on the hip. The relative size
of the hip joint in humans compared with that in apes
appears to be an important structural difference that
enhances humans’ ability to withstand bipedal ambulation.

Instrumented femoral or acetabular implants installed at
the time of a hip joint arthroplasty allow researchers to meas¬
ure directly the pressures (force/area) on the hip joint surfaces
during a variety of activities [2,14,31,35,38,40]. Peak acetabu¬
lar stresses of approximately 4.0-7.0 MPa are reported on the
posterosuperior surfaces of the hip during slow, fast, and free
speed walking [23,31]. The high stresses sustained in walking
occur in the areas of the hip joint where the articular cartilage
is very thick, and very small stresses occur on the anterior lat¬
eral surface of the hip where the cartilage is thinner. Direct
measurements reveal that the use of a cane reduces peak
stresses to 3.0-4.0 MPa, a finding that is consistent with the
static analysis described earlier in this chapter. Rising from
or descending into a chair and stair climbing increase joint
stresses on the hip to approximately 5.0-9.0 MPa [2,40].

Active exercise during the acute phase of rehabilitation
following hip fracture generates peak pressures similar to
those reported during gait [38]. Non-weight-bearing ambu¬
lation appears to produce higher pressures on the hip than
touch-down weight bearing in an individual with a femoral
head replacement following a hip fracture [14]. It appears
that the co-contraction of muscles needed to hold the limb
off the ground also approximates the joint surfaces, increas¬
ing the contact forces. Stair ascent and descent produce even
higher hip joint stresses than walking, with peak stress of 15
MPa reported during descent [31].

Although direct measurement of joint stresses has
occurred in only a few individuals and from individuals with
specific pathology, studies such as these shed new light on the
demands on the hip joint during activity. They offer insights
regarding how loads are distributed over a surface, which sur¬
faces sustain large stresses and for how long, and which sur¬
faces bear little or no load. Such evidence helps clarify the
relationships among activity, joint loads, and articular integrity.
These joint pressure studies also offer an important perspec¬
tive for the clinician. The data suggest that activities long
believed to exert low loads on the hip may actually load the
hip quite significantly. These data also provide more informa¬
tion to help therapists develop effective rehabilitation regi¬
mens and patient education programs that protect the joint
from excessive loads.

Clinical Relevance

HIP JOINT STRESSES AND CLINICAL OUTCOMES
IN AVASCULAR NECROSIS: Avascular necrosis of the
femoral head produces painful degenerative changes in the
femoral head resulting from death of trabecular and sub¬
chondral bone. As the disorder progresses, some of the
femoral head is no longer weight bearing and loading
occurs over a smaller surface area (increased stress).
Treatment includes femoral head arthroplasty and femoral
osteotomies to realign the weight-bearing surface so that
weight bearing occurs over the undamaged area. In a
study of 30 hips that were treated with intertrochanteric
osteotomies for avascular necrosis , Dolinar and colleagues
report that those individuals with successful outcomes from
9 to 26 years following surgery exhibited an average
decrease in peak femoral head stress of 0.2 MPa while those
who had an unsuccessful outcome had an average increase
in stress of 0.08 MPa [11]. These data suggest that surgical
realignment that minimizes the stress on the femoral head
may actually improve clinical outcomes. This study demon¬
strates the direct clinical applicability of biomechanical
measures such as bone stress.

PRACTICAL APPLICATIONS OF FORCE
ANALYSIS

Osteoarthritis is the most common rheumatic disease in the
world, found in approximately one third of adults aged
65 years or older [3,24,30]. The hip joint is one of the most
commonly affected joints [17,21]. Mechanical factors such as
the magnitude of the loads on the joints as well as the fre¬
quency and duration of loading have long been implicated in
degenerative joint disease [33]. The most important risk fac¬
tors for osteoarthritis of the hip include obesity and occupa¬
tions that require repeated lifting, providing more evidence
linking loads and loading patterns to osteoarthritis [3,9,41].
Thus a common goal of conservative treatment in individuals
with arthritis is to reduce the loads on the involved joints. An
analysis of forces and pressures applied to the hip offers the
clinician direct evidence by which to evaluate such joint pro¬
tection programs. The examples presented so far provide con¬
crete clinical applications: (a) individuals with a painful hip
can benefit from the use of a cane in the opposite hand; (b) if
there are no other problems, using a cane in the ipsilateral
hand can worsen the gait in an individual with a painful hip;
and (c) the femoral head of an individual following hip frac¬
ture sustains significant pressures, even during non-weight¬
bearing ambulation.

The ability to analyze loads on the hip allows the clinician
to evaluate most situations and provide advice to help an indi¬
vidual decrease the loads on the hip. For example, carrying a
load in the ipsilateral hand reduces the abductor forces used

Chapter 40 I ANALYSIS OF THE FORCES ON THE HIP DURING ACTIVITY

735

Figure 40.6: A. A weight on the side of the weight-bearing limb
produces an abduction moment (M AB ) on the stance hip, reduc¬
ing the force required of the hip abductors. B. A weight on the
side opposite the weight-bearing limb produces an adduction
moment (IVI AD ) on the stance hip, increasing the force required
of the hip abductors.

to stabilize the trunk and pelvis and thus decreases the joint
reaction force, and carrying loads in the contralateral hand
has the opposite effect [27,29]. A brief consideration of the
mechanics of the situation reveals that this conclusion is a
direct outgrowth of principles of static equilibrium. The load
in the ipsilateral hand creates an abduction moment on the
stance hip, thereby reducing the need for the abductor mus¬
cles (Fig. 40 . 6 ). Conversely, a load in the contralateral hand
creates an adduction moment and increases the need for the
abductors. This analysis can be used to evaluate the load on
the hip in industrial settings where workers are required to lift
or carry loads repetitively Similarly, the single-limb
stance analysis can be applied to situations requiring
prolonged asymmetrical support. Thus it behooves
the clinician to analyze the mechanics of an activity and use
the results of that analysis to optimize an intervention.

SUMMARY

This chapter uses the principles of static equilibrium to ana¬
lyze the forces involved in the case of single-limb support.
The abductor force required during single-limb stance is
approximately two times body weight, and the resulting joint

reaction force is approximately 2.5 times body weight.
Estimates of the joint reaction force on the femur during
locomotion vary but are likely to be two to three times body
weight. Mechanical analysis demonstrates that the use of a
cane in the opposite hand is quite effective in reducing the
joint reaction force on the femoral head. Because the joint
reaction force is largely a function of the abductor muscle
force, procedures to improve the mechanical advantage of the
muscles or strategies to decrease the external moments on the
hip are effective in reducing the loads on the hip.

This chapter also examines the stresses applied to the hip
and demonstrates that the femoral head sustains large stresses
(4-6 MPa) during walking and even during non-weight-bearing
gait. Descending stairs generates much larger peak stresses.
The chapter also demonstrates that the stresses are applied
unevenly across the joint surface and are highest where the
articular cartilage is the thickest. Understanding the forces and
stresses to which the hip is subjected every day allows the cli¬
nician to quantify the impact of structural abnormalities or
muscular impairments. These concepts should help guide the
clinician to develop more directed, efficient, and successful
interventions.

References

1. Anderson D, Hillberry B, Teegarden D, et al.: Biomechanical
analysis of an exercise program for forces and stresses in the hip
joint and femoral neck. J Appl Biomech 1996; 12: 292-312.

2. Bachtar F, Chen X, Hisada T: Finite element contact analysis of
the hip joint. Med Bio Eng Comput 2006; 44: 643-651.

3. Berenbaum F: Osteoarthritis A. Epidemiology, pathology, and
pathogenesis. In: Klippel JH, ed. Primer on the Rheumatic
Diseases. Atlanta: Arthritis Foundation, 2001; 285-289.

4. Bergmann G, Deuretzbacher G, Heller M, et al.: Hip contact
forces and gait patterns from routine activities. J Biomech 2001;
34: 859-871.

5. Bergmann G, Graichen F, Rohlmann A: Hip joint loading dur¬
ing walking and running, measured in two patients. J Biomech
1993; 26: 969-990.

6. Bergmann G, Kniggendorf H, Graichen F, Rohlmann A:
Influence of shoes and heel strike on the loading of the hip joint.
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7. Blount W: “Don’t throw away the cane.” J Bone Joint Surg 1956;
38: 695.

8. Chan GNY, Smith AW, Kirtley C, Tsang WWN: Changes in knee
moments with contralateral versus ipsilateral cane usage in
females with knee osteoarthritis. Clin Biomech 2005; 20:
396 ^ 04 .

9. Cooper C, Campbell L, Byng P, et al.: Occupational activity and
the risk of hip osteoarthritis. Ann Rheum Dis 1996; 55: 680-682.

10. Crowninshield RD, Johnston RC, Andrews JG, Brand RA: A
biomechanical investigation of the human hip. J Biomech 1978;
11: 75-85.

11. Dolinar D, Antolic V, Herman S, et al.: Influence of contact hip
stress on the outcome of surgical treatment of hips affected by
avascular necrosis. Arch Orthop Trauma Surg 2003; 123:
509-513.

12. Duda GN, Schneider E, Chao EYS: Internal forces and moments
in the femur during walking. J Biomech 1997; 30: 933-941.

736

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

13. Genda E, Iwasaki N, Li G, et al.: Normal hip joint contact pres¬
sure distribution in single-leg standing—effect of gender and
anatomic parameters. J Biomech 2001; 34: 895-905.

14. Givens-Heiss DL, Krebs DE, Riley PO, et al.: In vivo acetabu¬
lar contact pressures during rehabilitation, Part II: Postacute
phase. Phys Ther 1992; 72: 700-710.

15. Heller MO, Bergmann G, Deuretzbacher G, et al.: Musculo¬
skeletal loading conditions at the hip during walking and stair
climbing. J Biomech 2001; 34: 883-893.

16. Heller MO, Bergmann G, Kassi JP, et al.: Determination of
muscle loading at the hip joint for use in pre-clinical testing.
J Biomech 2005; 38: 1155-1163.

17. Hochberg MC: Osteoarthritis. B. Clinical features. In: Klippel
JH, ed. Primer of the Rheumatic Diseases. Atlanta: Arthritis
Foundation, 2001; 289-293.

18. Hurwitz DE, Foucher K, Sumner DR, et al.: Hip motion and
moments during gait relate directly to proximal femoral bone
mineral density in patients with hip osteoarthritis. J Biomech
1998; 31: 919-925.

19. Hurwitz DE, Hulet C, Andriacchi T, et al.: Gait compensations
in patients with osteoarthritis of the hip and their relationship to
pain and passive hip motion. J Orthop Res 1997; 15: 629-635.

20. Iglic A, Antolic V, Srakar F, et al.: Biomechanical study of vari¬
ous greater trochanter positions. Arch Orthop Trauma Surg
1995; 114: 76-78.

21. Ivan D: Pathology for the Health-Related Professions.
Philadelphia: WB Saunders, 1996.

22. Jungers W: Relative joint size and hominoid locomotor adapta¬
tions. J Hum Evol 1988; 17: 247.

23. Krebs DE, Robbins CE, Lavine L, Mann RW: Hip biomechanics
during gait. J Orthop Sports Phys Ther 1998; 28: 51-59.

24. Martin DF: Pathomechanics of knee osteoarthritis. Med Sci
Sports Exerc 1994; 26: 1429-1433.

25. McLeish R, Charndey J: Abduction forces in the one-legged
stance. J Biomech 1970; 3: 191-209.

26. Murray MP, Gore DR, Clarkson BH: Walking patterns of
patients with unilateral hip pain due to osteo-arthritis and avas¬
cular necrosis. J Bone Joint Surg 1971; 53A: 259-274.

27. Neumann D: An electromyographic study of the hip abductor
muscles as subjects with a hip prosthesis walked with different
methods of using a cane and carrying a load. Phys Ther 1999; 79:
1163-1173.

28. Neumann DA: Hip abductor muscle activity as subjects with hip
protheses walk with different methods of using a cane. Phys
Ther 1998; 78: 490-501.

29. Neumann DA, Cook TM: Effect of load and carrying position
on the electromyographic activity of the gluteus medius muscle
during walking. Phys Ther 1985; 65: 305-311.

30. O’Sullivan S, Schmitz T: Physical Rehabilitation: Assessment
and Treatment. Philadelphia: FA Davis, 1988.

31. Park S, Krebs DE, Mann RW: Hip muscle co-contraction: evi¬
dence from concurrent in vivo pressure measurement and force
estimation. Gait Posture 1999; 10: 211-222.

32. Pedersen D, Brand R, Davy D: Pelvic muscle and acetabular
contact forces during gait. J Biomech 1997; 30: 959-965.

33. Radin EL, Orr RB, Kelman JL, et al.: Effects of prolonged walk¬
ing on concrete on the knees of sheep. J Biomech 1982; 15:
487-492.

34. Rohrle H, Scholten R, Sigolotto C, Sollbach W: Joint forces in
the human pelvis-leg skeleton during walking. J Biomech 1984;
17: 409-424.

35. Rydell N: Intravital measurements of forces acting on the hip-
joint. Biomech Related Eng Top 1965; 351-357.

36. Schmalzried TP, Szuszczewicz ES, Northfield MR, et al.:
Quantitative assessment of walking activity after total hip or
knee arthroplasty. J Bone Joint Surg 1998; 80: 54-59.

37. Sequeira MM, Rickenbach M, Wietlisbach V, et al.: Physical
activity assessment using a pedometer and its comparison with a
questionnaire in a large population survey. Am J Epidemiol
1995; 142: 989-999.

38. Strickland EM, Fares M, Krebs D, et al.: In vivo acetabular con¬
tact pressures during rehabilitation, Part I: Acute phase. Phys
Ther 1992; 72: 691-699.

39. Witte H, Eckstein F, Recknagel S: A calculation of the forces
acting on the human acetabulum during walking. Acta Anat
1997; 160: 269-280.

40. Yoshida H, Faust A, Wilckens J, et al.: Three-dimensional
dynamic hip contact area and pressure distribution during activ¬
ities of daily living. J Biomech 2006; 39: 1996-2004.

41. Yoshimura N, Sasaki S, Iwasaki K, et al.: Occupational lifting is
associated with hip osteoarthritis: a Japanese case-control study.
J Rheumatol 2000; 27: 434-440.

42. Zijlstra W, Bisseling R: Estimation of hip abduction moment
based on body fixed sensors. Clin Biomech 2004; 19: 819-827.

UNIT 7

KNEE UNIT

T he previous unit presents the structure and mechanics of the hip joint. The present unit describes the struc¬
ture and function of the knee joint as well as factors that contribute to its dysfunction. Like the elbow in the
upper extremity, the primary function of the knee is to lengthen and shorten the limb, thus assisting the hip
in positioning the foot. For example, the knee shortens the lower extremity to assist in foot clearance during the
swing phase of gait and lengthens the limb as it extends toward the ground for the stance phase of gait. However, the
knee's role in telescoping the limb is complicated by several factors: (a) The knee is weight-bearing; ( b) it is located
between the two longest bones of the body, the femur and tibia; and (c) the motion of the foot on the ground causes
a twisting motion of the tibia and, consequently, of the knee. These factors require the knee to possess more capabili¬
ties than a pure hinge joint has. In fact, the knee exhibits complex three-dimensional motion. The purposes of the unit
on the knee are to

■ Describe the structure of the bones and articulations of the knee joint and their affects on the mobility and
functional capacity of the knee

■ Discuss the contribution of the muscles of the knee to the normal mechanics and pathomechanics of the knee
■ Examine the forces sustained by the knee during normal function and consider the role of these forces in knee
joint pathology

737

CHAPTER

Structure and Function of the
Bones and Noncontractile
Elements of the Knee

CHAPTER CONTENTS

BONES OF THE KNEE JOINT.739

Shaft and Distal Femur.739

Proximal Tibia .741

Effects of the Shapes of the Articular Surfaces on Tibiofemoral Joint Motion .741

Tibiofemoral Motion.742

Patella .744

Proximal Fibula .744

Palpable Landmarks of the Knee.745

ARTICULAR STRUCTURES OF THE KNEE.745

Organization of the Trabecular Bone and Articular Cartilage Found in the Knee .745

Menisci.746

Motion of the Menisci on the Tibia.747

Noncontractile Supporting Structures.748

NORMAL ALIGNMENT OF THE KNEE JOINT.755

Frontal Plane Alignment .756

Sagittal Plane Alignment .756

Transverse Plane Alignment.757

ALIGNMENT OF THE PATELLOFEMORAL JOINT.757

Medial-Lateral Alignment .758

Proximal-Distal Alignment.758

Angular Positioning of the Patella .758

MOTION OF THE KNEE .759

Normal Range of Motion of the Knee in the Sagittal Plane.759

Transverse and Frontal Plane Rotations of the Knee.759

Patellofemoral Motion.760

SUMMARY .761

T he primary function of the knee to alter the length of the lower extremity requires motion of only a simple
hinge joint. However, the motion of the tibia caused by the foot on the ground and the location of the
knee joint at the center of a long weight-bearing limb place unusual additional demands on the knee joint.

738

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

739

These demands require a delicate balance between the stability needed for weight-bearing and the mobility required
for bipedal ambulation. The purposes of this chapter are to

■ Discuss the structure of the bones of the knee and how the structure affects the mobility and stability of
the knee joint

■ Examine the complex three-dimensional movement of the tibiofemoral and patellofemoral articulations

■ Examine the normal alignment of the bones of the knee joint

■ Consider the articular structures that contribute to the stability of the knee joint

■ Review the normal ranges of motion of the knee

BONES OF THE KNEE JOINT

The knee joint is composed of the distal femur, proximal
tibia, and the patella. This chapter presents the character¬
istics of each bone that affect the mechanics of the knee
joint, including the femoral shaft and distal femur. The
proximal femur is discussed in the preceding unit on the
hip (Chapter 38). Similarly, the present chapter describes
the proximal tibia. Descriptions of the tibial shaft and distal
tibia are furnished in the ankle unit (Chapter 44). Although
the fibula does not participate directly in the mechanics
of the knee joint, some muscles that cross the knee attach
to the fibula. Consequently, the proximal fibula also is
described in this chapter.

Shaft and Distal Femur

The shaft of the femur has three surfaces, anterior, medial,
and lateral (Fig. 41 . 1 ). The medial and lateral surfaces are
separated from each other posteriorly by the linea aspera,
the prominent posterior crest that gives rise to much of the
quadriceps femoris muscle. The linea aspera splits distally,
contributing to the medial and lateral supracondylar lines
and demarcating a posterior surface for attachment of the
popliteal muscle. Distally, the femoral shaft flattens in
an anterior-posterior direction and widens medially and lat¬
erally to form the medial and lateral supracondylar lines.
The supracondylar lines terminate in the expanded distal
end of the femur, which provides the articular surfaces for
the knee joint.

The distal end of the femur consists of two large condyles
that are continuous with each other anteriorly but are separated
by an intercondylar notch posteriorly. The anterior portions of
the articular surfaces of both the medial and lateral condyles
combine to provide articulation for the patella. Although this
patellar surface is continuous with the rest of the articular sur¬
faces of the medial and lateral condyles, it is distinguished
from the tibiofemoral articular surfaces by a very slight medi-
olateral groove [146]. The articular surface for the patella is
concave in the medial-lateral direction with a distinct longitu¬
dinal groove through its midline. It is convex in a superior-
inferior direction. The anterior surface of the lateral condyle,
which articulates with the patella, extends farther anteriorly

Figure 41.1: A. An anterior view of the femur reveals the
medial and lateral condyles with their respective epicondyles.
B. A posterior view of the femur reveals the linea aspera, the
medial and lateral supracondylar lines, the popliteal surface,
and the intercondylar fossa.

than the anterior surface of the medial condyle, forming a but¬
tress against lateral dislocation of the patella [191].

The medial and lateral condyles are separated from one
another by the intercondylar fossa on their distal and pos¬
terior surfaces where they articulate with the tibia. The
medial and lateral walls of the intercondylar fossa provide
attachments for the posterior cruciate ligament (PCL) and
anterior cruciate ligament (ACL), respectively. The sur¬
faces of the two condyles are quite different from one
another, which helps explain the complex motions of the
tibiofemoral articulation. The unique characteristics of
each condyle are described below.

740

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MEDIAL CONDYLE

The medial condyle extends farther distally than the lateral
condyle. However, because in the normal knee the two
condyles lie on the same horizontal plane, the shaft of
the femur forms a slight angle with the vertical (Fig. 41 . 2 ).
The proximal surface of the medial condyle is marked by the
adductor tubercle, a palpable landmark where the adductor
magnus attaches. The medial aspect of the medial femoral
condyle offers an easily palpated apex known as the medial
epicondyle.

The shape and size of the tibiofemoral articular surface of
the medial condyle distinguish it from the lateral condyle and
influence the motions of the tibiofemoral articulation. The
medial condyle is slightly curved in the transverse plane, as
though it lies on a circle that surrounds the lateral condyle
(Fig. 41 . 3 ). The medial condyles articular surface for the tibia
is longer from anterior to posterior than that of the lateral
condyles articular surface. In addition, although the medial
condyle is convex from anterior to posterior, its curvature is
variable. It is flattest on its most distal surface and is more
curved posteriorly [44,81,86,146,191]. The articular surface
for the patella also is more curved than the distal surface.
Radius of curvature describes the curvature of a surface
(Chapter 7). In general, the radius of curvature is the radius of
the circle from which the articular surface can be derived.
Therefore, a flat surface is a segment of a very large circle with
a large radius. A curved surface is part of a smaller circle with
a smaller radius (Fig. 7 . 10 ). Thus the radius of curvature of the

Figure 41.2: A. The larger medial condyle projects beyond the
horizontal plane when the femur is vertical. B. When the
condyles are aligned horizontally as they are in vivo, the femoral
shaft is projected laterally.

Figure 41.3: A distal view of the femur shows that the medial
femoral condyle is curved in the transverse plane, and the lateral
femoral condyle projects posteriorly close to the sagittal plane.

medial condyle is greatest distally and is smaller on its posteri¬
or surface (Fig. 41 . 4 ). This asymmetry in curvature contributes
to the complex motion between the femur and tibia.

LATERAL CONDYLE

The lateral condyle s articular surface for the tibia projects pos¬
teriorly, more in the sagittal plane than the medial condyle.
Like the medial condyle, the articular surface presents vari¬
able curvatures and, like the medial condyle, is flattest distally.
The lateral femoral condyle is flatter distally than the medial
condyle and hence has a larger radius of curvature [132].
In the frontal plane, both condyles are slightly convex, but the
lateral condyle is flatter than the medial one. The lateral aspect
of the lateral condyle forms a prominent projection, the lateral

Figure 41.4: The radii of curvature of the medial (A) and lateral
(B) femoral condyles vary across the surface of the condyle,
longer distally and shorter anteriorly and posteriorly.

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

741

epicondyle, which is an important palpable landmark. The
knee joints axis of flexion and extension passes approximately
through the lateral and medial epicondyles [33,179].

Proximal Tibia

The tibia is the second longest bone of the body, exceeded
only by the femur. It is characterized by an expanded proxi¬
mal end that consists of medial and lateral condyles, or
plateaus, separated by a nonarticulating intercondylar
region (Fig. 41.5). This nonarticular region is roughened
and consists of an intercondylar eminence and smooth inter¬
condylar areas anterior and posterior to the eminence.
Medial and lateral intercondylar tubercles, or spines, project
proximally from the eminence. The intercondylar region
provides attachment for the medial and the lateral menisci
and the ACL and PCL. The anterior surface of the proximal
tibia is marked by the tibial tuberosity, readily palpated
since it is covered by only skin and the infrapatellar bursa.
Just distal to the lateral tibial plateau and lateral to the tib¬
ial tuberosity is another tubercle, the tubercle of the lateral
condyle of the tibia, also known as Gerdy’s tubercle. A facet
for the head of the fibula is located on the inferior surface of
the lateral condyle. The facet faces laterally, distally, and
slightly posteriorly.

ARTICULAR SURFACES OF THE PROXIMAL TIBIA

The articular surfaces of the proximal tibia for the femoral
condyles consist of medial and lateral facets on the tibial
plateaus. The proximal articular surfaces of the tibia are con-

Figure 41.5: The proximal tibia contains the tibial plateaus with
their articular facets. The medial articular facet is concave from
medial to lateral; the lateral articular facet is concave medial to
lateral but slightly convex anterior to posterior.

siderably smaller than the respective articular surfaces on the
femur. Additionally, the articular surface on the medial tibial
plateau is larger than the articular surface of the lateral tibial
plateau, decreasing the stress (force/area) applied to the
medial tibial plateau, which bears more force than the lateral
plateau in upright stance [12,148].

The tibia’s medial articular surface is slightly concave.
However, it has a very large radius of curvature, indicating
that it is relatively flat [187,195]. The shape of the lateral
articular surface is more variable. It is concave in the medial-
lateral direction, but, like the femur, the lateral tibial plateau
is flatter than the medial plateau. Although some authors
report that the lateral articular surface also is concave in
the anterior-posterior direction [191], direct measurements
of cadaver knees suggests that the surface actually is flat or
even convex throughout most of its anterior-posterior sur¬
face [12,50,187,195]. Thus it is apparent that not only do the
medial and lateral articular surfaces of the tibia differ from
each other, they also differ from the respective articular
surfaces of the femur. The differences in shapes of the artic¬
ular surfaces of the tibiofemoral joint influence the loading
pattern across the joint. Although these differences are
modulated somewhat by the intervening menisci, which are
discussed later in this chapter, the remaining differences
among the articular surfaces influence the motion of the
tibiofemoral joint.

Effects of the Shapes of the Articular
Surfaces on Tibiofemoral Joint Motion

Three factors regarding the shapes of the knee’s articular sur¬
faces affect the motion of the tibiofemoral joint:

• The different size of the articular surfaces of the femoral
condyles and the tibial condyles

• The different size of the articular surface of the medial
femoral condyle and the lateral femoral condyle

• The variation in curvature from anterior to posterior in all
of the articular surfaces

Each of these factors has a different impact on the motion
that occurs at the tibiofemoral joint, and together they help to
explain the complex three-dimensional motion that occurs
during flexion and extension of the knee.

DISPARITY BETWEEN THE TIBIAL
AND FEMORAL SURFACES

Because there is more articular surface on the femoral side of
the knee joint than on the tibial side, pure rolling motion is
impossible. As described in Chapter 7, pure rolling occurs
when for every point of contact on one surface there is a
unique point of contact on the other surface (Fig. 7.3). Thus
rolling requires equal articular surfaces. If, during knee flex¬
ion, the femur underwent pure rolling on the tibia it would roll
off the tibial surface (Fig. 41.6). During flexion, the contact
between the femur and tibia moves progressively posteriorly

742

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 41.6: If flexion of the knee occurs with pure rolling
with no translation of the femur, the femur will "roll off"
the tibia.

on the tibia, indicating some rolling [49,170,192,195].
However, the magnitude of the difference in articular sur¬
faces between the tibia and femur dictates that in knee flex¬
ion, the femur must undergo additional motions as it rolls into
flexion. Conversely, in extension, the contact between femur
and tibia moves progressively anteriorly as the knee moves
from flexion to extension, but the femur exhibits additional
motion as the femur rolls into extension.

DISPARITY BETWEEN THE SIZE OF THE MEDIAL
AND LATERAL FEMORAL CONDYLES

Because the lateral femoral condyle has a shorter articular
surface for the tibia than the medial condyle, pure flexion and
extension movements fail to use the entire articular surface of
the medial condyle. Only additional movement in the trans¬
verse and frontal planes allows full use of the medial condyle s
articular surface.

VARIABILITY OF CURVATURE IN ALL
OF THE ARTICULAR SURFACES OF THE
TIBIOFEMORAL JOINT

The variability in shape of the individual articular surfaces
from anterior to posterior suggests that the relative motion
between tibia and femur depends on which part of the
condyles are actually in contact. Consequently, the knees
relative motion is a function of its position. Thus the shapes of
the femoral and tibial articular surfaces have a direct impact
on the relative motion of the tibiofemoral joint [19].
Ligamentous supports also influence tibiofemoral joint
motion. The contributions made by these structures are dis¬
cussed later in this chapter.

Tibiofemoral Motion

The complex shapes and incongruities of the tibiofemoral
joint surfaces contribute to intricate three-dimensional
movement of the femur and tibia during knee flexion and
extension. The classic view of tibiofemoral motion is based
on two-dimensional analyses that suggest that knee flexion
begins with lateral rotation of the femur and continues with
posterior rolling of the femur and concomitant anterior
gliding of up to 2 cm [49].

More recent three-dimensional analyses confirm the three-
dimensional nature of the tibiofemoral movement during flex¬
ion and extension but provide more precise measurements of
the frontal and transverse plane movements [33,72,81,193].
These studies demonstrate that lateral rotation of the femur
with respect to the tibia accompanies knee flexion reaching
approximately 20° of lateral rotation as the knee moves from
full extension to at least 90° of flexion (Fig. 41.7). In addition,
femoral abduction with respect to the tibia also occurs with
knee flexion, although this excursion is much smaller, on the
order of 5° [134,162]. Extension from the flexed position com¬
bines the opposite motions: anterior rolling, medial rotation,
and adduction of the femur.

Translation of the femoral condyles also accompanies knee
flexion and extension. During flexion the lateral femoral
condyle translates posteriorly [38,134,162]. Translation of the
medial condyle is less well understood and appears to be less
than that of the lateral condyle. The traditional view of
femoral or tibial translation during knee flexion and extension
appears to require revision. The traditional view, based on
two-dimensional analysis, describes knee movement accord¬
ing to the so-called concave-convex rule. The concave-convex
rule suggests that a convex surface (the femoral condyles)
rolling on a concave surface (the tibial plateaus) will roll in
one direction and glide, or translate, in the opposite direction.
Applied to the knee, this rule would dictate that during
flexion the femoral condyles would roll posteriorly and trans¬
late anteriorly Existing data convincingly refute this belief.
Using three-dimensional imaging techniques, investigators
consistently demonstrate substantial posterior translation of
the lateral femoral condyle during knee flexion. Translation of
the medial femoral condyle is reported by some as minimal

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

743

Figure 41.7: A. Flexion of the knee occurs with rolling, lateral
rotation and abduction of the femur, and at least some transla¬
tion. B. Extension reverses the motions.

[38], as anterior by some [162], and as posterior by others
[68,134]. Some of the posterior translation of the lateral
femoral condyle probably reflects lateral femoral rotation but
may include independent posterior translation of the femoral
condyle. Studies also demonstrate that the contact point on
the tibia moves posteriorly during flexion, particularly on the
lateral femoral plateau [38].

The confusion regarding knee motion likely stems from
the two-dimensional images that were the primary source of
information for most of the twentieth century and the erro¬
neous interpretation of femoral rotation as translation.
Regardless of the source of the misconception, the twenty-
first century understanding of knee motion acknowledges
complex three-dimensional motion that consists mainly of
flexion or extension with significant longitudinal rotation,
slight frontal plane motion, and very slight translation, most of
which is posterior.

The timing of the medial or lateral rotation also remains
disputed. While the traditional view that rotation occurs only
at the beginning of flexion or end of extension has been refuted,
some investigators suggest that there is an initial rotation at
the beginning of flexion (or end of extension), which then
ceases until at least 45° of flexion. Others suggest that the
rotation continues smoothly through at least the first 90° of

the motion [193]. The screw home mechanism describes
the final medial rotation of the femur as the knee reaches full
extension. Whether this is a distinct femoral movement or the
continuation of the femoral rotation throughout the range
is unresolved.

Despite the existing controversies, the tibiofemoral move¬
ment during knee flexion and extension exhibits characteristic
components:

• During flexion, as the femur rolls into flexion, it rotates lat¬
erally with respect to the tibia. Conversely, the femur
rotates medially as it rolls into extension.

• Contact between the femur and tibia migrates posteriorly
on the tibia during flexion and anteriorly during extension.

• There appears to be some anterior-posterior translation
between the tibia and femur during some portions of flex¬
ion and extension, although this translation may be small.

Thus far this chapter has described flexion and extension
of the knee as motion of the femur on the tibia. Such motion
occurs when sitting into, or rising from, a chair. In these cases,
the foot is fixed on the floor and the thigh moves over the leg.
This is known as a closed chain activity. However, during the
swing phase of gait, the leg moves more than the thigh. In this
case the tibia can be described as moving on the femur. This
movement is known as an open chain activity, characterized
by the foots ability to move freely in space. Regardless of
whether the thigh moves on the leg or the leg on the thigh,
the relative motion of femur and tibia remains the same during
flexion and extension of the knee. These motions are listed
in Table 41.1.

It is clear from the description of the motions that occur
at the tibiofemoral joint, that the knee does not function as
a pure hinge joint. It allows significant motion about the
three axes, medial-lateral, anterior-posterior, and longitudi¬
nal. Although the motion about the medial-lateral axis far
exceeds the motions about the other two axes, all of the
motions play a significant role in the function of the
tibiofemoral articulation. In addition, the tibiofemoral joint
allows translation along all three axes. Although only the
anterior-posterior gliding that is limited by the cruciate
ligaments is well described, there is potential for a small
amount of medial and lateral translation and slight distrac¬
tion of the joint along its long axis [134,162]. Therefore, the
motion of the tibiofemoral joint is an example of a joint with
six degrees of freedom (DOF), allowing rotation about,
and translation along, three axes (Fig. 41.8).

TABLE 41.1: Relative Motion of the Femur
and Tibia during Knee Flexion and Extension

Femoral Motion

Tibial Motion

Rolling

Rotation

Rolling

Rotation

Flexion

Backward

Lateral

Forward

Medial

Extension

Forward

Medial

Backward

Lateral

744

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Clinical Relevance

TIBIOFEMORAL JOINT MOTION: The motion of the
tibiofemoral joint is quite complex. The restoration of full knee
flexion or extension range of motion (ROM) in a patient with
limited motion requires that the clinician facilitate rotations
and translations in all three planes. Common manual therapy
techniques are aimed at enhancing tibiofemoral glide and
rotation. It is important for the clinician to restore the neces¬
sary accessory motions of the joint to improve flexion and
extension flexibility.

Figure 41.8: The knee is capable of rotation and translation
about three axes and therefore has six degrees of freedom.

Patella

The patella is the largest sesamoid bone in the human body,
imbedded in the tendon of the quadriceps femoris muscle. It
is triangular, with its apex pointing distally (Fig. 41.9). Only
its posterior surface is articular. The articular surface is oval,
with a central ridge that runs from proximal to distal. This
ridge creates a medial and larger lateral facet for articulation
with the medial and lateral femoral condyles, respectively
A third facet, known as the odd facet, or border facet, is
found on the medial border of the medial facet. The ridge on
the posterior surface of the patella glides in the reciprocal
groove, or sulcus, on the anterior surface of the distal femur.

Although the patella protects the quadriceps tendon from
excessive friction from the femur during knee flexion, its pri¬
mary function is to increase the angle of application and, con¬
sequently, the moment arm of the quadriceps tendon.
Estimated reductions of 33 to almost 70% in the quadriceps
muscle s moment arm with the knee extended are predicted
with removal of the patella [88,186].

Clinical Relevance

PATELLECTOMY: Removal of the patella is known as a
patellectomy ; Severe comminuted fractures of the patella
occasionally require the removal of the fragments because
satisfactory repair is not possible. However ; the functional
impairments resulting from such surgery cause the proce¬
dure to be considered a procedure of last resort [35,100].

A patellectomy places two significant mechanical challenges
on the extensor mechanism. The first is the reduction of the
moment arm of the quadriceps. As a result of the removal
of the patella, the quadriceps muscle must generate a larger
force to produce a moment (M) than the force needed to
generate the same moment in the presence of the patella
[168] (Fig. 41.10).

The patella also serves to lengthen the quadriceps mus¬
cle. Consequently, without the patella the extensor muscle is
functionally longer and unable to shorten adequately to
extend the knee through its entire excursion. Thus the
quadriceps exhibits active insufficiency, which in the knee
is referred to as an extensor lag. As a result of these
mechanical deficits, a patellectomy typically is accompanied
by reconstruction of the extensor mechanism, such as surgi¬
cal shortening or distal advancement of the distal attach¬
ment to stretch the muscle, thus increasing its strength and
its ability to move the knee through its full ROM [35,143].

Proximal Fibula

The fibula does not participate directly in knee joint function.
However, muscles affecting the knee attach to it.
Consequently, its proximal end is reviewed here. The slightly
enlarged proximal end of the fibula consists of a head and

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

745

Figure 41.9: The patella is triangular and contains a medial, lateral,
and odd facet on its articular surface.

smaller neck. The head contains an articular facet on its medi¬
al aspect to articulate with the corresponding facet on the
tibia. The proximal end of the fibula ends in a projection
known as the styloid process. Both the head and its styloid
process are palpable just distal to the lateral aspect of the
knee joint.

Figure 41.10: A. One role of the patella is to lengthen the moment
arm fma 7 J of the quadriceps femoris muscle. B. Removal of the
patella results in a significant reduction of the muscle's moment
arm (ma 2 ), and hence, the moment produced by the muscle.

Palpable Landmarks of the Knee

A thorough examination of the knee depends considerably on
the clinicians ability to palpate and identify many of the indi¬
vidual components of the knee. Unlike most joints, many of the
connective tissue structures associated with the knee also are
directly palpable in addition to the important bony
landmarks. The relevant palpable structures of the
knee are listed below.

• Medial epicondyle of the femur

• Adductor tubercle of the femur

• Lateral epicondyle of the femur

• Tibial plateaus

• Tibial tubercle

• Tubercle of the lateral condyle of the tibia

• Borders of the patella

• Apex of the patella

• Head of the fibula

• Anterior margins of the menisci

• Medial collateral ligament (MCL)

• Lateral collateral ligament (LCL)

ARTICULAR STRUCTURES OF THE KNEE

The knee joint complex consists of the tibiofemoral and
patellofemoral articulations. The proximal tibiofibular joint
has an indirect effect on the knee as it functions to absorb
motion at the foot and ankle. However, since its motions are
best explained in the context of the foot and ankle, it is dis¬
cussed in full in that chapter. Although the tibiofemoral joint
is often described as a hinge joint, it is more precisely a com¬
bination of hinge and pivot joints and is sometimes called a
modified hinge joint [156]. The patellofemoral joint is a
gliding joint. The tibiofemoral and patellofemoral articula¬
tions share the same supporting structures but also exhibit
unique features and motions. The following describes the
functionally relevant characteristics of the articular cartilage,
the menisci, and the noncontractile supporting structures of
the entire knee joint complex.

Organization of the Trabecular Bone
and Articular Cartilage Found in the Knee

Like the bony surfaces of the hip, the architecture of the
bones involved in the knee appears to follow Wolffs Law
[64,65,79,86,122]. The distal femur, proximal tibia, and patel¬
la all demonstrate trabecular bone whose organization is cor¬
related with the forces and stresses applied to each bone. The
organization observed in each bone suggests that the bones
develop according to the forces applied to them and that each
bone is specialized to sustain very large loads.

The knee joint also possesses the thickest articular carti¬
lage found anywhere in the body, even thicker than that
found in the hip joint (Fig. 41.11 ) [1]. Average thicknesses of
between 2 and 3 mm are reported for the patellar and tibial

746

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 41.11: Knee joint cartilage. In a lateral view MRI, the
manually drawn outline of the articular cartilage of the knee
demonstrates the significant thickness of the articular cartilage of
the knee, particularly on the patella and tibia. (Reprinted from Clin
Biomech, Li G, Park SE, DeFrate LE, et al. The cartilage thickness
distribution in the tibiofemoral joint and its correlation with
cartilage-to-cartilage contact, 736-744, 2005; 20: with permission.)

surfaces, with only slightly less on the distal femur [12,107].
Peak thicknesses of approximately 6 mm are reported on the
patella and tibia. The presence of such thick articular carti¬
lage provides further evidence that these articulations sustain
large forces. Thick articular cartilage allows considerable
deformation of the articulating surface as well. The previous
section describes the incongruity between the articular sur¬
faces of the femur and tibia. The curvature of the patella in
the superior and inferior directions is larger than the patellar
surface of the femur. The compliance of the thick articular
cartilage on the patella and the tibia helps improve the con-
gruity between the articulating surfaces of the patellofemoral
and tibiofemoral joints [67,143]. Improved congruence
increases the area of contact and thus reduces the stress
(force/area) applied to the surface. The knee exhibits addi¬
tional specializations that appear designed to help minimize
the stress across the tibiofemoral joint, namely, the menisci.

Menisci

STRUCTURE

The two menisci are fibrocartilaginous discs seated on the
medial and lateral tibial plateaus [63]. The medial meniscus is
larger in diameter than the lateral meniscus, consistent with
the larger medial tibial plateau (Fig. 41.12). The menisci
cover more than 50% of the tibial plateaus, with the lateral
covering a greater percentage of the plateau than the medial
meniscus [12,51]. As a result, there is more direct contact
between the femur and tibia in the medial joint compartment
than in the lateral compartment.

When viewed from above, each meniscus forms part of a
circle, the medial meniscus completes approximately a half

. .w.

Anterior poles

Lateral

meniscus

Figure 41.12: Superior view of the menisci shows that the lateral
meniscus completes most of a circle, while the medial meniscus
forms approximately half a circle. The menisci are attached at
their anterior and posterior poles to the tibia.

circle, while the lateral forms almost a complete circle. The
anterior and posterior ends of the arcs in each meniscus are
known as anterior and posterior poles, or horns. The poles of
the lateral meniscus are close to each other, while the poles
of the medial meniscus are farther apart. Viewed in the
frontal plane, each meniscus is wedge shaped, thicker on its
periphery and thin centrally, creating a concave surface for
the femoral condyles (Fig. 41.13).

Lateral

meniscus

Medial

meniscus

Figure 41.13: In a frontal plane view, the menisci are wedge
shaped, thicker on the periphery, creating a concave surface for
the femoral condyles.

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

747

Although the menisci are frequently described as “washers,”
they are firmly attached to the tibial plateaus. Ligaments bind
both menisci to the tibia at their anterior and posterior horns.
In addition, each meniscus attaches to the joint capsule and to
the periphery of the tibia by coronary ligaments. The medial
meniscus is more firmly attached and also is connected to the
MCL. In contrast, the lateral meniscus has no attachment with
the LCL. Rather it is attached to the tendon of the popliteus
muscle, which may help pull the meniscus posteriorly during
knee flexion [23,119]. The lower mobility of the medial menis¬
cus than that of the lateral meniscus may help explain why it is
more frequently injured than its lateral counterpart [63].

The menisci receive their nutrition through synovial diffu¬
sion and from a blood supply to the horns of the menisci and
the peripheral one quarter to one third of each meniscus [56].
Therefore, lesions along the periphery demonstrate healing
and even regeneration of meniscus-like tissue. The periphery
of the menisci also appears to have sensory innervation that
may extend into the more central portion of the discs [11,119].
The sensory function appears to be mostly proprioceptive.

Clinical Relevance

TREATMENT OF MENISCAL TEARS: A tear in
the avascular central region of a meniscus is unable to
undergo spontaneous healing. Surgical repair in this
region is reported, particularly in young athletes [15,63,160].
Unfortunately, most tears occur in the avascular region of
the meniscus [119]. If a surgical repair is not possible, tears
in the avascular area of a meniscus usually result in partial
meniscectomies, although meniscal transplants may also be
considered.

FUNCTION OF THE MENISCI

Several functions are ascribed to the menisci, including shock
absorption [96,184], knee joint lubrication, and stabilization
[105,119,137,139]. However, the primary function of the
menisci is to increase the contact area between the femur and
tibia, thereby reducing the stress sustained by the articular
cartilage [51,119,144].

Each meniscus is concave on its superior surface but rela¬
tively flattened inferiorly, reflecting the shapes of the femoral
condyle and tibial plateau contacting it. Without the menisci,
contact between the differently curved femoral condyle and
the tibial plateau occurs over a very small area, leading to large
stresses applied to the bones (Fig. 41.14). The addition of a
meniscus between the femoral condyle and tibial plateau
approximately doubles the area of contact between the femur
and tibia [51]. As a result, the menisci significantly reduce the
stress between the femur and tibia. Conversely, removal of a
meniscus increases the stresses applied to the tibial plateau
and femoral condyle [104,135]. The stresses increase as more
meniscal tissue is removed [104].

Figure 41.14: The menisci increase the contact area between the
tibia and femur. Absence of a meniscus decreases the area of
contact between the two bones.

Clinical Relevance

MENISCECTOMY: Meniscal tears or progressive degenera¬
tion is common and can interrupt the normal function of
the knee. A common treatment for a torn meniscus is a
complete or partial meniscectomy. However, concern for the
long-term consequences of meniscus removal remains.
Studies suggest that total removal of a meniscus can lead
to accelerated articular cartilage damage [119]. A 15-year
follow-up study of 146 patients suggests that accelerated
cartilage degeneration is less likely following partial menis¬
cectomies performed arthroscopically [26]. However, other
studies continue to report articular degeneration after even
partial meniscectomies [115,145]. Because the amount of
stress applied to the tibia is related to the amount of menis¬
cal tissue present, these studies suggest a strong link
between the stresses applied to a joint and the potential for
articular degeneration. This link provides a powerful ration¬
ale for identifying treatments that preserve or replace an
injured meniscus [32].

Motion of the Menisci on the Tibia

The complex motion between the femur and tibia applies
similarly complex loads to the menisci lying between the two
long bones. These forces cause the menisci to deform and
to glide on the tibia during knee motion. The motion of the
menisci is consistent with their role as washers between the

748

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 41.15: The menisci glide posteriorly with knee flexion and
anteriorly with knee extension.

Figure 41.16: Medial meniscus tear. A sagittal view MRI of the
medial compartment of the knee reveals a tear of the posterior
horn of the medial meniscus. (From Chew FS, Maldjian C, Leffler
SG. Musculoskeletal Imaging: A Teaching File, Baltimore:
Lippincott Williams & Wilkins; 1999)

two bony surfaces. The menisci move in concert with the
rolling femoral condyles (Fig. 41.15). As the femur rolls poste¬
riorly in knee flexion, the menisci are pushed posteriorly ahead
of the rolling condyles. Similarly they glide anteriorly ahead of
the anteriorly rolling condyles during knee extension [143].
The lateral meniscus moves farther than the medial meniscus
because the latter is stabilized by attachments to the medial
knee joint capsule, collateral ligament, and the tibial plateau by
the coronary ligaments [23,27]. Because the menisci remain
attached at their poles as they slide posteriorly and anteriorly
on the tibia, they also undergo considerable distortion in shape.
This strain may contribute to eventual tears [149].

MENISCAL LESIONS

There are several reasons for the high incidence of meniscal
injury. They are located between the two longest bones of the
body in a large, weight-bearing joint. Compression forces of
several times body weight are reported at the tibiofemoral
joint and must be borne by the menisci. In addition, the glid¬
ing and rotary motion between the femur and tibia applies
large shear forces on the menisci. Finally, the menisci are
attached to the tibial plateaus so that the twisting motion of
the femur and tibia cause large deformations of the fibrocar-
tilages. All of these factors conspire to cause acute tears
and fraying, or fibrillation, of the central border. (Fig. 41.16)
Fragments from large tears can cause predictable mechanical
problems in the knee by becoming dislodged and relocating
in the center of the tibial plateau, interrupting the normal
rolling and gliding motions of the tibia and femur. A classic
complaint of an individual with a meniscal tear is that the joint

“locks,” particularly when he or she attempts to extend the
knee from a position of weight-bearing, such as rising from a
seated position or climbing stairs.

Clinical Relevance

TESTING FOR MENISCAL TEARS: There are several
different clinical tests used to identify a tear in a meniscus.

A goal of many of the tests is to dislodge the torn fragment ,
causing it to interrupt smooth knee motion. A positive test
result frequently consists of producing an audible click or
a mechanical block to motion [114,176].

Noncontractile Supporting Structures

The noncontractile supporting structures of the knee
include the typical supporting structures, the capsule, and
the MCL and LCL. The ACL and PCL provide additional
support, playing a role in supporting and guiding the com¬
plex translatory and rotary motions of the knee. The capsule
also is reinforced posteriorly by additional small ligaments.
Each structure is presented below to understand its effects
on knee joint stability and mobility. However, it is important
to recognize that these supporting structures work in con¬
cert to stabilize the knee. Although each ligament appears to
serve a primary role in stabilizing some movement, other
ligaments provide secondary support [129].

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

749

ARTICULAR CAPSULE OF THE KNEE JOINT

The knee joint capsule is the largest joint capsule of the
human body [62]. In most joints the two primary layers of the
joint, fibrous and synovial, adhere to one another. In the knee,
however, these two layers adhere only in parts of the joint. In
other areas of the knee, the two layers follow different courses
around the joint.

The fibrous capsule is attached posteriorly to the posterior
margins of the femoral and tibial condyles and spans the inter¬
condylar notch (Fig. 41.17). This layer continues medially and
laterally, attached along the borders of the articular surfaces of
the femur and tibia. Anteriorly, the fibrous layer merges with
the tendinous expansions of the vastus medialis and lateralis
and attaches to the margins of the patella. These expansions
are known as the medial and lateral patellar retinaculi. Each
retinaculum is reinforced by patellofemoral and patellotibial
ligaments. The lateral patellar retinaculum also receives rein¬
forcement from the iliotibial band. The capsule and retinaculi
discontinue proximal to the patella, with no anterior attach¬
ment on the femur [156,191].

The synovial layer of the knee joint capsule is larger and
more complex than the fibrous layer. It creates the largest
and most extensible synovial cavity in the body, able to hold

Figure 41.17: The attachments of the fibrous and synovial
capsule separate from one another posteriorly, creating an
extrasynovial space. The fibrous capsule is absent anteriorly,
proximal to the patella.

up to almost a quarter of a cup of fluid without damage
[62,173]. Posteriorly, the synovial capsule attaches to the
articular margins of the femoral and tibial condyles.
However, unlike the fibrous layer, the synovial lining follows
the contours of the condyles and thus invaginates in the
intercondylar notch. As a consequence, the intercondylar
notch and eminence are enclosed by the fibrous capsule but
lie outside the synovial space. The superior portion of the
posterior synovial lining extends proximally slightly beyond
the posterior aspect of the condyles, forming small pouches
proximal to each condyle. It may also expand distally and
laterally as the popliteal tendon bursa.

The synovial lining continues medially and laterally with
the fibrous capsule. Anteriorly, the synovial layer continues
with the fibrous layer and attaches to the borders of the patella.
However, the synovial lining again diverges from the fibrous
layer proximal to the patella. The synovium is attached to the
superior border of the patella and the anterior margins of the
femoral condyles. It then forms a large pocket that extends
proximally a few centimeters between the anterior surface of
the femur and the posterior surface of the quadriceps muscle
(Fig. 41.18). This proximal expansion, known as the suprap¬
atellar pouch, is essential for the full movement of the patella
and thus for full excursion of the knee.

Figure 41.18: The suprapatellar pouch of the synovial capsule is
an expansion of the synovial lining proximally between the
femur and quadriceps femoris muscle.

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Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 41.19: Distal glide of the patella must accompany knee
flexion and requires unfolding of the suprapatellar pouch.

Clinical Relevance

EFFECTS OF LIMITED PATELLOFEMORAL JOINT
MOTION: As the knee flexes , the patella glides to the distal
end of the femur (Fig. 41.19). This distal migration is allowed by
the unfolding of the suprapatellar pouch. Inability of the supra¬
patellar pouch to unfold or the presence of adhesions between
the patella and the femur limits the distal glide of the patella
on the femur. If the patella is prevented from gliding , knee flex¬
ion ROM is limited [180]. The clinician must restore distal glide
of the patella to restore normal knee flexion excursion.

The extensive pouches of the synovial lining complicate
the assessment of joint swelling, or effusion. Excess joint fluid
is sequestered in the suprapatellar pouch when the knee is
extended and moves to the posterior spaces when the knee is
flexed. Assessment of knee joint swelling requires that the
joint fluid be concentrated anteriorly under the patella. This
can be accomplished by positioning the patient with the knee
extended to extrude any joint fluid from the posterior spaces.
Then manual pressure is used to milk the fluid from the
suprapatellar pouch.

Clinical Relevance

KNEE JOINT SWELLING AND FLEXION
CONTRACTURES: The knee joint capsule is most relaxed
with the knee flexed between 75 and 30° [23]. Consequently ,

individuals with knee joint swelling frequently rest with the
knee slightly flexed. This position reduces the tension in
the capsule and increases the patient's comfort. However ;
if slight flexion of the knee is maintained for a prolonged
period , patients are likely to develop knee flexion contrac¬
tures. Consequently , a patient with knee joint swelling must
be instructed to extend the knee fully several times during
the day or to use a resting splint to maintain full knee
extension ROM and to prevent flexion contractures.

The synovial lining of the knee joint is characterized by
multiple folds referred to as plicae [63]. Some of these folds
are large and may become calcified or fibrotic. These folds
may also impinge on the patella or femoral condyle, especially
during motion. Consequently, the plicae, particularly on the
medial side of the joint, may cause knee joint pain producing
the plica syndrome [14,41].

In conclusion, the capsule of the knee joint is large and
complex. It is an essential contributor to the integrity of the
knee. However, it may also contribute to impairments in the
normal movement of the knee.

COLLATERAL LIGAMENTS

There are two collateral ligaments of the knee, the medial
and lateral. These two ligaments provide important rein¬
forcement to the fibrous capsule of the knee joint. The
medial (tibial) collateral ligament is more extensive than
the lateral (fibular) collateral ligament. It forms a broad, flat,
triangular fibrous band covering most of the medial aspect
of the joint (Fig. 41.20). It consists of two parts, an anterior,
more superficial portion and a posterior, deeper portion.
Both segments are attached to the medial femoral
epicondyle. The superficial and deep layers blend together
posteriorly and form the posteromedial joint capsule [155].
The anterior segment is several centimeters long and
extends distally and slightly anteriorly to attach to the medial
surface of the shaft of the tibia. The posterior segment is
shorter and projects distally and posteriorly to attach to the
tibial condyle. The posterior segment also attaches to the
joint capsule and to the medial meniscus. The anterior bor¬
der of the ligament is palpable along the medial joint line
when the knee is flexed.

The LCL is a cordlike structure passing from the lateral
epicondyle to the head of the fibula. The LCL is easily pal¬
pated when a varus (adduction) force is applied to a flexed
knee. Sitting with one foot resting on the opposite knee
applies a varus stress to the knee, making the LCL prominent
(Fig. 41.21).

Several studies have investigated the role of the collateral
ligaments and the effect of knee joint position on their func¬
tion. As described in Chapter 11, a valgus force tends to
abduct the distal segment of a joint, and varus forces tend to
adduct the distal segment. The location of the MCL and LCL
on the medial and lateral sides of the joint makes them

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

751

Figure 41.20: A. The MCL is large, extending distally beyond the
tibial condyle as well as anteriorly and posteriorly. Its deep por¬
tion attaches to the medial meniscus. B. The LCL is a narrow cord
from the lateral epicondyle to the head of the fibula.

well-suited to stabilize the knee against valgus and varus
stresses, respectively (Fig. 41.22). However, other ligaments
also contribute to medial and lateral stability. Therefore, there
continues to be some controversy regarding the relative
importance of the collateral ligaments in supporting the
knee against medial and lateral loads [23,139,167]. Although
a classic anatomical study by Brantigan and Voshell [23] sug¬
gests that there is no significant increase in medial and lateral
instability with the sectioning of the collateral ligaments in
cadaver specimens, more recent studies indicate that the
MCL contributes the primary protection against valgus forces
[139,167]. The superficial portion of the medial collateral
ligament is considerably stronger than the deep portion,
exhibiting almost twice the load to failure, and is composed of
longer fibers [153]. It provides the primary support against

Figure 41.21: The LCL is readily palpated when the knee is flexed
and a varus force is applied. The left knee applies a medial force
to the right tibia, imparting a varus force to the right knee.

Figure 41.22: The MCL and LCL protect against valgus and varus
stresses, respectively.

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Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

valgus stresses from 0° to at least 90° of knee flexion [154].
However, significant support also is provided by the deep
MCL, the PCL and ACL, and by the menisci [71,80,116].

Clinical Relevance

TEARS OF THE DEEP OR SUPERFICIAL PORTION OF
THE MEDIAL COLLATERAL LIGAMENT: The deep MCL is
weaker and consists of shorter fibers than the superficial MCL
Consequently , a vaigus movement of the knee applies a larger
strain (relative change in length) to the deep MCL than to the
superficial MCL. With a lower ultimate strength (load to fail¬
ure), the deep MCL is ruptured more often than the superficial
portion. However, because the superficial portion provides the
majority of the valgus stability to the knee, a patient with a
tear of only the deep portion of the MCL may not exhibit val¬
gus laxity. A patient who does exhibit valgus laxity is likely to
have an extensive injury to the whole MCL.

Although the LCL provides significant support against
varus forces, important support also comes from the
popliteal tendon, both cruciate ligaments, the menisci, and
the iliotibial band [101,139,167]. Regardless of the combina¬
tion of ligamentous support, varus and valgus stability of the
knee in individuals with intact ligaments depends more on
noncontractile tissues rather than muscular support [109].
The collateral ligaments also provide support against medial
and lateral rotation of the tibia. The MCL appears to resist
both medial and lateral rotations [78,167], while the LCL
primarily resists lateral rotations [150,157,167].

The amount of knee flexion influences the roles of the col¬
lateral ligaments [23,43,78,109,139,167]. This is not surpris¬
ing because of the complex structure of the MCL and the
irregularity of the femoral condyles. The more extensive
MCL is variably affected by knee flexion. The posterior portion
of the ligament is stretched more with the knee extended, and
the anterior portion is stretched with the knee flexed
[23,109]. Although both collateral ligaments appear to be
most taut in extension [23,43,89] their relative contribution to
medial-lateral stability increases as the knee flexes until at
least 30° [139,167].

Clinical Relevance

TESTING THE INTEGRITY OF THE COLLATERAL
LIGAMENTS OF THE KNEE: The standard test for
integrity of the MCL and LCL is the manual application of
a valgus and varus force, respectively. The test is frequently
performed with the knee in 15-30° of flexion (Fig. 4L23).
The rationale for the knee flexion stems from studies that
demonstrate that in slight knee flexion, the collateral liga¬
ments are the more important stabilizers in the medial and

Figure 41.23: Stress tests of the MCL and LCL are performed with
the knee slightly flexed. Shown here is the test for the MCL.

lateral directions. Instability medially or laterally with the
knee flexed slightly is more likely to indicate damage to
a collateral ligament. Medial or lateral instability of the
knee with the knee completely extended indicates more-
extensive ligamentous damage and perhaps more gross
articular damage [77].

CRUCIATE LIGAMENTS

The two cruciate ligaments are essential for normal function
of the knee joint and affect both the stability and mobility of
the joint. The ACL attaches to the tibia anterior and just lat¬
eral to the intercondylar eminence. It attaches to the femur
posteriorly on the medial surface of the lateral condyle. The
PCL attaches on the posterior surface of the proximal tibia
posterior to the intercondylar space and to the posterior
aspect of the lateral surface of the medial femoral condyle
[55] (Fig. 41.24). The PCL has a larger cross-sectional area
and is stronger than the ACL [55,178,191]. The two cruciate
ligaments are found in the space between the synovial and
fibrous layers of the knee joint capsule. Therefore, they are
intracapsular but extrasynovial.

The role of the cruciate ligaments has been studied exten¬
sively, and their contributions to knee joint stability are com¬
plex [7,13,16,17,23,43,53-55,71,90,130,133,178,197]. Their
oblique lines of pull and their complex structures complicate
analysis of their functions. Both the ACL and the PCL can be
described as consisting of at least two segments. The ACL is
composed of an anteromedial and a posterolateral bundle
[13,55,178]. Intermediate bundles also are described in the
ACL [71,99,158]. The PCL is composed of multiple bundles,
most commonly described as an anterior, or anterolateral,
bundle and a posterior, or posteromedial, bundle [48,55,85].
Some reports in the literature examine the function of the

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

753

Figure 41.24: The ACL and PCL prevent anterior and posterior
glide, respectively, of the tibia on the femur.

cruciate ligaments as a whole, while others examine individual
segments of the ligaments. This methodological difference
helps explain the disagreements found in the literature about
the functions of these ligaments.

The ACL limits anterior glide of the tibia on the femur
[21,48,143]. However, the degree of anterior laxity resulting
from a disruption of the ACL depends on

• The position of knee flexion or extension at which the laxity
is assessed

• The portion of the ACL disrupted

• The external load applied to the knee

• The integrity of the surrounding tissue

The effect of knee flexion and extension on the tension in
the ACL is well studied, and there is consensus that the ACL
is pulled tightest in extension of the knee [16,17,43,54,55,
90,106,133] (Fig. 41.25). In fact, disruption of the ACL
appears to increase extension or hyperextension ROM [55].

Studies of the restraining role played by discrete portions
of the ACL suggest that tension in both the small antero¬
medial and the larger posterolateral bundles is greatest
when the knee is extended. Similarly, tension in both bun¬
dles decreases as the knee flexes. However, with increasing
flexion, from approximately 30°, tension increases in the
anteromedial bundle [13,16,55,99]. Some authors suggest

that sectioning the posterolateral segment of the ACL pro¬
duces anterior instability when the knee is extended and
that anterior instability with the knee flexed to 90° indicates
a lesion in the anteromedial bundle of the ACL [53,114].
However, studies investigating the tension or load in these
segments in 15° or more of knee flexion report that the
anteromedial bundle sustains substantially larger loads than
does the posterolateral bundle [158,197]. These data produce
an important clinical question: What test is best at identifying
a lesion in the ACL?

Clinical Relevance

ANTERIOR DRAWER TEST AND THE LACHMAN TEST:

Two classic tests for ACL integrity are the anterior drawer
test and the Lachman test (Fig. 41.26). The anterior drawer
test is performed with the patient's knee flexed to 90°. The
examiner attempts to pull the tibia anteriorly on the femur. In
the Lachman test the examiner performs the same maneuver
with the patient's knee flexed to 20°. An in vivo study of 20
young adults reports that the Lachman test produces maxi¬
mum tension through a larger proportion of the entire ACL
ligament than does the anterior drawer test. However ; more
tension occurs in the anteromedial than in the posterolateral
bundle in both tests [158]. Thus while the Lachman test may
stress more of the overall ACL, it may not be any more specific
for the posterolateral bundle of the ACL.

754

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

To clarify the role of the ACL and to improve the clinical
tests to identify lesions of the ACL, its contributions to
rotary stability also have been examined. The ACL is pulled
taut with both medial and lateral rotations of the tibia [13].
Some studies indicate that rotations in both directions
increase with sectioning of the ACL in cadaver specimens
[53,55,167]. Others report only an increase in medial rota¬
tion [178], and still others report no significant increase in
either direction [75]. There also is evidence to suggest that
the ACLs role in stabilizing medial and lateral rotation of
the knee depends upon other stresses placed on the knee.
Many authors suggest that what appears to be instability in
rotation is more accurately described as an anterior sublux¬
ation of either the medial or lateral plateau about a long axis
or a pivot point in the center of the knee. The addition of
varus or valgus stresses as well as compressive loads alters
the ligaments ability to limit rotations, or pivots, about the
long axis of the knee [30,84,103,117]. Clinical tests assessing
both anteromedial and anterolateral subluxations are
described for the ACL [77,114].

Clinical Relevance

PIVOT SHIFT TEST OF THE ANTERIOR CRUCIATE
LIGAMENT: The pivot shift test is a common test for
tears of the ACL. Although there are several versions of the
test, it typically adds a medial rotation torque and valgus
stress to the anterior force of the original drawer sign [114[.
The examiner watches for anterior subluxation of the lateral
tibial plateau with a medial pivot of the tibia.

There appears to be no single, broadly accepted, definitive
physical examination maneuver to establish the integrity of
the ACL. Clinicians are urged to use more than one test to

evaluate the ACL and to include tests that examine com¬
bined, or coupled, motions of the knee [4,77,126,130]. The
sensitivity and specificity of an assessment that uses multiple
tests are greater than those for individual tests [172].
Continued study using improved three-dimensional motion
analysis and imaging techniques is needed to understand fully
the complex role of the ACL.

Like the ACL, the PCL has a complex role in stabilizing
the knee and contributes to stability in several directions.
The PCL limits posterior glide of the tibia on the femur
[23,55]. Although the PCL appears to be taut when the knee
is extended [54,55,148], studies repeatedly demonstrate that
knee flexion increases the tension in the PCL [3,28,37,48]
(Fig. 41.27). As in the ACL, the position of the knee in the
sagittal plane appears to affect the anterior and posterior seg¬
ments of the PCL slightly differently [3,16,48,178].

The PCL also contributes to varus, valgus, and rotational
stability [34,71,80,116,127]. Like the ACL, the PCL is likely
to contribute to both medial and lateral rotational stability of
the knee, depending on knee position [34,198]. Its role in
stabilizing all motions appears coupled with the surrounding
ligaments, particularly the MCL and LCL [23,123,127,150,
181,183]. Thus the clinician again must use a combination of
test movements to ascertain the integrity of the PCL. In
conclusion, it is clear that the cruciate ligaments play a crit¬
ical role in stabilizing the knee in multiple directions. In
addition, it appears that each cruciate ligament is composed
of multiple fiber bundles that make slightly different contri¬
butions to the function of the whole ligament [3,34,106].

ACCESSORY LIGAMENTS OF THE KNEE

Although the collateral and cruciate ligaments are the pri¬
mary connective tissue supports in the knee, other smaller lig¬
aments provide some additional support. These are found on
the posterior and lateral aspects of the knee. The oblique

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

755

popliteal and arcuate popliteal ligaments attach to the lat¬
eral joint capsule and are closely associated with the
popliteal tendon [191]. They reinforce the LCL, providing
additional posterolateral support [127,139,167]. Smaller
meniscofemoral ligaments are reported but are inconsis¬
tently present. Cadaver data suggest that over 90% of
knees contain at least one meniscofemoral ligament
[101,124]. Their contributions to overall joint stability
remain controversial but they may provide secondary rein¬
forcement to the PCL [5,55,150]. They may also assist and
control the motion of the lateral meniscus [61,124].

CONCLUSIONS REGARDING THE CONNECTIVE
TISSUE SUPPORT OF THE KNEE

The roles of the collateral and cruciate ligaments are complex
and interdependent [136]. The following generalizations are
useful in explaining their functions:

• Although the collateral ligaments provide the primary sup¬
port to control mediolateral stability of the knee joint, the
cruciate ligaments supply important secondary support.

• Similarly, the cruciate ligaments stabilize the knee in the
anterior and posterior directions but are reinforced by the
collateral ligaments.

• Rotary stability is provided by all of the cruciate and col¬
lateral ligaments.

• The integrity of the menisci and the articular surfaces also
directly affects the stability of the knee joint.

Correct diagnosis of ligamentous injuries is essential to
optimizing the function of the knee and to limiting the
chances of future joint deterioration resulting from the
altered mechanics of a ligament-deficient knee. The func¬
tional consequences of injury to any of the primary ligaments
of the knee are intimately related to the integrity of the sur¬
rounding ligamentous structures.

Clinical Relevance

INJURIES TO THE PRIMARY LIGAMENTS OF THE

KNEE: An obvious consequence of tears to any of the col¬
lateral or cruciate ligaments is instability of the knee in multi¬
ple directions. However ; an additional and perhaps even more
troubling impairment is documented with injuries to any of
these ligaments. The cruciate ligamentsparticularly , contribute
to the complex rotational and translational movements of the
knee that occur with knee flexion and extension [52]. Although
these complex motions of the knee are primarily the result
of the shapes of the articulating surfaces, studies demonstrate
that the loss of either the ACL or PCL can alter the normal
mechanics of the knee during passive and active flexion and
extension of the knee [85,87,98,117,168,192]. In addition, ACL
deficiencies alter the walking patterns and postural control of
many individuals [8,10,22,40,74,113,161,182]. Some individu¬
als change their activities and lifestyles after ligamentous
injuries at the knee [60].

Altered motion can lead to abnormal loading of the
articular surfaces of the knee joint and perhaps to accelerat¬
ed joint degeneration [147]. Increased degenerative changes
are reported in the articular cartilage and menisci of individ¬
uals with injuries to the cruciate or collateral ligaments com¬
pared with those of healthy controls [83,112]. Degenerative
changes appear related to the amount of ligamentous
damage [111]. Consequently, treatments are designed to
restore normal joint mechanics through the use of muscular
control, orthotic devices, or joint reconstruction [131,171].

NORMAL ALIGNMENT OF THE KNEE JOINT

Alignment of the knee is affected by the alignment of the hip,
ankle, and foot. This interaction is the result of the knees loca¬
tion between the ground on which the subject stands and the
superimposed weight of the head, arms, trunk, and opposite
lower extremity (HAT-L weight). Malalignment of the knee
can result from malalignment of the hip, ankle, or foot joints,
from muscle imbalances, and from abnormal loads on the
knee joint [36,118]. Conversely, there is evidence that knee
joint deformities cause abnormal stresses on the joint and can
lead to joint degeneration [9,92]. Accurate identification of
malalignments of the knee and associated deformities of adja¬
cent joints is an essential part of a thorough musculoskeletal
evaluation.

756

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Frontal Plane Alignment

The unique angulation of the knee joint in the frontal plane is
considered a hallmark of bipedal ambulation [142,166]. As
noted earlier in this chapter, the medial femoral condyle
extends farther distally than the lateral femoral condyle.
However, in the normally-aligned joint, the distal surfaces of
the two condyles lie on the same horizontal plane.
Consequently, the shaft of the femur projects laterally from
the vertical, putting the knees and feet closer together than
the hip joints in normal erect standing. Frontal plane align¬
ment is described by the terms varus and valgus. Valgus is
the alignment in which the angle between the proximal and
distal segments opens laterally. In varus alignment, the angle
opens medially (Fig. 41.28).

The exact value of valgus or varus of the knee depends on
the method of measurement. The measure can be made using
the anatomical or the mechanical axes of the knee
(Fig. 41.29). The anatomical method uses the long axes of the
femur and the tibia. The mechanical method uses the
mechanical axes of the lower extremity. A radiological study of
120 adults reports approximately 5° of valgus using the
anatomical axes [31]. However, values of up to 10° of valgus
are reported in individuals without knee pathology [86]. Using
the anatomical axes, varus alignment is abnormal in adults and
is often associated with degenerative joint disease [9].
However, using the mechanical axes, the normal alignment of
the knee is in approximately 2° of varus [31,76]. Location of
the mechanical axes requires radiographic assessment.

Figure 41.28: A. In varus alignment of the knee, the angle
formed by lines through the femur and tibia opens medially.
B. In valgus alignment of the knee, the angle formed by lines
through the femur and tibia opens laterally.

Figure 41.29: The anatomical axis of the knee is projected
through the shafts of the femur and tibia. The mechanical axis
projects through the centers of the hip, knee, and ankle joints.

Therefore, normal frontal plane alignment measured in a
physical examination is slight valgus.

Newborns and young children normally exhibit genu
varum [93]. This varus alignment disappears and is replaced
by a valgus alignment that reaches a peak of about 12° by the
age of 3 years. There is a gradual reduction of valgus, which
finally plateaus at the adult values by the time the child is 6 or
7 years of age. Normal valgus alignment of the knee results in
a narrower base of support during stance, requiring less lat¬
eral shift to keep the body’s center of mass over its base of
support during single-limb stance and gait [166] (Fig. 41.30).

Sagittal Plane Alignment

Normal erect standing posture of the knee in the sagittal
plane consists of a vertically aligned femur and tibia, together
forming a 180° angle. However, hyperextension of the knee in
standing can occur, and the associated postural alignment is
known as genu recurvatum (Fig. 41.31). It frequently
results from muscle imbalances at the ankle or knee. Such a
posture applies increased stress to the posterior joint capsule
of the knee and to the ACL [110].

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

757

Figure 41.30: Valgus of the knee allows the feet to be closer
together in standing, narrowing the base of support and requir¬
ing less lateral shift to stand on one leg.

Transverse Plane Alignment

In a knee free from joint pathology, the tibial plateaus and
femoral condyles are aligned so that with the knee extend¬
ed, the transverse axes of the proximal tibia and distal femur
are parallel [42]. This is described as 0° of version of the

Figure 41.31: Genu recurvatum is sagittal plane alignment of the
knee in the hyperextended position.

Figure 41.32: A. A view of the normally aligned knee in the
transverse plane reveals that the femoral and tibial condyles are
aligned together along the medial-lateral axis of the knee. B. In
lateral version of the knee, the tibial condyles are rotated later¬
ally with respect to the femoral condyles.

knee (Fig. 41.32). In individuals with osteoarthritis of the
knee, 5° of lateral tibial version with respect to the femur
is reported.

ALIGNMENT OF THE PATELLOFEMORAL
JOINT

As noted above, malalignment of the tibiofemoral joint may
be the manifestation of altered mechanics at joints proximal
and distal to the knee. It may also indicate abnormal stresses
on the joint. Finally, malalignment may signify the presence
of damaging loads that may precipitate or continue destruc¬
tion of the joint. Malalignments of the patellofemoral joint
may indicate similar scenarios. Consequently, it is essential to
recognize the abnormal position of the patella with respect to
the femur, to understand and affect the underlying pathome-
chanics of patellofemoral joint pain.

Alignment of the patella on the femur typically is
described in terms of a medial-lateral alignment and a
proximal-distal position. These positions are altered by
translation of the patella on the femur. In addition, the
position of the patella is described in terms of rotations.
Several angular orientations are reported in the literature.
Some of the most common are described here. Patellar
tilt describes a rotation about a superior-inferior axis.
Additional measures of patellofemoral alignment include
the sulcus angle and the congruence angle. Although
patellar translation and patellar tilt are assessed visually
in the clinical setting, such observations are reportedly
unreliable [140,177,189]. However, assessment of these
measures using a variety of radiographic or magnetic reso¬
nance imaging (MRI) techniques yields clinically useful
information. The descriptions of the patellofemoral joint
alignment described below are based on data obtained
from radiographic and MRI studies.

758

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Medial-Lateral Alignment

In general, the clinical perception is that in the normal
patellofemoral joint, the patella lies centered in the trochlear
notch, equidistant from the medial and lateral epicondyles
[140]. However, reports suggest that slight lateral deviation of
the patella as seen with imaging techniques is normal [59,97].
This deviation is no more than a few millimeters and may be
less when viewed by MRI [58]. Excessive deviation medially
or, more commonly, laterally is known as medial or lateral
tracking. Excessive lateral tracking is associated with chon¬
dromalacia patellae [165] and patellofemoral pain [18,196].
These disorders may be the result of abnormal stresses on the
patella resulting from the malalignment.

Proximal-Distal Alignment

The proximal-distal position of the patella is described by
a ratio of the distance between the patella and the tibia to
the length of the patella [2,82,128] (Fig. 41.33). The exact
distances used vary among specific methods. However, an
increase in the ratio, indicating an increase in the distance
between the tibia and the patella is noted as patella alta.
Patellar baja (infera) describes a decrease in the distance
between patella and tibia. Both patella alta and baja are
associated with anterior knee pain and, like abnormal patellar
tracking, are likely to result in abnormal loading of the

Figure 41.33: The proximal-distal alignment of the patella is
described by a ratio of the length of the patella (a) and the
distance between the distal patella and the tibial tubercle (b).

patellar articular surface [47,73,120,169]. Individuals with
patella alta exhibit decreased contact area and increased
stress of the patellofemoral joint during fast speed walking
[188]. Elevated stress may contribute to pain and degenera¬
tive changes in the patellofemoral joint.

Angular Positioning of the Patella

PATELLAR TILT

Patellar tilt is the angle formed by a line drawn through the
largest width of the patella and a line touching the most
anterior surfaces of the medial and lateral femoral condyles
(Fig. 41.34). Studies using MRI and computed tomography
(CT) suggest that with the knee extended, the patella is
positioned in slight lateral tilt [24,138,141].

SULCUS ANGLE

The sulcus angle is the angle formed by lines drawn from
the deepest point of the femoral sulcus to the highest point
on each condyle. The location on the femur at which the
measurement is made alters the angle, indicating that the
depth of the sulcus varies over the femoral surface [97].
Reported means vary from approximately 125° to 155°, with
no significant difference found between men and women
[2,97,174].

Figure 41.34: Patellar tilt is the angle formed by a line drawn
through the largest width of the patella and a line touching
the most anterior surfaces of the medial and lateral femoral
condyles. The sulcus angle is the angle formed by lines drawn
from the deepest point of the femoral sulcus to the highest
point on each condyle. The congruence angle is formed by
a line bisecting the sulcus angle and another line that
projects from the apex of the sulcus angle through the
lowest point on the patellar ridge.

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

759

CONGRUENCE ANGLE

The congruence angle is a measure of how well the patella fits
into the trochlear notch of the femur. It is formed by a line
bisecting the sulcus angle and another line that projects from
the apex of the sulcus angle through the peak of the patellar
ridge. A wide variation in the congruence angle is reported in
the literature, from an average of 8° ± 6°, larger in women
than in men [2], to means of approximately 14° to 18°
[138,196]. Further research establishing normative values for
the congruence angle is needed to understand its relationship
to patellofemoral joint pathology.

Clinical Relevance

ASSOCIATIONS BETWEEN PATELLAR MALALIGNMENTS
AND JOINT PATHOLOGY AND PAIN: It is commonly
believed that pathomechanics contributes to the complaints
of pain and dysfunction at the patellofemoral joint. One man¬
ifestation of abnormal mechanics at the patellofemoral joint is
malalignment. A decrease in patellar tilt is reported in a small
sample of individuals with anterior knee pain [196]. An asso¬
ciation between an increase in the congruence angle and
anterior knee pain also is reported [196]. Patellar alignment
also appears to predict lateral patellar subluxations. Escala et
al. report odds ratios of 8.7 and 4.5 for increased patellar tilt
and patella alta respectively [45]. These data can be interpreted
to mean that an individual with increased patellar tilt is 8.7
times as likely to have a lateral subluxation of the patella , and
an individual with patella alta is 4.5 times as likely to sublux
laterally as individuals without these malalignments. This
increased risk may be due to the decreased and laterally dis¬
placed contact area between femur and patella found in indi¬
viduals with lateral subluxation [69].

All of these associations support the notion that abnormal
patellofemoral alignment is involved in patellofemoral joint dis¬
orders. However ; the exact nature of the relationships remains
unclear. Surgical corrections of malalignments are accepted
treatment approaches. However ; quadriceps strength also is a
recognized factor influencing patellar alignment and
patellofemoral joint pain. Although malalignments are impor¬
tant clinical findings; clinicians are cautioned to consider them
within the context of the entire clinical picture.

MOTION OF THE KNEE

The motion of the whole knee joint complex is characterized
primarily by the flexion and extension of the tibiofemoral
joint. However, this apparently simple knee motion involves
complex three-dimensional motion of the tibiofemoral joint.
In addition, normal knee motion depends upon the motion of
the patellofemoral joint.

Normal Range of Motion of the Knee
in the Sagittal Plane

The normal ROM of the knee reported in the literature is
presented in Table 41.2. All ranges are passive except those
reported by Roach and Miles [151]. Although reports of
hyperextension of the knee are found in the literature, the
data presented here demonstrate that significant hyperexten¬
sion in adult subjects without knee pathology is uncommon.
However, hyperextension occurs commonly in young chil¬
dren, then gradually disappears in adolescence [57]. In adults,
age and gender appear to have little effect on knee ROM
[151,185], but obesity is negatively associated with knee flex¬
ion ROM [46].

Studies report that knee excursions during gait range from
almost complete extension (approximately 1° in midstance)
to 65-75° in midswing [125,194]. However, many com¬
mon activities of daily living require more knee flex¬
ion. Stair ascent and descent use between 90 and 110°
of flexion [159,182], rising from a chair requires approximately
90° [25], getting in and out of a bath tub requires approxi¬
mately 130° [140], and squatting can use up to 165° [159].

Clinical Relevance

KNEE ROM IMPAIRMENTS: There are many disorders
that can lead to reduced knee flexion ROM. The functional
significance of such limitations varies in individual patients.
Data suggest that only large reductions of flexion ROM will
directly affect locomotor patterns. Yet even a slight loss in
flexion excursion may have profound repercussions in an
individual who must squat or knee1, such as a carpet layer.
The clinician must consider the patient's flexibility within the
context of the patient's own lifestyle and career require¬
ments to grasp the significance of altered knee flexion ROM.

In conclusion, malalignments of the tibiofemoral and
patellofemoral joints are associated with a number of disorders
of the knee joint complex. Further research is needed to define
these links clearly enough to establish optimal therapeutic
interventions and perhaps to identify effective preventive
strategies. An appreciation of the normal alignment of the
individual components of the knee joint also is essential to
understanding knee motion.

Transverse and Frontal Plane Rotations
of the Knee

The discussion throughout this chapter clearly indicates that
the knee joint allows, actually requires, medial and lateral
rotation and abduction and adduction. However, there are
limited and varied data describing the normal available ROM
in subjects without knee pathology. The challenge in estab¬
lishing normative values of transverse and frontal plane

760

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

TABLE 41.2: Normal ROM of the Knee Reported in the Literature

Boone and

Azen [20] a

Walker et al.

[185] b

Roach and

Miles [151 ] c

Roass and

Andersson [152] d

Escalante, et al. e [46]

Flexion

141.2 ± 5.3

133 ± 6

132 ± 10

144 ± 6.5

137 ± 15

Hyperextension f

-1.1 ± 2.0

-1.0 ± 2

HR g

-1.6 ± 2.9

a Based on 56 male subjects aged 20 to 54 years.
fa Based on 30 men and 30 women aged 60 to 84 years.
c Based on 1683 men and women aged 25 to 74 years.
d Based on 180 knees from 90 male subjects aged 30 to 40 years.
e Based on 687 men and women aged 64 to 79 years.

^Negative numbers indicate that the mean end range is in flexion rather than hyperextension.
9 NR: not reported

motion of the knee stems from the technical difficulty of
quantifying three-dimensional motion of such small excur¬
sions. In addition, these motions are significantly smaller than
flexion and extension but are directly influenced by the posi¬
tion of the knee in the sagittal plane [6,23,85,121].
Consequently, only a few studies measure knee motion in all
three planes, and these few studies use very different tech¬
niques. Some studies assess these motions during walking,
while others assess them actively or passively with the sub¬
jects at rest. Only one known study describes the loads used
to rotate the knee. Therefore, the data provide, at best, a per¬
spective for the clinician to appreciate the relative mobility of
the knee.

Despite the limited data it is useful for the clinician to have
a general concept of the potential flexibility of the knee in
these planes. Reported mean values of total medial and later¬
al rotation excursion vary from 12° to 80° when the knee is
flexed [6,21,23,121,181]. Despite this wide variation in
reported means, studies consistently demonstrate a signifi¬
cant decrease in total rotation excursion when the knee is
extended [6,23,121,181]. Peak medial and lateral rotation also
are approximately equal to one another [121]. However, stud¬
ies suggest that much less rotation occurs during normal loco¬
motion. Reported total rotations during locomotion range
from approximately 8° to 15°, with medial rotation occurring
in stance and lateral rotation occurring in the swing phase of
gait [91,102,175].

Reports of frontal plane motion of the knee have the same
limitations as those reports of rotation. However, the reports
of frontal plane motion consistently demonstrate less motion
than reported for transverse plane motion. Reports range
from approximately 10 to 20° [21,121]. Reported abduction
and adduction excursion in gait is even less, approximately 5°
[91,102].

Patellofemoral Motion

Proper motion of the patellofemoral joint is critical to the nor¬
mal function of the tibiofemoral joint. However, the
patellofemoral joint is itself subject to pathology. Abnormal
movement of the patella during knee flexion and extension is
considered by many to be an important contributing factor to

patellofemoral disorders [24]. The patella glides distally on
the femur during flexion and recoils proximally during knee
joint extension. However, its motion during knee flexion and
extension is considerably more complex than mere proximal
and distal glides. It is important for the clinician to recognize
normal and abnormal patellofemoral motion to understand
and alter the underlying pathomechanics of knee pain.

When the knee is extended, the patella has only slight con¬
tact with the femur [164]. As a result it is freely movable.
Although not well studied, the patella appears able to move a
few centimeters medially, laterally, and distally in full knee
extension with the quadriceps relaxed. Some suggest that the
patella should move no more than one half its width in the
medial and lateral directions [29]. Medial and lateral transla¬
tions of the patella with the knee extended are limited by the
pull of the retinacular ligaments [39,66]. As flexion begins,
the patella slides into the femoral trochlea, and the bony
contact greatly reduces its mobility. Although not thoroughly
studied, more investigations describe the motion of the patella
during knee flexion. The motion consists of both translation
and rotation, which are presented separately below.

TRANSLATION OF THE PATELLA DURING
KNEE FLEXION

Translations of the patella during flexion and extension of the
knee occur in both proximal-distal and medial-lateral direc¬
tions. The distal glide of the patella during knee flexion is
reportedly 5-7 cm, allowed by the unfolding of the suprap¬
atellar pouch [67]. There also is a slight medial translation of
the patella at the beginning of flexion [59,94,108,141]. The
magnitude and duration of this medial translation remains in
dispute. However, there appears to be general agreement
that by 30° of knee flexion the patella has begun lat¬
eral translation that continues to increase at least until
45° of flexion, when it plateaus.

ROTATION OF THE PATELLA DURING
KNEE FLEXION

Rotations of the patella during knee motion include medial
or lateral tilt, which is defined as it is for the alignment of
the patella. The patella also rotates about a medial-lateral

Chapter 41 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE KNEE

761

Figure 41.35: The patella is capable of three-dimensional motion
with translation along, and rotation about, the cardinal axes.

axis (Fig. 41.35). This motion is termed flexion and exten¬
sion of the patella. Finally, the patella undergoes medial
and lateral rotation about an anterior-posterior axis [94].
These motions are inadequately studied; however, there are
some consistencies in the literature. The patella appears to
be in a slight lateral tilt that increases slightly as the knee
flexes [94,141]. The patella also appears to undergo flexion
defined as the inferior pole tipping toward the tibia
[70,94,108]. Rotations of the patella about the anterior-
posterior axis appear negligible [94,108].

These data reveal that the patella undergoes small but
systematic movements during knee flexion and extension.
The movement of the patella produces significant changes
in the location and total area of contact between the patella
and the femur. The changes in contact location and area of
contact alter the stress applied to the joint surface.

Femoral contact on the patella occurs on the lateral
facet at the inferior pole of the patella when the knee is in
complete extension. The area of contact steadily increases
and includes the medial facet as the knee flexes, moving
proximally on the patella [59,67,69,164] (Fig. 41.36). The
patella contacts the femur on the femoral condyles and
patellar surface. Later in flexion, the patella contacts only
the femoral condyles, and the odd facet (the medial seg¬
ment of the patella’s medial facet) contacts the lateral
aspect of the medial femoral condyle [67].

The large changes in contact location and area between
the femur and patella produce large changes in stress (force/
area). It is believed that abnormal stresses contribute to
patellofemoral joint dysfunction.

Figure 41.36: The area of contact on the patella increases and
moves proximally as the knee flexes to approximately 90°.

Clinical Relevance

PATELLOFEMORAL CONTACT AREA WITH PATELLAR
SUBLUXATION: A study of eight individuals with a history
of patellar subluxation revealed a significant decrease in
contact area between the femur and patella [69]. Perhaps
more importantly , the contact area was shifted almost
entirely to the lateral femoral condyle during knee flexion
from 0-90°. Such changes in patellar movement patterns
are likely to produce abnormal stresses and lead to pain
and degenerative changes. Careful observation of patellar
motion and close evaluation of the structures affecting
patellar mobility are essential ingredients to a thorough
assessment of the patellofemoral joint.

SUMMARY

This chapter describes the structure of the bones composing
the knee joint. The complex and irregular shapes of these
bones are the primary explanation for the complex pattern
of movement at the tibiofemoral and patellofemoral articula¬
tions during knee flexion and extension. The soft tissue
supporting structures of the knee are the primary source of
stability of the knee along with the muscles that are presented
in the following chapter. The MCL and LCL contribute the
primary medial and lateral support, and the cruciate
ligaments provide stability in the anterior-posterior direc¬
tions. However, all four ligaments participate in concert to
offer three-dimensional stability.

This chapter also provides a detailed description of the
complex motions occurring at the tibiofemoral and patello¬
femoral joints during flexion and extension of the knee.
Although the knee is often described as a hinge joint, its

762

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

motion includes translations and rotations about the three axes
of the body. Flexion occurs with lateral rotation and abduction
of the femur with respect to the tibia as well as some transla¬
tion. Extension reverses these motions. Thus the knee actually
exhibits six DOF in its movements. The importance of restor¬
ing mobility in all six directions also is noted. The patella also
exhibits three-dimensional movements that are essential com¬
ponents of normal knee flexion and extension.

The complexity of the knees motion puts an enormous
burden on the connective tissue supporting structures of the
knee to provide sufficient stability even when the knee partic¬
ipates in vigorous activities such as running and twisting. The
importance of the muscles surrounding the knee to stabilize
and mobilize the knee cannot be understated. The following
chapter presents the muscles of the knee and describes their
participation in the movement and stability of the knee joint.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity at the Knee

CHAPTER CONTENTS

EXTENSORS OF THE KNEE.768

Rectus Femoris.768

Vastus Intermedius .770

Vastus Lateralis .771

Vastus Medialis .771

Functional Considerations for the Quadriceps Femoris Muscle.774

FLEXORS OF THE KNEE .775

Hamstrings.776

Mechanics of Two-Joint Muscles at the Knee.779

Popliteus .780

Functional Implications of Flexion Contractures of the Knee.780

MEDIAL ROTATORS OF THE KNEE.780

Sartorius.780

Gracilis.782

Pes Anserinus.782

LATERAL ROTATORS OF THE KNEE.783

Tensor Fasciae Latae.783

STRENGTH OF THE FLEXOR AND EXTENSOR MUSCLES OF THE KNEE.785

Comparisons between Extension and Flexion Strength at the Knee .786

Factors Influencing Muscle Strength at the Knee .786

Effects of Joint Position on Muscle Strength at the Knee.786

SUMMARY .787

T he preceding chapter describes the bony architecture of the knee joint and the connective tissue structures
supporting the knee. The present chapter discusses the muscles that move the knee. It is important to recog¬
nize that these muscles also provide substantial stabilization, reinforcing the supporting role of the various
ligaments. Thus the function and dysfunction of the muscles affect both the mobility and stability of the knee.

The present chapter focuses on the role of these muscles at the knee. However, the vast majority of these muscles cross
the hip joint and play an important role at the hip as well. Therefore, the current chapter presents the functions of the
muscles that cross the knee as they relate to both the knee and the hip. The specific purposes of this chapter are to

767

768

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

■ Explain the normal function of the muscles that cross the knee

■ Explore the functional effects of strength and flexibility impairments in these muscles

■ Consider the contributions made by these muscles to the pathomechanics of the knee joint

■ Describe the relative strengths of these muscle groups

Although there are a variety of ways to classify the muscles crossing the knee, including their location or their innerva¬
tion, for the purposes of this chapter, the muscles are organized by their actions at the knee. Thus they are grouped as
extensors, flexors, medial rotators, and lateral rotators. Each group is presented separately. However, it is important to
remain aware that many of the muscles of one group also contribute to the actions of another group.

EXTENSORS OF THE KNEE

Rectus Femoris

The quadriceps femoris muscle represents the primary knee
extensor, although the tensor fasciae latae also contributes to
knee extension (Fig. 42.1). The quadriceps femoris is com¬
posed of four separate heads that are discussed individually
below.

The rectus femoris is the only head of the quadriceps group
that is a two-joint muscle, crossing both the hip and the knee
joint (Muscle Attachment Box 42.1). It is a bipennate muscle.
It also is one of two heads of the quadriceps located centrally
on the anterior thigh.

Figure 42.1: The quadriceps femoris with its four heads—the
rectus femoris, vastus intermedius, vastus lateralis and vastus
medialis—is the primary extensor of the knee, but the tensor
fasciae latae also extends the knee.

ACTIONS

MUSCLE ACTION: RECTUS FEMORIS

Action

Evidence

Knee extension

Supporting

Hip flexion

Supporting

Hip lateral rotation

Supporting

Hip abduction

Supporting

There is no doubt that the rectus femoris contributes to knee
extension and hip flexion [5,53]. However, it is important to
understand the circumstances under which the rectus
femoris contributes to these motions. The rectus femoris is

MUSCLE ATTACHMENT BOX 42.1

ATTACHMENTS AND INNERVATION
OF THE RECTUS FEMORIS

Proximal attachment: Anterior inferior iliac spine and
a groove on the ilium, superior to the acetabulum

Distal attachment: The aponeurosis of the quadri¬
ceps attaching to the superior border of the patella

Innervation: Femoral nerve, L2-L4

Palpation: The muscle can be palpated proximally
between the tendons of the sartorius and tensor
fasciae latae and distally between the vastus medi¬
alis and vastus lateralis. Subcutaneous fat may make
these palpations difficult.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

769

active in knee extension with the hip flexed or extended
[23,27,113]. However, studies demonstrate that it is more
active during a straight leg raise than when performing an iso¬
metric contraction of the quadriceps muscle as a whole in the
supine position [107,108]. Electromyographic (EMG) data
suggest that the muscle participates in active pure hip flexion
only in the middle and end of the flexion range of motion
(ROM). However, its activity increases with both lateral rota¬
tion and abduction of the hip joint [16]. Biomechanical analy¬
sis reveals that the rectus femoris possesses a significant
abduction moment arm at the hip [72]. These findings have
important implications for the clinician.

Clinical Relevance

EXERCISING THE QUADRICEPS FEMORIS MUSCLE:

A very common exercise to strengthen, or prevent weakness
of, the quadriceps femoris muscle is to perform an isometric
contraction or to "set" the muscle. The subject is positioned
with the knee extended and is instructed to contract the
muscle on the anterior surface of the thigh. Data suggest
that this exercise is less effective in recruiting the rectus
femoris portion of the quadriceps femoris. The clinician must
consider additional exercises using hip positions to recruit
the rectus femoris optimally.

EFFECTS OF WEAKNESS

Although isolated weakness of the rectus femoris is unusual,
it causes a reduction in knee extension strength as well as
some decrease in hip flexion strength. The physiological
cross-sectional area of the rectus femoris is approximately

15% of the total quadriceps femoris muscle mass [28,118].
Direct electrical stimulation of the rectus femoris suggests
that the muscle produces approximately 20-25% of the total
extensor torque, during submaximal contractions [128]. Thus
the loss of rectus femoris strength alone may produce up to a
25% loss in extension strength.

EFFECTS OF TIGHTNESS

Unlike weakness of the rectus femoris, isolated tightness of
the rectus femoris is common, since the position to stretch
the muscle (hip extension with knee flexion) is an uncom¬
mon position. Tightness of the rectus femoris limits ROM in
the combined movements of knee flexion and hip extension.
Identification of tightness of the rectus femoris requires
examination of knee flexion mobility with simultaneous hip
extension. Hip flexion puts the rectus femoris on slack,
allowing more knee flexion ROM (Fig. 42.2). The role of the
rectus femoris in hip abduction and lateral rotation can
make assessment of tightness of the rectus femoris difficult.
Hip position must be standardized to obtain repeatable
measures.

Clinical Relevance

ASSESSING TIGHTNESS OF THE RECTUS FEMORIS:

Tightness of the rectus femoris is assessed typically with the
subject prone to extend the hip. The knee is flexed until ten¬
sion is felt in the anterior thigh from the stretch of the rectus
femoris. However, inadvertent abduction of the hip slackens
the muscle and may mask the muscle tightness (Fig. 42.3).
The clinician must take care to maintain the limb in the
sagittal plane while extending the hip and flexing the knee.

Figure 42.2: A. Tightness of the rectus femoris is assessed by stretching the muscle with combined hip extension and knee flexion.
B. Allowing the hip to flex puts the rectus femoris on slack and permits full knee-joint flexion.

770

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 42.3: A. The rectus femoris is stretched by combining hip
extension and knee flexion while maintaining the hip in neutral
abduction. B. Allowing hip abduction puts the rectus femoris on
slack and allows more knee flexion.

MUSCLE ATTACHMENT BOX 42.2

ATTACHMENTS AND INNERVATION
OF THE VASTUS INTERMEDIUS

Proximal attachment: The anterior and lateral sur¬
faces of the upper two thirds of the femoral shaft.
The articularis genu arises from the anterior surface
of the lower shaft of the femur.

Distal attachment: The deep portion of the aponeu¬
rosis of the quadriceps attaching to the lateral bor¬
der of the patella and the lateral tibial condyle. The
articularis genu inserts into the suprapatellar pouch.

Innervation: Femoral nerve, L2-L4

Palpation: Not palpable in the normal thigh.

Vastus Intermedius

The other centrally positioned quadriceps muscle is the vas¬
tus intermedius. It is a unipennate muscle and is the deepest
muscle of the quadriceps group [28] (Muscle Attachment Box
42.2). It is not palpable in the normal thigh.

ACTIONS

MUSCLE ACTION: VASTUS INTERMEDIUS

Action

Evidence

Knee extension

Supporting

Like the other vasti, the undisputed action of the vastus inter¬
medius is knee extension. EMG studies repeatedly demon¬
strate activity of the vastus intermedius during knee extension
throughout the full excursion of extension. The deepest part
of the vastus intermedius is associated with another muscle,
the articularis genu. These muscles may be distinct from
one another or blended together [120]. The articularis genu
attaches with or without the vastus intermedius to the supra¬
patellar pouch. Its role is to pull the pouch proximally during
knee extension, thus preventing impingement of the pouch in
the patellofemoral joint.

EFFECTS OF WEAKNESS

Weakness of the vastus intermedius by itself is unlikely
However, in some individuals it represents a substantial pro¬
portion of the entire quadriceps femoris muscle. Estimates of
the percentage of the quadriceps femoris formed by the vas¬
tus intermedius based on physiological cross-sectional area
range from approximately 15 to 40% of the total muscle bulk
[2,8,118]. During direct electrical stimulation in submaximal
contractions, the vastus intermedius produces approximately
40-50% of the total extensor torque [128]. Thus weakness of
the vastus intermedius results in a substantial decrease in
knee extension strength.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

771

EFFECTS OF TIGHTNESS

Tightness of the vastus intermedius, alone, also is unlikely.
However, tightness of the vasti together contributes to
decreased knee flexion ROM. It is important to recognize
that tightness of the three vasti muscles that cross only the
knee results in diminished knee flexion ROM regardless of
the position of the hip, unlike the rectus femoris, whose tight¬
ness is noted when the hip is extended.

Vastus Lateralis

The vastus lateralis is a large, pennate muscle (Muscle
Attachment Box 42.3). In many individuals it is the largest of
the quadriceps femoris muscles [28,35].

submaximal contractions produced contributions similar to
the rectus femoris, approximately 20-25% of the total exten¬
sion moment [128]. Both of these estimates suggest that
weakness in the vastus lateralis produces substantial weakness
in knee extension.

EFFECTS OF TIGHTNESS

Isolated tightness of the vastus lateralis is not common.
However, tightness limits knee flexion ROM. Tightness may
also contribute to an increase in the force of contact between
the patella and femur during knee flexion. Thus tightness of
the quadriceps may contribute to patellofemoral joint pain.

Vastus Medialis

ACTIONS

MUSCLE ACTION: VASTUS LATERALIS

Action

Evidence

Knee extension

Supporting

The uncontested action of the vastus lateralis is knee exten¬
sion. It is active throughout the excursion of knee extension,
and the amount of its recruitment is proportional to the
amount of resistance to extension [63].

EFFECTS OF WEAKNESS

Weakness of the vastus lateralis reduces knee extension
strength. Isolated weakness of the vastus lateralis is not com¬
mon. However, loss of strength in the vastus lateralis can be
expected to produce substantial reductions in extension
strength. Estimates based on physiological cross-sectional
area suggest that in some individuals, the vastus lateralis may
contribute 40% of the extension strength of the knee [28].
However, electrical stimulation of the vastus lateralis during

MUSCLE ATTACHMENT BOX 42.3

ATTACHMENTS AND INNERVATION
OF THE VASTUS LATERALIS

Proximal attachment: The intertrochanteric line, the
anterior and inferior borders of the greater
trochanter, the lateral border of the gluteal tuberosity,
and the proximal one half of the lateral lip of the
linea aspera and the lateral intermuscular septum

Distal attachment: Quadriceps aponeurosis, attach¬
ing to the lateral border and base of the patella
and the patellar tendon

Innervation: Femoral nerve, L2-L4

Palpation: The vastus lateralis is palpated on the
anterolateral surface of the thigh.

The vastus medialis is the most studied of the four heads of
the quadriceps femoris muscle (Muscle Attachment Box
42.4). In the classic study by Lieb and Perry [63], the vastus
medialis is described in two parts, the vastus medialis longus
(VML) and the vastus medialis oblique (VMO) (Fig. 42.4).
This division, based on both anatomical and mechanical
analysis, has helped to clarify the role of the vastus medialis
and to dispel long-held beliefs regarding its functional role. It
is estimated that the vastus medialis is approximately 20 to
35% of the overall physiological cross-sectional area of the
quadriceps femoris [28,35,118].

ACTIONS

MUSCLE ACTION: VASTUS MEDIALIS

Action

Evidence

Knee extension

Supporting

Patellar stabilization

Supporting

MUSCLE ATTACHMENT BOX 42.4

ATTACHMENTS AND INNERVATION
OF THE VASTUS MEDIALIS

Proximal attachment: The VML arises from the distal
half of the intertrochanteric line, the medial lip of
the linea aspera, the proximal two thirds of the
medial supracondylar line, and the medial intermus¬
cular septum. The VMO arises from the tendon of
the adductor magnus.

Distal attachment: The quadriceps aponeurosis,
attaching to the medial border of the patella and
the patellar tendon. The VMO attaches directly into
the medial border of the patella.

Innervation: Femoral nerve, L2-L4

Palpation: The vastus medialis is readily palpated on
the anteromedial side of the thigh. The oblique por¬
tion lies just proximal and medial to the patella.

772

The role played by the vastus medialis in knee extension was
at one time quite controversial [109]. However, repeated
analysis of its activity by EMG demonstrates that the vastus
medialis is active with the other heads of the quadriceps
femoris throughout the entire excursion of active knee exten¬
sion [63,77,109]. Direct stimulation of the vastus medialis
suggests that the vastus medialis provides approximately
10-12% of the total extension torque in submaximal contrac¬
tions [128]. The myth that the vastus medialis is responsible for
the final 15° of knee extension has been refuted convincingly!

The second important function of the vastus medialis is to
stabilize the patella during active extension of the knee. To
appreciate the importance of this function, it is necessary
to examine the overall architecture of the quadriceps femoris
muscle. The rectus femoris and the vastus intermedius are
centrally located, and their pulls on the patella are exerted
along the long axis of the femur. Because the femur deviates
laterally from the tibia, they pull proximally and laterally on
the patella (Fig. 42.5). In addition, the pull of the vastus lat¬
eralis on the patella actually is directed slightly laterally with
respect to the femur. However, the patellar ligament pulls on

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 42.5: The pull of the rectus femoris (RF) and vastus inter¬
medius (VI) is parallel to the shaft of the femur. The line of pull
of the vastus lateralis is lateral to the femur. The patellar tendon
pulls the patella distally. The sum of these forces is a proximal
and lateral pull on the patella.

the patella in a distal direction. Addition of these forces on the
patella yields a force that is directed laterally on the patella.

The Q angle estimates the lateral pull of the quadriceps
muscle [90]. It is formed by the intersection of a line drawn
from the anterior superior iliac spine (ASIS) of the pelvis to the
center of the patella and another drawn from the center of the
patella to the center of the tibial tuberosity [1,104] (Fig. 42.6).
Although estimates of the magnitude of the Q angle found
in the healthy population vary, there is general agreement
that normal values range from approximately 10 to 20°
[1,45,86]. Studies investigating the differences in Q angles
between men and women report statistically larger Q angles
in women, with values ranging from 15 to 20°. Reported val¬
ues for men range from approximately 10 to 15°.

In the individual without lower extremity pathology, the Q
angle and the measure of valgus of the knee using the long
axis of the femur and the tibia are very similar (Fig. 42.7).
However the Q angle is a function of the location of the tibial
tuberosity rather than the shaft of the tibia. Therefore,

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

773

Figure 42.6: The Q angle is formed by a line through the ASIS
of the pelvis and the center of the patella and a line from the
center of the patella to the tibial tubercle.

torsional deformities of the tibia and femur and rotary
malalignments of the foot can alter the Q angle without
changing the valgus alignment of the knee. (The effects of
foot alignment on the mechanics of the knee are examined
more closely in Chapter 44.) An increased Q angle indicates
an increased lateral pull on the patella and appears to increase
the risk of anterior knee pain [1,86].

The Q angle indicates that active contraction of the quadri¬
ceps femoris muscle applies a lateral pull on the patella as it
pulls the patella proximally and extends the knee. Indeed,
reported changes in patellar tilt and congruence angles dur¬
ing contraction of the quadriceps femoris in individuals with¬
out knee pathology indicate the tendency of the patella to be
pulled laterally, although no true lateral translation is reported
[11,123].

There are three systems of protection to stabilize the
patella and prevent its lateral deviation [48]. One source of
protection is the expanded surface on the lateral condyle of
the femur described in the preceding chapter. This bony
expansion serves as a buttress against lateral displacement of

Figure 42.7: A. In normal alignment, the Q angle is approximately
equal to the valgus angle of the knee. B. Lateral torsion of the
tibia can increase the Q angle while the valgus angle remains
unchanged.

the patella. The medial extensor retinaculum also provides
passive resistance to the lateral pull on the patella. Finally,
dynamic protection is offered by the vastus medialis, particu¬
larly by the fibers of the VMO. The fiber arrangement of the
VMO makes the muscle ideally suited to provide a stabilizing
force, since the fibers run almost completely in a medial and
lateral direction and insert directly on the patella (Fig. 42.8).
Thus the primary function of the VMO appears to be stabi¬
lization of the patella against the normal lateral pull of the
other heads of the quadriceps femoris muscle.

EFFECTS OF WEAKNESS

Simulated weakness of the vastus medialis in cadaver speci¬
mens leads to a lateral shift of the patella during terminal
extension [102]. However, the effect of vastus medialis weak¬
ness in vivo remains controversial, although some consistency
in the data is beginning to emerge. First, it is important to
reiterate that the vastus medialis appears to participate with
the other heads of the quadriceps femoris muscle throughout
knee extension [63]. This participation is independent of the
angular position of the knee, the speed of contraction, and the

774

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Rectus

femoris

Figure 42.8: The horizontal pull of the VMO on the patella bal¬
ances the lateral pull of the rest of the quadriceps femoris
muscle.

mode of contraction (concentric vs. eccentric) [27,42,77].
Thus it is difficult to identify a scenario in which the vastus
medialis is weak in isolation. Weakness of the vastus medialis
in conjunction with the rest of the quadriceps femoris muscle
leads to decreased strength in knee extension.

Yet there remains a strong clinical impression that weak¬
ness of the vastus medialis contributes to anterior knee pain
by allowing excessive lateral tracking of the patella. Abnormal
lateral displacement of the patella during quadriceps contrac¬
tion is reported in individuals with anterior knee pain [11].
However, data to demonstrate that specific weakness of the
vastus medialis contributes to the abnormal tracking of the
patella and thus to anterior knee pain have been elusive. In
contrast, quadriceps femoris strength as a whole is correlated
with the presence or absence of anterior knee pain [79,98,99].
Quadriceps strength also appears to be a powerful predictor
of the success or failure of an intervention regimen for ante¬
rior knee pain [85].

Clinical Relevance

STRENGTHENING THE VASTUS MEDIALIS: Abnormal
tracking of the patella is linked to anterior knee pain. In
addition, the role of the VMO is to stabilize the patella dur¬
ing contraction of the quadriceps femoris. These two factors
have led clinicians and researchers alike to seek means of
strengthening the VMO selectively on the belief that frank
weakness or, at least , relative weakness of the VMO com¬
pared with the rest of the quadriceps femoris contributes to
faulty patellofemoral joint mechanics and thus to anterior
knee pain. However ; attempts to develop strengthening exer¬
cises that recruit the VMO selectively are; at best , confusing.
One study denies any difference in recruitment with knee
position and velocity of contraction [65]. Another suggests
there may be a slight increase in the ratio of VMO to vastus
lateralis electrical activity during concentric contractions
while stair climbing. Conflicting data exist regarding whether
hip position alters the relative activity of the VMO and vas¬
tus lateralis during knee extension, but most studies show
little effect [7,17,26,44,60]. Delayed onset of vastus medialis
activity has also been suggested as a contributor to
patellofemoral joint pain since delayed onset of medialis
activity could allow the vastus lateralis to contribute to
excessive lateral glide or tilt of the patella during functions
requiring quadriceps contraction. Although most studies
suggest that onset of activity is similar for the vastus later¬
alis and medialis in individuals without knee pain [43],
investigators disagree on the presence of vastus medialis
activity onset delay in individuals with patellofemoral joint
syndrome [8,19,20,98]. Yet even those who report onset
delays demonstrate improved recruitment with strengthen¬
ing programs for the whole quadriceps muscle [8[.
Generalized strengthening combined with biofeedback for
the vastus medialis may have additional benefits [88]. At the
present time, the best scientific data available suggest that
generalized strengthening of the whole quadriceps femoris
muscle is the most successful exercise regimen for anterior
knee pain [40,85].

EFFECTS OF TIGHTNESS

There are no reports of specific tightness of the vastus medi¬
alis. Tightness in conjunction with the rest of the quadriceps
femoris muscle decreases knee flexion ROM.

Functional Considerations for
the Quadriceps Femoris Muscle

From the preceding discussion, it is clear that the heads of the
quadriceps femoris function together to extend the knee. In
activities of daily living, contraction of the quadriceps femoris
is required primarily to raise and lower the weight of the body

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

775

while upright. Such activities include getting in or out of a
chair or climbing stairs. In contrast, extending the knee dur¬
ing the swing phase of gait requires little or no activity
from the quadriceps femoris. This extension occurs
primarily as the result of momentum.

Clinical Relevance

EFFECT OF CHAIR HEIGHT ON FUNCTIONALLY
IMPAIRED ELDERLY INDIVIDUALS: More quadriceps
force is required to rise from a low chair than from a higher
chair (Fig. 42.9). A study of young healthy individuals and
frail elders with muscle weakness demonstrates that the eld¬
ers use a greater percentage of their total strength to rise
from the chair [47]. In addition , this study predicts the limit
of the chair height from which these individuals can rise.
Careful analysis of an individual's home setting and simple
alterations in chair height can significantly improve an indi¬
vidual's independence in the home.

Quadriceps weakness is a strong predictor of impaired per¬
formance in many activities of daily living [97]. Quadriceps
strength also appears to be a critical factor in rehabilitation of
the knee joint following ligamentous injuries [54,82,92,
101,121]. Similarly, knee extension strength is positively corre¬
lated with function and negatively correlated with symptoms
in individuals with osteoarthritis of the knee [30,32]. There is
even evidence that quadriceps strength offers protection from

Figure 42.9: The extension moment needed to rise from a high
chair is less than that needed to rise from a low chair.

joint degeneration [75,105]. Thus the clinician is urged to eval¬
uate knee extension strength carefully in patients with lower
extremity impairments.

FLEXORS OF THE KNEE

The hamstring muscles represent the primary flexors of the
knee (Fig. 42.10). However, there are several other muscles
capable of flexing the knee. The muscles that contribute to
flexion of the knee are: the biceps femoris longus and brevis,
semimembranosus, semitendinosus, popliteus, gracilis, sarto-
rius, and gastrocnemius. The hamstrings and popliteus are
discussed below. Although the sartorius and gracilis are dis¬
cussed in a later section of this chapter as rotators of the knee
joint, it is important to recognize that these two muscles also
undoubtedly participate in knee flexion. The gastrocnemius
also produces flexion at the knee. However, it plays an essen¬
tial role at the ankle and is discussed in Chapter 45 with the
other muscles of the leg and foot.

Figure 42.10: The primary knee flexors include the biceps
femoris, short head; biceps femoris, long head; semimembra¬
nosus; semitendinosus, and popliteus. Additional flexors include
the sartorius and gracilis.

776

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 42.5

ATTACHMENTS AND INNERVATION
OF THE BICEPS FEMORIS

Proximal attachment: The long head of the biceps
femoris attaches to the medial surface of the ischial
tuberosity. The short head attaches to the lateral lip
of the linea aspera, the proximal half of the lateral
supracondylar line, and the lateral intermuscular
septum.

Distal attachment: Head of the fibula, the lateral
collateral ligament, and the lateral condyle of the
tibia

Innervation: The long head is innervated by the tib-
ial division of the sciatic nerve, and the short head is
innervated by the common peroneal division of the
sciatic nerve, L5, SI, and S2.

Palpation: The tendon of the biceps femoris is readily
palpated as it inserts into the fibular head during
knee flexion.

Hamstrings

The hamstrings comprise the biceps femoris longus and bre¬
vis, forming the lateral mass of the hamstrings, and the semi¬
membranosus and semitendinosus, making up the medial
mass (Muscle Attachment Boxes 42.5-42.7). All of these mus¬
cles flex the knee, and all but the biceps femoris brevis con¬
tribute to hip extension. Consequently, their actions and the
effects of impairments in strength and flexibility are discussed
together. The individual traits of each muscle also are
presented.

MUSCLE ATTACHMENT BOX 42.6

ATTACHMENTS AND INNERVATION
OF THE SEMIMEMBRANOSUS

Proximal attachment: The lateral facet of the ischial
tuberosity

Distal attachment: Posterior and medial surfaces of
the medial tibial condyle

Innervation: Tibial division of the sciatic nerve,

L5, SI, and S2

Palpation: The muscle belly can be palpated distally
on the posterior surface of the knee on either side
of the semitendinosus tendon.

MUSCLE ATTACHMENT BOX 42.7

ATTACHMENTS AND INNERVATION
OF THE SEMITENDINOSUS

Proximal attachment: Inferior and medial surface of
the ischial tuberosity

Distal attachment: Via a flattened aponeurosis to
the proximal aspect of the medial surface of the
shaft of the tibia

Innervation: Tibial division of the sciatic nerve, L5,
SI, and S2

Palpation: The semitendinosus tendon is the most
lateral of the muscles on the posteromedial aspect
of the knee. It is typically the most prominent ten¬
don in this region.

ACTIONS

MUSCLE ACTION: HAMSTRINGS

Action

Evidence

Knee flexion

Supporting

Hip extension

Supporting

Knee medial rotation

Supporting

Knee lateral rotation

Supporting

Hip medial rotation

Supporting

Hip lateral rotation

Supporting

Hip adduction

Supporting

The role of the hamstring muscles as knee flexors is incon¬
trovertible. However, it is important to recognize that
because these muscles attach on both the medial and lateral
aspects of the knee joint, pure knee flexion requires activity
of both the medial and lateral muscle mass. Contraction of
only the medial hamstrings produces knee flexion with medial
rotation of the knee; contraction of only the lateral muscle
mass produces knee flexion with lateral rotation of the knee
joint (Fig. 42.11). Yet, EMG studies of lateral and medial
rotations of the knee without concomitant knee flexion pro¬
duce inconsistent activity of the hamstrings [5], emphasizing
the importance of the other rotators in moving the knee joint
in the transverse plane.

In addition to flexing and rotating the knee, the hamstrings
reportedly contribute to the stability of the knee. The ham¬
strings provide active resistance to anterior glide of the tibia
on the femur. Thus they are described as important adjuncts
to the anterior cruciate ligament (ACL) and perhaps a critical
substitute in the ACL-deficient knee [52,62,65,70,126]. The
role of the hamstrings in stabilizing the knee against varus and
valgus stresses is less clear. The semimembranosus has an

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

111

Figure 42.11: When contracting alone, the medial hamstrings
produce medial rotation of the knee with knee flexion; the
lateral hamstrings produce lateral rotation with knee flexion.

The hamstring muscles also play an essential role in exten¬
sion of the hip. EMG data reveal that the hamstring muscles
are active in hip extension even when the knee is flexed [29].
A study examining the residual hip extension strength after a
sciatic nerve block that incapacitated the hamstrings and
adductor magnus suggests that the hamstrings provide
between 30 and 50% of hip extension strength. Other studies
reveal activity in the hamstring muscles during forward bend¬
ing and lifting [5,87]. Chapter 39 discusses the gluteus maxi-
mus and presents data suggesting that it is best suited for
hyperextension of the hip. The data presented here suggest
that the hamstring muscles play a critical role in hip extension
throughout the range of hip motion. EMG evidence also sug¬
gests that the hamstring muscles can contribute to adduction
of the hip [5]. The biceps femoris longus demonstrates activ¬
ity during lateral rotation of the hip. Analysis of the medial
hamstring muscles’ moment arms reveals that with the hip in
neutral, the medial hamstrings exhibit very small (<1.0 cm)
medial rotation moment arms that increase as the hip laterally
rotates [4].

The hamstrings are active during normal locomotion.
The most prominent period of activity is at the transition
between the swing and stance periods of the gait cycle
[25,122] (Fig. 42.12). The role of this activity is to slow the
extension of the knee in late swing and to help extend the hip
in the stance phase. Although a detailed description of

expanded insertion about the medial aspect of the knee, with
attachments onto the medial collateral ligament and medial
meniscus. Tears of the semimembranosus can accompany
medial collateral ligament injuries [6]. Although EMG data
are conflicting, there is evidence of hamstring activity during
the application of varus and valgus stresses to the knee
[3,13,66]. These data suggest that hamstring muscles have the
mechanical potential, at least, to help stabilize the knee in the
frontal plane.

Clinical Relevance

ROLE OF HAMSTRING MUSCLE ACTIVITY IN THE
DYSFUNCTIONAL KNEE: There is considerable evidence
from cadaver studies indicating that the hamstring muscles
decrease the strain on the ACL There also is evidence that
individuals with ACL insufficiencies increase the activity of
their hamstring muscles in some activities such as hill climb¬
ing [52]. Decreased strength in the hamstrings also is associ¬
ated with poor functional outcomes in individuals following
patellectomies [59]. Therefore , while quadriceps strength is
well-recognized as important in knee function, careful con¬
sideration of hamstring function appears indicated in indi¬
viduals with disorders of the knees.

Figure 42.12: The hamstring muscles help to slow the knee's
extension and the hip's flexion in the late swing phase of gait.

778

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 42.13: The weight of the head, arms, and trunk (HAT)
when sitting into or rising from a chair produces a flexion
moment (M) at the knee that must be balanced by quadriceps
femoris muscle contraction. The same weight produces a flexion
moment (M) at the hip that must be balanced by hamstring
muscle contraction.

locomotion is found in Chapter 48, it is worth noting that the
knee flexion that occurs in late stance and early swing usually
occurs without the activity of the hamstrings [25,122]. It also is
important to recognize that many activities in the erect posture
that require knee flexion such as descending stairs and sitting
down use the quadriceps femoris to control the flexion rather
than the hamstrings to produce the flexion. The weight of the
head, arms, and trunk creates an external flexion moment at
the knee that is resisted by an internal extension moment gen¬
erated by the quadriceps femoris muscle (Fig. 42.13).

EFFECTS OF WEAKNESS

Weakness of the hamstring muscles produces a significant
loss of knee flexion strength. However, from the preceding
discussion, it is clear that knee flexion in the erect posture is
often the result of a superimposed weight and controlled by
an eccentric contraction of the quadriceps femoris.
Consequently, weakness in knee flexion in the erect posture
produces little disability. However, weakness of the ham¬
strings may produce more functional impairment at the hips,
where the hamstring muscles provide a substantial part of
extension strength. The superimposed weight that creates a
flexion moment at the knee during a squat produces a flexion
moment at the hip. That flexion moment is resisted by a mus¬
cular extension moment generated at least in part by the ham¬
string muscles. Consequently, weakness of the hamstrings
may result in significant difficulty in bending and lifting.

Clinical Relevance

CAN HAMSTRING STRENGTHENING PREVENT LOW
BACK INJURIES DURING LIFTING TASKS?: Improper
bending techniques during lifting are associated with low
back injuries. Bending the knees when lifting reduces the
external flexion moment on the lumbar spine. However ; it
produces flexion moments at both the hips and the knees
that must be resisted by extensor muscles at these joints.

The role of the strength of the quadriceps femoris muscles in
limiting lifting ability is widely accepted [90,103]. However ;
since the hamstring muscles are important hip extensors ,
they too may contribute to an individual's ability to bend
and lift correctly. Yet the role of hamstring muscle weakness
in reduced bending and lifting capacity is not well studied.
Additional research is needed to identify any relationship
between hamstring strength and lifting ability and then to
determine if strength training for the hamstring muscles can
improve an individual's ability to lift. In addition , studies are
needed to determine the effect of hamstring muscle
strengthening on the incidence of low back injury [56].

EFFECTS OF TIGHTNESS

The effects of tightness of the hamstrings are complex
because all but the biceps femoris brevis cross both the hip
and knee joints. As in the rectus femoris, the assessment of
tightness in the hamstring muscles must account for their
two-joint muscle construction. Tightness of the hamstrings
results in limitations in knee extension ROM when the hip is
flexed (Fig. 42.14) or limited hip flexion with the knee
extended. A study of over 200 individuals without lower limb
dysfunction reports an average hip flexion ROM of 68.5° ±
6.8° in men and 76.3° ± 9.5° in women with the knee extended
[127]. When the hip is extended, the hamstring muscles are
put on some slack and allow full knee extension ROM. If the
position of the hip has no effect on the range of knee exten¬
sion, any limitations in knee extension range are the result of
one-joint structures such as the joint capsule and ligaments of
the knee.

Large amounts of hamstring tightness can produce knee
flexion contractures, an inability to reach full knee extension.
Knee flexion contractures secondary to hamstring dysfunc¬
tion are found commonly in individuals with overactivity, or
spasticity, of the hamstrings. Overactivity of the hamstring
muscles can produce decreased knee extension in late swing
and at ground contact during gait [18]. Lesser amounts of
hamstring tightness are reportedly associated with a posterior
rotation of the pelvis in standing (Fig. 42.15). A posterior rota¬
tion of the pelvis tends to flatten the lumbar spine, which may
increase the risk of low back pain. However, the associations
among hamstring tightness, postural abnormalities, and low
back pain are not well-established. Research is needed to
identify and explain any links that may exist.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

779

Figure 42.14: A. Hip flexion and knee extension put the ham¬
strings on stretch so that the knee cannot be fully extended.

B. Putting the hip in neutral puts the hamstrings on slack, and
full knee joint ROM is allowed.

Mechanics of Two-Joint Muscles
at the Knee

The knee is controlled mostly by two-joint muscles that cross
either the hip and the knee or the knee and the ankle.
Contraction of one of these muscles alone produces move¬
ment at all of the joints that the muscle crosses. To isolate
movement at a single joint, the two-joint muscle must contract
with other muscles, frequently with a one-joint synergist.

Figure 42.15: Tightness of the hamstrings can pull the pelvis into
a posterior pelvic tilt, flattening the lumbar spine.

This is seen at the wrist and finger where the carpi muscles
contract with the extrinsic finger muscles to produce pure fin¬
ger motion (Chapter 15). Such synergists also are available at
the knee. The iliopsoas and the hamstrings together produce
isolated knee flexion by canceling each others effect at the hip.
Similarly, simultaneous contraction of the gluteus maximus
and quadriceps femoris produces knee extension without hip
flexion. However, the knee more frequently displays simulta¬
neous contraction of the quadriceps and hamstrings. This
unusual pattern of simultaneous contraction of two-joint mus¬
cles appears to increase the ability of the knee and hip to gen¬
erate the large moments needed during many activities. Some
of the activities that exhibit this behavior include walking, run¬
ning, cycling, jumping, bending, and lifting [50,95,114,122].
Co-contraction of the hamstrings and quadriceps also seems to
help stabilize the knee and protect the ligamentous structures
[66,73]. Although this pattern of a co-contraction increases

780

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 42.8

ATTACHMENTS AND INNERVATION
OF THE POPLITEUS

Proximal attachment: Lateral aspect of the lateral
femoral condyle, popliteus tendon, arcuate liga¬
ment, and the fibrous capsule of the knee. The ten¬
don may have an attachment to the lateral menis¬
cus as well.

Distal attachment: A triangular area on the posterior
surface of the tibia proximal to the soleal line

Innervation: Tibial nerve, L4, L5, and SI

Palpation: Not easily palpated but may be felt in
some people just posterior to the lateral collateral
ligament during knee flexion.

the efficiency of producing large muscular moments
at both the hip and the knee, it also causes a rise in
the loads sustained by the knee.

Popliteus

The popliteus is a small muscle with an unusual pattern of
attachments (Muscle Attachment Box 42.8). It appears to play
a unique role at the knee.

ACTIONS

MUSCLE ACTION: POPLITEUS

Action

Evidence

Knee flexion

Supporting

Tibial medial rotation

Supporting

The physiological cross-sectional area of the popliteus is
quite small compared with the rest of the knee flexor muscles
[118]. Therefore, its contribution to knee joint flexion torque
is small. EMG data reveal only slight activity of the popliteus
during knee flexion [5,71]. Its activity increases significantly
when knee flexion is accompanied by medial rotation of the
tibia, and isolated medial rotation of the knee elicits signifi¬
cant activity in the popliteus [71]. The popliteus also contracts
during gait when the tibia is medially rotating.

Studies suggest that the popliteus serves an important role
as a dynamic stabilizer of the knee [37,115]. These cadaver
studies demonstrate the muscle s ability to reinforce the pos¬
terior cruciate ligament, preventing posterior glide of the tibia,
as well as rotations into varus and lateral rotation. Finally, the
muscle s attachment on the lateral meniscus suggests that it
may assist in pulling the meniscus posteriorly during knee flex¬
ion, perhaps providing additional protection from tears
[10,74]. In summary, the primary role of the popliteus seems

to be to medially rotate the tibia and to protect the integrity
of the tibiofemoral joint. It does not contribute substantially
to knee flexion strength.

EFFECTS OF WEAKNESS

Identification of weakness in the popliteus is difficult, since it is
covered by the large and powerful hamstring muscles.
Consequently, the effects of weakness can only be theorized.
However, rupture of the popliteus is reported in conjunction
with extensive injuries to other posterolateral ligamentous
structures including the lateral collateral and arcuate ligaments
and the posterolateral joint capsule. Injuries to this entire com¬
plex lead to significant instability of the knee joint [37,91,116].

EFFECTS OF TIGHTNESS

As in weakness, discrete tightness of the popliteus is difficult
to identify and is not likely to occur. However, the popliteus is
likely to be tight with other structures in the presence of a
flexion contracture.

Functional Implications of Flexion
Contractures of the Knee

Tightness of the flexors of the knee and the posterior connec¬
tive tissue structures can result in flexion contractures of the
knee. The inability to reach full extension ROM can signifi¬
cantly increase the functional demands on the body in erect
posture. In normal erect posture, the knee is extended, and
the superincumbent weight exerts an extension moment on it.
As a result, no muscle activity is required to support the knee
in erect stance [111]. However, the presence of a knee flexion
contracture precludes the use of this passive support mecha¬
nism. With the knee flexed, the weight of the head, arms, and
trunk produces a flexion moment at the knee, and contraction
of the quadriceps femoris muscle is required to maintain the
standing position. This greatly increases the metabolic cost of
erect standing. It also alters the magnitude and direction of
the forces at the knee and may contribute to further damage
of the knee joint complex. The mechanics and pathomechan-
ics of posture are discussed in greater detail in Chapter 47.
However, it is clear that a knee flexion contracture signifi¬
cantly increases the challenge of erect posture.

MEDIAL ROTATORS OF THE KNEE

The medial rotators of the knee are the semimembranosus,
semitendinosus, and popliteus, which were described in the
preceding section, and the sartorius and gracilis, described
below (Fig. 42.16).

Sartorius

The sartorius is a strap muscle and contains some of the
longest muscle fibers found in the human body [46] (Muscle
Attachment Box 42.9). Reports suggest that the fibers are

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

781

Figure 42.16: The medial rotators of the knee include the sarto-
rius, gracilis, semitendinosus, semimembranosus, and popliteus.

MUSCLE ATTACHMENT BOX 42.9

ATTACHMENTS AND INNERVATION
OF THE SARTORIUS

Proximal attachment: Anterior superior iliac spine
and the proximal half of the notch below it

Distal attachment: Proximal aspect of the medial
surface of the shaft of the tibia.

Innervation: Femoral nerve, L2, and L3

Palpation: The strap-like muscle belly of the sarto-
rius is palpable on the medial aspect of the knee
during contraction. It is the most anterior of the pes
anserinus muscles.

Palpation: The sartorius is readily palpated at its
proximal end. It also is palpated distally, anterior to
the tendon of the gracilis.

approximately 90% of the length of the muscle [118]. The sar¬
torius is one of the three muscles composing the pes anseri¬
nus that crosses the medial aspect of the knee.

ACTIONS

MUSCLE ACTION: SARTORIUS

Action

Evidence

Hip flexion

Supporting

Hip lateral rotation

Supporting

Hip abduction

Supporting

Knee flexion

Supporting

Tibial medial rotation

Insufficient

EMG data and electrical stimulation studies support the role of
the sartorius as a hip flexor [2,5,16,72]. Analysis of its moment
arms reveals mechanical potential for flexion, abduction, and
lateral rotation of the hip [24,72]. Lateral rotation of the hip
elicits EMG activity of the sartorius and increases its activity
when combined with hip flexion. Hip abduction and abduction
with lateral rotation also produce activity of the sartorius. The
sartorius has a large moment arm for each motion at the hip.
Consequently, despite its small cross-sectional area, the sarto¬
rius can generate considerable moments at the hip [72,106].

Knee flexion with the subject prone is accompanied by
some sartorius activity, although the activity appears later in
the motion than activity of the other muscles such as the ham¬
strings [16]. This response is consistent with mechanical
analyses that show that the muscles flexion moment arm
increases with knee flexion from 0 to 90° [93]. EMG analysis
reveals no activity during isolated medial rotation of the knee
with the knee slightly flexed and the subject upright [16]. The
rotation moment arm of the sartorius changes very little with
knee flexion [93]. Studies report activity in early swing with
other hip flexors [51,122]. Activity of the sartorius in the early
portion of the swing phase of gait is consistent with its role as
a hip flexor. Studies differ on the muscle s activity during the
stance phase. Although the sartorius is not well studied in
locomotion, most studies report activity in early stance, which
may reflect its role as a hip abductor with the gluteus medius
and minimus [51,122].

EFFECTS OF WEAKNESS

The cross-sectional area of the sartorius is a small fraction
of that of the other muscles that flex, abduct, or laterally rotate
the hip [64,118]. These muscles include the iliopsoas, glutei,
and rectus femoris. Similarly, the hamstrings, the primary flex¬
ors of the knee, are much larger and stronger than the sarto¬
rius. Consequently, the effects of isolated weakness of the sar¬
torius on the strength of hip and knee flexion may be small.

EFFECTS OF TIGHTNESS

There are no known reports of isolated tightness of the sarto¬
rius in the literature. However, it may contribute to a hip

782

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

flexion contracture, although its size suggests that other mus¬
cles may be larger factors in the contracture.

generate torque at either the knee or hip. Thus the effects of
weakness are unknown.

Gracilis

EFFECTS OF TIGHTNESS

The gracilis is described here because of its role at the knee,
although it often is described as part of the hip adductor
group (Muscle Attachment Box 42.10).

ACTIONS

MUSCLE ACTION: GRACILIS

Action

Evidence

Knee medial rotation

Supporting

Knee flexion

Supporting

Hip adduction

Supporting

The gracilis is rather poorly studied by EMG analysis.
However, there are consistent reports that the muscle con¬
tributes to both medial rotation and flexion of the knee
[5,106]. Analysis of the moment arms of the gracilis support
its role as both a flexor and medial rotator of the knee
[14,22,93]. Mechanical analyses demonstrate a large adduc¬
tion moment arm at the hip, with a very small moment arm
for lateral rotation.

EFFECTS OF WEAKNESS

The physiological cross-sectional area of the gracilis is very
small compared with that of the other knee flexors and hip
adductors. However, the gracilis has a substantial moment
arm for hip adduction. In addition, it has a large moment arm
for knee flexion. Thus the muscle appears capable of gener¬
ating considerable hip and knee moments; however, there are
no known studies that examine the capacity of the gracilis to

MUSCLE ATTACHMENT BOX 42.1

ATTACHMENTS AND INNERVATION
OF THE GRACILIS

Proximal attachment: Via a thin aponeurosis from
the medial surfaces of the inferior half of the body
of the pubis, the inferior pubic ramus, and the
ischial tuberosity

Distal attachment: Proximal aspect of the medial
surface of the shaft of the tibia.

Innervation: Obturator nerve, L2, and L3

Palpation: The tendon of the gracilis is palpated on
the medial aspect of the knee anterior to the semi-
tendinosus tendon during contraction.

Isolated tightness of the gracilis is unlikely. Tightness is most
likely to occur in the presence of tightness of the entire group
of hip adductor muscles. The effects of hip adductor tightness
are detailed in Chapter 39 but include reduced hip abduction
ROM.

Pes Anserinus

The sartorius and gracilis, along with the semitendinosus, insert
together forming the pes anserinus. Although these muscles per¬
form the same action at the knee, each has a different function
at the hip. Each also has a different innervation. Comparison of
these muscles at the knee reveals that the semitendinosus has the
largest flexion moment arm, while the sartorius has the smallest
[22,93] (Fig. 42.17). The semitendinosus also has the largest

Figure 42.17: The sartorius has the shortest moment arm of the
muscles of the pes anserinus, followed by the gracilis, and then
the semitendinosus.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

783

Figure 42.18: Many activities that require quick turns such as
playing tennis can put large valgus stresses on the knee.

physiological cross-sectional area of the three and thus is able to
generate the largest knee flexion moment.

These three muscles together appear to contribute to the
dynamic stabilization of the knee against valgus and rotary
forces. EMG data reveal increased electrical activity in each
of these muscles with the addition of a valgus load to the knee
[3,61]. The knee is frequently subjected to such forces. It is
intriguing to notice that at least one of these three muscles of
the pes anserinus is available to provide support to the knee
regardless of the hip position. Similarly, each of the primary
motor nerves innervates one of these muscles. Thus there
appears to be an organization established to guarantee the
availability of some dynamic stabilization of the medial side of
the knee. Many sports activities require running and quick
turning, including soccer and tennis (Fig. 42.18). Such move¬
ments require increased stabilization of the knee. When these
motions are repeated frequently enough or when the partici¬
pant is inadequately trained, inflammation of, or around, the
pes anserinus can develop.

Clinical Relevance

CASE REPORT: A 25-year-old female tennis player sought
a physical therapy consultation , complaining of medial knee

pain of increasing intensity She denied any specific trauma
and reported a gradual onset of pain while playing tennis.
She was an elite player ; having competed on the professional
tour. However ; she had discontinued her tennis to begin
physical therapy education. She was playing tennis sporadi¬
cally at the time of the evaluation.

Palpation revealed tenderness on the medial aspect of
the knee slightly distal to the joint line. There was no evi¬
dence of joint inflammation or joint line tenderness. ROM of
the hip and knee joints was full and pain free. Strength of
hip flexion , abduction , and medial and lateral rotation was
within normal limits and comparable to that of the opposite
limb. Similarly , knee flexion and extension strengths were
within normal limits and pain free. There was no medial
joint instability. However ; a valgus stress to the knee pro¬
duced slight pain. Palpation of the medial collateral liga¬
ment was pain free.

Muscle testing of the sartorius by combining resisted hip
flexion and abduction with knee flexion produced pain that
the patient identified as her chief complaint. Further isolated
testing revealed slight discomfort with resisted contractions
of the gracilis and semitendinosus muscles. These results
suggested that the patient had an inflammation of the pes
anserinus bursa that lies deep to the three tendons as they
pass over the medial tibiaI plateau. Her tennis play was con¬
sistent with this conclusion , since the activity required
repeated rapid movements and pivots on the affected limb.
Treatment with ice and rest alleviated the patient's
complaints.

LATERAL ROTATORS OF THE KNEE

The lateral rotators of the knee are the biceps femoris longus
and brevis, which were described earlier in this chapter, and
the tensor fasciae latae described below (Fig. 42.19).

Tensor Fasciae Latae

The tensor fasciae latae lies just anterior to the gluteus
medius and minimus (Muscle Attachment Box 42.11).

ACTIONS

MUSCLE ACTION: TENSOR FASCIAE LATAE

Action

Evidence

Hip flexion

Supporting

Hip abduction

Supporting

Hip medial rotation

Supporting

Knee extension

Supporting

Tibial lateral rotation

Supporting

784

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 42.19: The lateral rotators of the knee include the tensor
fasciae latae and the biceps femoris, long and short heads.

MUSCLE ATTACHMENT BOX 42.11

ATTACHMENTS AND INNERVATION
OF THE TENSOR FASCIAE LATAE

Proximal attachment: Anterior aspect of the lateral
surface of the iliac crest and anterior superior iliac
spine and from the fascia lata

Distal attachment: Via the iliotibial band (ITB) to the
lateral tubercle of the tibia. The ITB also attaches to
the lateral condyle of the femur and head of the
fibula and blends with the extensor expansion of
the vastus lateralis.

Innervation: Superior gluteal nerve, L4, L5, and SI

Palpation: The tensor fasciae latae muscle belly can
be palpated at the ASIS of the pelvis during
contraction.

EMG data consistently reveal activity of the tensor fasciae
latae during hip flexion, abduction, and medial rotation
[5,16]. Although the cross-sectional area of the tensor fasciae
latae is considerably smaller than that of the iliopsoas or glu¬
teus medius and minimus, it has a large hip abduction
moment arm. Its hip flexion moment arm is larger than the
moment arm of the iliopsoas [24,46]. Consequently, the ten¬
sor fasciae latae is able to produce substantial hip abduction
and flexion moments although still not equal to those of the
primary abductors or flexors [72].

Reports also reveal electrical activity of the tensor fasciae
latae during knee extension which is unchanged by rotation of
the knee in either the medial or lateral direction [16,53,80].
In addition, the tensor fasciae latae helps produce lateral rota¬
tion of the knee. Like the muscles of the pes anserinus on the
medial side of the knee, the tensor fasciae latae provides
dynamic stabilization to the knee joint via its attachment into
the iliotibial band, increasing its activity in the presence of
forces tending to adduct the knee [3,13,61].

During locomotion, activity of the tensor fasciae latae is
reported in both the stance and swing phases [36,122]. In the
stance period, it most likely contributes to stabilizing the pelvis
with the other hip abductors. Chapter 38 demonstrates that
medial rotation of the hip can occur either by movement of the
femur on the pelvis or by movement of the pelvis on the femur
when the lower extremity is fixed. Thus as a medial rotator, the
tensor fasciae latae may also help to advance the pelvis on the
unsupported side (Fig. 42.20). Finally, activity of the tensor fas¬
ciae latae during swing occurs with activity of the iliacus and is
consistent with the tensor fasciae latae s role as a hip flexor.

Tensor

fasciae

latae

Figure 42.20: In the stance phase of gait, the tensor fasciae latae
can pull on the pelvis, causing medial rotation and advancing the
pelvis on the opposite side.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

785

EFFECTS OF WEAKNESS

Although the tensor fasciae latae contributes to activities at
the hip and knee, its estimated peak force is only approxi¬
mately 16% of the peak force of the iliopsoas and approxi¬
mately 12% of the estimated peak force of the gluteus medius
[46]. The difference between the estimated peak force of the
tensor fasciae latae and the estimated peak in the quadriceps
muscles is even larger. Thus isolated weakness of the tensor
fasciae latae is unlikely to produce significant disability. A case
report of an individual with isolated paralysis of the tensor fas¬
ciae latae reports slight but detectable weakness in hip abduc¬
tion and flexion strength and in medial rotation strength of
the hip with the knee extended [80]. A slight reduction in
knee extension strength also is reported. Despite these minor
weaknesses, the author reports a negative Trendelenburg test
result and an unremarkable gait pattern. This case suggests
that in the absence of other abnormalities, weakness of the
tensor fasciae latae produces little functional loss.

EFFECTS OF TIGHTNESS

Tightness of the tensor fasciae latae reduces the ROM in com¬
bined hip extension, adduction, and lateral rotation when the
knee is extended. The complex three-dimensional
motion at both the hip and the knee resulting from ten¬
sor fasciae latae contraction and, therefore, from tight¬
ness makes identification of such tightness a clinical challenge.

Clinical Relevance

OBER'S TEST: The classic test to determine tightness of the
tensor fasciae latae is the Oder's test in which the subject's
test hip is extended and the amount of adduction available
at the hip is observed (Fig. 42.21). Although the original test
calls for the knee to be flexed, the test is more often

Figure 42.21: A. In a positive Ober's test result, adduction is limited
the hip to roll into medial rotation puts the tensor fasciae latae on
response to the Ober's test.

performed with the knee extended. One of the challenges of
this test for the clinician is to control the limb to prevent
medial rotation of the hip , since medial rotation puts the
muscle in a slackened position and can produce a false¬
negative response.

Tightness of the tensor fasciae latae is associated with both
lateral and anterior knee pain. Iliotibial band friction syn¬
drome is an irritation of the iliotibial band (ITB) from repeated
rubbing and excess friction between the band and the lateral
epicondyle [94]. It is a common complaint in runners and typ¬
ically is reported during the stance phase of gait when the
muscle is active in supporting the hip. Complaints are report¬
edly diminished by decreasing the amount of knee flexion
used during this phase of the gait cycle. Reduction of the
knee flexion excursion decreases the stretch on the muscle
during active contraction.

Tightness of the tensor fasciae latae and the ITB also are
linked to excessive lateral deviation of the patella which is
associated with anterior knee pain [38]. Treatments for exces¬
sive lateral tracking or excessive lateral tilting of the patella
include patellar taping or bracing and surgical release of the
lateral patellar retinaculum into which the ITB inserts [38].
The mechanical effects of patellar malalignment and conser¬
vative treatments such as bracing are discussed in more detail
in Chapter 43.

STRENGTH OF THE FLEXOR AND
EXTENSOR MUSCLES OF THE KNEE

Knee joint strength is well recognized as an important factor
influencing functional capacity [69,97,100]. Therefore, meas¬
urement of muscular strength at the knee is a critical clinical
tool. However, the interpretation of strength measurements

when the hip is held in extension and neutral rotation. B. Allowing
slack and allows the hip to adduct, producing a false-negative

786

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

may be even more critical in identifying abnormalities and
developing strategies to ameliorate the problem. Thus an
understanding of the relative strengths of the muscle groups
of the knee is particularly useful to the clinician. In addition,
appreciation of the factors that affect muscular force produc¬
tion at the knee assists the clinician in distinguishing normal
variability from pathology. The data reviewed here are
designed to provide a perspective from which the clinician
can judge a patients strength at the knee.

Comparisons between Extension
and Flexion Strength at the Knee

It is widely recognized that extension strength at the knee is
significantly greater than flexion strength. This finding is con¬
sistent with the data that demonstrate that the mass of the
extensors is significantly larger than the mass of the flexors
[9,118]. Studies demonstrate that the ratio of hamstring
strength to quadriceps femoris strength ranges from approxi¬
mately 0.45 to 0.65. In other words, the maximum strength of
the hamstring muscles is approximately 45-65% of the maxi¬
mum strength of the quadriceps femoris [15,21,33,34,83,84].
This ratio persists throughout the aging process and is present
in children from at least the age of six [39,83,84]. However,
the magnitude of the ratio is affected by gender and knee
joint position, as well as by the speed and mode of contrac¬
tion. The clinician is cautioned to consider these effects when
making judgments about the adequacy of strength in either
muscle group. When possible, comparisons with the unaf¬
fected limb may be more useful than a target ratio.

Factors Influencing Muscle Strength
at the Knee

Age and sex have significant effects on knee joint strength.
Reports consistently describe up to 50% less flexion and
extension strength in adults over the age of 70 years than in
young adults [31,83,84,112]. However, there is less agree¬
ment regarding the pattern of strength decline. Some authors
report declines from young adulthood to middle age and a
larger decline in later years [49,83,84]. Others report less
decline until after the age of 50 [31].

Clinical Relevance

DECREASED KNEE STRENGTH IN OLD AGE: The impor¬
tance of quadriceps femoris and hamstring strength to ris¬
ing from a chair, walking up a hill, or getting on and off a
toilet is undeniable. The reports of declining strength in the
knee musculature with age are worrisome, since they sug¬
gest that there may be a concomitant loss in function. It is
important for the clinician to recognize the possibility of
declining strength in elder patients and to consider the pos¬
sible functional implications. Fortunately, there is strong

evidence that muscle strengthening is possible at any age
[12,81]. Perhaps the loss of strength at the knee with age is
preventable, reversible, or at least able to be slowed.

Not surprisingly, men exhibit significantly greater strength
than women in both knee flexion and extension [31,76,83,84].
Genetic, hormonal, and cultural factors may all contribute to
this difference [76]. Studies are needed to determine if dif¬
ferences in strength contribute to the increased incidence of
some musculoskeletal problems in women, such as ACL tears
and osteoarthritis.

Effects of Joint Position on Muscle
Strength at the Knee

Several studies examine the effects of joint position on knee
extension and flexion strength. Chapter 4 describes the basic
relationship between joint position and muscle strength in
detail. The primary effects result from changes in muscle
length and in the moment arm of the muscle. The following
presents the available data on the changes in muscle force
output at the knee with joint position and provides some data
on muscle moment arms to help explain the data.

QUADRICEPS FEMORIS

Most of the studies assessing the effect of joint position on
extension strength at the knee report isometric strengths with
the subject seated. These data generally demonstrate that
quadriceps femoris strength peaks in midrange somewhere
between 50 and 80° of knee flexion [21,55,58,96] (Fig. 42.22).
These findings are explained by the effects of both muscle

£=

_C/5

1 - 1 - 1 - 1 - 1 - 1 - 1 - 1 -r

10 20 30 40 50 60 70 80 90

Knee flexion angle (degrees)

Figure 42.22: A plot of quadriceps femoris muscle strength
against knee flexion ROM reveals that the quadriceps femoris
generates a maximum force of contraction in the midrange of
knee motion, where neither the muscle's length or moment arm
are maximized.

Chapter 42 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE KNEE

787

length and moment arm. With the hip fixed in flexion, the
length of the quadriceps increases as the knee flexes. If
the length-tension relationship dominated the results, exten¬
sion strength would increase with knee flexion and reach a
maximum at maximum knee flexion rather than in midrange.
Although there is some disagreement in the literature about
where the maximum moment arm of the extensor muscles
occurs in the ROM of the knee, most studies suggest that the
peak occurs at less than 50° of knee flexion [14,57,67,89,
110,117,125]. If the output of the quadriceps femoris were dic¬
tated by the muscle s moment arm, maximum extension force
would occur earlier in extension. Since peak force occurs some¬
where between 50 and 80° of flexion, it appears that like the
biceps brachii at the elbow, knee extension strength is a func¬
tion of both muscle length and moment arm. Hip position can
be expected to influence knee extension strength as well but is
less well studied.

HAMSTRING MUSCLES

In contrast to the quadriceps femoris muscle, most studies
suggest that the hamstring muscles exhibit a steady increase
in isometric force output from a position of knee flexion to
extension, although most studies examine strength only to 20°
of knee flexion [55,58,68,78]. However, some studies show a
peak or little change in hamstring strength in the middle
range of knee flexion (30-60°) [83,84,124]. Some of the dif¬
ferences in these reports are attributable to differences in hip
position, which significantly alters the length of the muscle
[58]. Most of the data suggest that hamstring performance is
influenced more by muscle length than by moment arm, since
the optimal moment arms for the hamstrings occur with the
knee flexed [14,67]. However, additional research is needed to
resolve the contradictory reports.

These data demonstrate the significant impact on the force
production capacity of the knee flexors and extensors made
by joint position. The clinician is reminded that valid strength
assessments depend on the successful control of factors influ¬
encing force production. Therefore, assessment of changes in
isometric strength at the knee requires consistency in the
joint position used for testing.

SUMMARY

This chapter presents the muscles of the knee and discusses
their functions at the knee and hip. These muscles play an
obvious role in moving the knee but also contribute signifi¬
cantly to the stability of the knee. In addition, the contribu¬
tion of these muscles to the pathomechanics of the knee joint
is described. Finally, the available data describing the relative
strengths of the flexors and extensors of the knee are
reviewed. Peak flexion strength is approximately 40-65% of
peak extension strength. Joint position, age, and gender all
significantly affect knee joint strength.

The muscles that move and stabilize the knee joint are
large and capable of producing large contractile forces.

In addition, the knee functions most frequently while bearing
at least half, if not all, of a person s body weight. Consequently,
the joint surfaces and surrounding connective tissue of the
tibiofemoral and patellofemoral joints are subjected to large
and repeated loads. The following chapter examines the loads
that the knee joint sustains under normal conditions and con¬
siders the impact of such loads in pathological conditions.

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CHAPTER

Analysis of the Forces on the Knee
during Activity

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE FORCE IN THE QUADRICEPS FEMORIS MUSCLE DURING KNEE EXTENSION . . . .791

Effect of Mode of Exercise on Quadriceps Femoris Force .792

FORCES AND MOMENTS ON THE STRUCTURES OF THE KNEE JOINT DURING ACTIVITY.796

Forces and Moments on the Tibiofemoral Joint.796

Forces on the Ligaments of the Tibiofemoral Joint.798

Co-contraction of Muscles across the Knee.799

Forces and Stresses at the Patellofemoral Joint.799

SUMMARY .803

T he preceding two chapters describe the structure of the knee and its impact on the knee's function as well
as the mechanics of muscle control across the knee. They emphasize the unusual mechanical demands of
the knee, a joint with complex mobility that also sustains large loads as a weight-bearing joint. The knee
is controlled by very large muscle groups that stabilize the joint and help move the superimposed weight of the
body over a foot fixed on the ground. Consequently, the knee is repeatedly subjected to very large forces throughout
a day. The mechanical stresses sustained by the knee are likely contributors to the osteoarthritis so commonly found
at the knee. Therefore, it is important for the clinician to be aware of the characteristics of the forces and the factors
that influence them. The purposes of this chapter are to

■ Present a two-dimensional analysis of the force required of the quadriceps during simple exercises

■ Examine the forces and stresses that are applied to the tibiofemoral joint and their relationship to osteoarthritis
of the knee

■ Consider the loads in the cruciate ligaments as a result of quadriceps femoris and hamstring muscle contraction

■ Analyze the forces at the patellofemoral joint under varying exercise strategies

TWO-DIMENSIONAL ANALYSIS OF THE
FORCE IN THE QUADRICEPS FEMORIS
MUSCLE DURING KNEE EXTENSION

A typical strengthening exercise for the quadriceps femoris
muscle is knee extension lifting a weight from the seated
position. Examining the Forces Box 43.1 presents a simple
two-dimensional analysis of this exercise. Although this
example is an oversimplification of the loads on the knee, it
provides an acceptable approximation of the force required

of the quadriceps femoris to hold the leg and foot at a 30°
angle of knee flexion with a 10-lb weight at the ankle [6].
The analysis reveals that the extensor muscles must gener¬
ate a force of 1.08 times body weight (BW) to maintain this
position!

Careful examination of the moment arm of the quadriceps
femoris muscle compared with the moment arms of the ankle
weight and the weight of the leg and foot explains why such a
large extensor force is needed. The moment arm of the ankle
weight is about 10 times greater than the muscles moment

791

792

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

EXAMINING THE FORCES BOX 43.1

CALCULATION OF THE QUADRICEPS FEMORIS
FORCE NEEDED TO HOLD THE KNEE
EXTENDED TO 30° WITH A 10-POUND
WEIGHT AROUND THE ANKLE

The following dimensions are based on a female who is
5 feet 8 inches tall (1.72 m) and weighs 140 lb (623 N).
The limb segment parameters are extrapolated from
the anthropometric data of Braune and Fischer [6]. The
geometry of the quadriceps femoris muscle is based on
data of Buford et al. [10]. The extension force is
assumed to be provided entirely by the quadriceps
femoris, with no co-contractions of other muscles.

Weight of the leg and foot: 6% of body weight

(BW)

Weight at the ankle: 10 lb (7% BW)

Length of the leg and foot: Approximately 29%
of the subject's height: 0.5 m
Center of gravity of the leg and foot: Located at
61 % of length of leg and foot from the knee joint
Ankle weight: Located 0.44 m from the knee joint
Moment arm of the quadriceps femoris: 0.04 m

Solve for the

quadriceps force (Q):

SM = 0

(Q X 0.04 m)

- (0.06 BW X

0.3 m x (sin 60°))

- (0.07 BW X

0.44 m x (sin 60°)) = 0

(Q x 0.04 m)

= (0.06 BW X

0.26 m)

+ (0.07 BW X

0.38 m)

Q = 1.06 BW

or 660 N

arm. Similarly, the moment arm of the weight of the leg and
foot is approximately 6.5 times larger than the moment arm
of the quadriceps femoris. The mechanical disadvantage pro¬
duced by the short moment arm of the quadriceps femoris
results in very large force requirements of the muscle.

The example provided in Examining the Forces Box 43.1
analyzes the force in the quadriceps femoris muscle at a sin¬
gle position of the knee. Changes in the position of flexion
and extension of the knee alter the moment arms of the
weights and also of the muscle. As the knee extends from 90°
to full extension, the moment arms of the ankle weight and
the limb weight steadily increase (Fig. 43.1). Thus the exter¬
nal moments that must be resisted by the quadriceps muscle
increase. As noted in Chapter 42, the moment arm of the
quadriceps femoris is greater in extension than in flexion
beyond 50°. However, this increase is relatively slight
and provides only a small improvement in the mechanical

advantage of the muscle [10,35,43,53,54,67,72]. The increases
in the moment arms of the external forces exceed any
increased mechanical advantage of the quadriceps femoris.
Consequently, the force required of the quadriceps
femoris progressively increases from 90° of knee flex¬
ion to complete extension [34] (Fig. 43.2).

Effect of Mode of Exercise on Quadriceps
Femoris Force

The examples described above examine the muscular forces
that are required to resist the weight of the leg and foot and
any additional applied weights. However, there are many
other strengthening devices available that exert resistance
on the knee in different ways. Each method may alter the di¬
rection of the external force or the mechanics of the muscle
output. It is important for the clinician to appreciate the

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

793

Figure 43.2: Force output of the quadriceps increases as
knee extension increases during knee extension against
a free weight.

influence that exercise mode has on muscle forces at the
knee. This section examines the varying effects of resistance
applied by

• A pulley, or cam system

• An isokinetic dynamometer

• A closed-chain exercise

KNEE EXTENSION RESISTANCE DELIVERED
BY A PULLEY SYSTEM

The critical difference between resistance from free weights
as demonstrated in Examining the Forces Box 43.1 and a
resistance applied through a pulley is the direction of the
external force. Weight is a force that, by definition, is exerted
in a vertical and downward direction. However, a pulley or
cam system is designed to deliver a force that is always
directed perpendicular to the limb (Fig. 43.3). In this case,

794

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

the external moment applied to the knee is constant through¬
out the range of extension. Since the moment arm of the
quadriceps femoris increases slightly in the last half of the
extension excursion, the extension muscle force needed to
generate the moment is slightly lower in this range. The mag¬
nitude of the quadriceps force, then, is affected more by the
magnitude of the resistance than by the position of the knee.

KNEE EXTENSION AGAINST AN
ISOKINETIC DYNAMOMETER

An isokinetic dynamometer differs from free weights and
from pulley systems by allowing a variable resistance. As in
the pulley system, the resistance is applied perpendicular to
the leg. However, the dynamometer offers an accommodat¬
ing resistance that matches the torque applied by the indi¬
vidual using it. The mechanics of the quadriceps femoris
muscle described in detail in Chapter 42 reveal that the
extensor muscle generates its peak moment in the middle
range of knee flexion, somewhere between approximately 50
and 80° of knee flexion [30,31,53,69]. Therefore, when the
individual applies a maximum force to the dynamometer
through the range of knee extension, the quadriceps force
peaks in midrange [30,53] (Fig. 43.4).

KNEE EXTENSION EXERCISES USING
A CLOSED-CHAIN FORMAT

A closed chain is a mechanical description of a system of
links in which both ends of the system are attached to
relatively fixed structures [44]. The knee participates in
closed-chain activities when the foot is fixed on the ground.
Since the hip is connected to a less movable torso superiorly,
the knee is situated between two relatively fixed ends and
functions in a closed chain. Squats are a common form of
closed-chain exercise (Fig. 43.5).

10 20 30 40 50 60 70 80 90

Knee angle (degrees)

Figure 43.4: Because the quadriceps are strongest in the
midrange of knee flexion, quadriceps force during a
maximum isokinetic exercise is greatest in the midrange.

Figure 43.5: A squat is an example of a closed-chain exercise,
since the knee moves between two relatively fixed points,
the foot and the torso.

Closed-chain exercise exhibits two substantial differences
from the other extension-strengthening exercises described
so far. First, the resistance is the weight of the head, arms, and
trunk (HAT). The other major difference is the relationship
between the moment arm of the resistance and the position
of the knee joint. In erect standing, the center of mass of the
HAT weight lies slightly anterior to the knee joint. In this
position, the moment arm of the HAT weight is very small
and actually produces a slight extension moment [55].
Consequently, in erect standing, there is no need for activity
of the knee extensors. However, as the subject squats, the
center of mass of the HAT moves posteriorly, producing a
flexion moment arm that increases with the knee flexion angle
(Fig. 43.6). As the squat increases, the magnitude of the exter¬
nal flexion moment increases, and the force required of the
quadriceps femoris muscle increases in concert [15]. Closed-
chain activities are common throughout daily life. Rising from
a chair, climbing stairs, and getting out of the bathtub
are only a few examples of the closed-chain activities
undertaken routinely.

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

795

Figure 43.6: In a closed-chain exercise of the knee, the moment
arm of the weight of the trunk increases as knee flexion increases.

The four exercise modes described here differ from one
another in the pattern of quadriceps femoris muscle force
required through the range of knee flexion and extension
excursion. These patterns are summarized below:

• In exercise with free weights, the required quadriceps
femoris force for a given weight peaks when the knee is in
full extension.

• Resisted knee extension using a pulley system produces an
almost constant quadriceps force (slightly lower, as the
quadriceps moment arm decreases somewhat when the
knee is flexed less than 50°). The magnitude of the quadri¬
ceps force depends primarily on the external force.

• Because isokinetic resistance is accommodating, the
quadriceps femoris force reflects the muscles intrinsic
mechanical capacity. Therefore, the peak force of the
quadriceps femoris occurs in the midrange of knee flexion.

• Closed-chain exercise requires increasing quadriceps
femoris force as flexion of the knee increases.

Several studies report quadriceps muscle force during
maximum isometric or isokinetic exercise [3,43,53].
Estimates of the muscle forces generated during maximum
efforts vary but are as large as nine times BW [3], or internal
moments of approximately 250 Nm [51,59]. For comparison,
Examining the Forces Box 43.2 reproduces the calculations in
Examining the Forces Box 43.1 using internal moments in
units of newton-meters (Nm). These calculations reveal that
lifting a 10-lb free weight may generate an extension load of
approximately 26.3 Nm.

Clinical Relevance

AVULSION FRACTURE OF THE TIBIAL TUBEROSITY:
A CASE STUDY: Analysis of resisted extension exercises
demonstrates the enormous toads that the extensor muscles
are capable of generating. MaffuUi and Grewal report avul¬
sion fractures sustained by two adolescent male gymnasts
during landing maneuvers [36]. These authors report that

796

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

both boys displayed greater strength in their uninjured
extensors than nonathletic adolescent males. The authors
suggest that the force in the quadriceps femoris muscle
developed during the landing may have exceeded the
strength of the tibiaI tuberosity's growth plate. These reports
are useful in demonstrating the force that the quadriceps
femoris muscle is capable of generating. They also serve
to warn the clinician that the underlying musculoskeletal
system must be capable of sustaining these loads.

Several studies provide estimates of moments or forces in
the extensor muscles during daily activities. In normal locomo¬
tion, quadriceps femoris forces of over 400 lb (1800 N) are
reported in male subjects [41]. In similar locomotion studies,
extensor moments of approximately 30 Nm are reported [33].
Kicking a ball reportedly requires extension moments of
approximately 260 Nm [61]. Moments produced during lifting
are varied and depend on how the lift is performed but also can
be very large [43,48]. Rising from a chair may require moments
over 200 Nm but can be reduced by using the upper extremi¬
ties for additional propulsion [43,49]. These data
demonstrate how many activities of daily living require
substantial forces from the knee extensor muscles.

FORCES AND MOMENTS ON THE
STRUCTURES OF THE KNEE JOINT
DURING ACTIVITY

Forces and Moments on the Tibiofemoral
Joint

The preceding discussion demonstrates the magnitude of the
extensor forces that can be generated. Examples throughout
this textbook demonstrate that muscle force is a major con¬
tributor to the joint reaction force sustained by any joint.
This is certainly the case at the knee. Once the muscle force
is determined at a joint, static equilibrium equations can be
used to calculate the joint reaction forces at the joint.
Examining the Forces Box 43.3 provides a simple two-dimen¬
sional solution for the joint reaction force at the tibiofemoral
joint during the free-weight knee extension exercise
described in Examining the Forces Box 43.1. This example
reveals that during the simple knee extension exercise of lift¬
ing a 10-lb load, the tibiofemoral joint sustains a joint reac¬
tion force of approximately 100% of BW. Since muscle loads
are a major contributor to joint reaction forces, it is not sur¬
prising that considerably higher loads of up to several times
BW are reported during activities such as walking, jogging,
lifting, sguatting, and ascending stairs (Table 43.1).

EXAMINING THE FORCES BOX 43.3

CALCULATION OF THE REACTION FORCES
ON THE TIBIOFEMORAL JOINT WHEN
HOLDING THE KNEE EXTENDED TO 30° WITH
A 10-POUND WEIGHT AT THE ANKLE.

The results and anthropometric data from Boxes 43.1
and 43.2 are used in this calculation.

2F X :

J x -Qx (cos 15°) + 0.06 X BW x (sin 30°)

+ 0.07 X BW x (sin 30°) = 0
where Q = 1.06 BW or 660 N
J x = 598 N

XF y :

J Y + Q X (sin 15°) - 0.06 X BW X (cos 30°)

- 0.07 X BW X (cos 30°) = 0

J Y = -100.6 N

Using the Pythagorean theorem:

J 2 = V + V

J « 606.4 N
J « 0.97 BW

Using trigonometry, the direction of J can be
determined:

cos 0 = J x /J

0 ~ 10° from the x axis

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

797

TABLE 43.1: Loads on the Tibiofemoral Joint during Functional Activities (BW = body weight)

Activity

Number of Subjects

Peak Joint Reaction Force

Authors

Level walking

3.03 BW

Morrison [41]

Stair climbing

4.25 BW

Morrison [40]

Lifting

2.12 BW

Nisell [43]

Jogging

12.4 BW

Scott and Winter [50]

Squatting

7.6 BW

Nagura et al. [42]

The joint reaction force at a joint frequently is reported in
terms of its components of axial, or compressive, loading as
well as its shear forces in the anterior-posterior and
medial-lateral directions. The compressive loads at the knee
are far greater than the shear forces [28,42,53,70]. The joint
reaction force is of particular interest because it is regarded as
an important contributing factor in the development of
osteoarthritis (OA). The knee joint is one of the most com¬
mon weight-bearing joints affected by OA, and knee OA is a
leading cause of disability in aging adults [4,16,23,32].
Therefore, it is important for the clinician to recognize the
relationship between joint and muscle forces and their possi¬
ble associations with OA and to consider how exercise affects
joint loads [57,68].

The tibiofemoral joint also sustains large moments during
functional activities. As noted in Chapter 41, the tibiofemoral
joint exhibits 6 degrees of freedom (DOF) and thus sustains
forces and moments along and about the medial-lateral,
anterior-posterior, and longitudinal axes. Moments about
the medial-lateral and anterior-posterior axes are particularly
relevant clinically. Moments about the medial-lateral axis tend
to produce flexion or extension. An internal extension
moment produced by the quadriceps balances the external
flexion moment exerted by the ground reaction force during
a squat. In the frontal plane, during normal locomotion the
ground reaction force applies an external adduction moment
on the knee during mid-stance [26]. This adduction moment
increases the forces applied to the medial tibial plateau and
femoral condyle. The adduction moment increases in individ¬
uals with varus alignment of the knee and is associated with
degenerative changes of the medial side of the knee joint,
medial compartment knee osteoarthritis.

In contrast, an individual who lacks adequate hip and knee
joint stabilization in the frontal plane may sustain large exter¬
nal abduction moments during weight bearing. Excessive
abduction moments are associated with medial knee pain
and tears of the anterior cruciate ligament [22].

Clinical Relevance

ADDUCTION AND ABDUCTION MOMENTS ON THE

KNEE: The knee joint typically sustains large adduction
moments during the stance phase of gait Adduction
moments lead to increased loading of the medial tibial

plateau and femoral condyle. Factors such as malalignment
and footware may increase the adduction moment Excessive
adduction moments may contribute to the development and
progression of knee osteoarthritis, particularly in the medial
compartment leading to the characteristic genu varum
deformity (Fig. 43.7). High tibial osteotomies and the use of
walking assists such as braces and canes can reduce the
adduction moment relieve pain, and perhaps protect the
joint from additional damaging loads [11,45].

Excessive abduction moments on the knee may be pro¬
duced during weight bearing when the frontal plane align¬
ment of the knee is compromised by weak abductors of the
hip (Fig. 43.8). Weak hip abductors are a common finding in
individuals with anterior knee pain or a torn ACL. The
increased abduction moment may contribute to excessive
Q-angles or produce excessive loads in the ACL. Treatments
to increase hip abduction strength may lead to decreased
abduction moments and damaging loads on the ligaments.

An understanding of the frontal plane moments applied to the
knee will allow clinicians to develop more effective prevention
and treatment strategies for joint degeneration and trauma.

One important element in linking joint forces and
moments with subsequent joint degeneration is the area over
which the force is applied. The ability of a joint to sustain joint
reaction forces depends not only on the magnitude of the
reaction force, but also on its location and how it is dispersed
across the joint surface. As defined in Chapter 2, the area over
which a force is applied determines the stress (F/area)
applied to the structure. The incongruity of the articular sur¬
faces of the tibiofemoral joint directly affects the contact area
of the knee and, consequently, the stress applied to the tibial
surfaces. Chapter 41 describes the articular surfaces of the
knee joint in detail. Studies indicate that the normal medial
compartment of the knee bears more of the joint reaction
force than the lateral compartment [24,29,41]. However, the
overall articular surface is greater on the medial side of the
joint than on the lateral surfaces [27,47]. Reports differ over
which tibial condyle sustains larger stress [27,47,62,63].
Reported magnitudes of peak stress vary from 4 to 9 MPa
under static loading conditions, compared with 4 to 7 MPa at
the hip during level walking [19,47]. Additional research is
needed to characterize the stresses at the knee in individuals
with and without pathology.

798

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 43.7: Genu varum. Excessive adduction moments may
contribute to the development and progression of the varus
deformity of the knee that is characteristic of medial compart¬
ment knee osteoarthritis.

Clinical Relevance

ALTERING THE STRESSES APPLIED TO THE KNEE:

Obesity is a significant risk factor for knee OA [17]. This
finding is logical since body weight is a contributor to the
compression forces on the knee. However ; other factors
including lower extremity alignment and walking patterns
also affect the stresses at the knee [58]. Surgical treatments
to realign the knee are designed specifically to alter the
stresses at the knee [65,66]. However, the use of canes and
other assistive devices may improve the loading pattern on
the knee [11,39]. In the absence of a cure for OA, an under¬
standing of the links among activity, knee joint loads,
and arthritis may lead to more effective treatments and
prevention strategies.

Forces on the Ligaments
of the Tibiofemoral Joint

The analysis of tibiofemoral joint forces presented in
Examining the Forces Box 43.3 reveals that the pull of
the quadriceps femoris muscle during contraction can be

Figure 43.8: Valgus stresses on the knee. Weakness of the hip
abductors may lead to excessive abduction moments at the knee
because of inadequate frontal plane stability of the femur.

decomposed into a compressive and a shear component
(Fig. 43.9). The compressive component contributes to the
large axial forces at the tibiofemoral joint described above.
The anterior shear force also has important clinical implica¬
tions. The pull of the muscle in the anterior direction tends
to slide the tibia anteriorly on the femur. Anterior shear
forces equal to body weight are reported during a vigorous
quadriceps contraction [43]. The anterior cruciate ligament
(ACL) provides the primary resistance to anterior transla¬
tion of the tibia. Therefore, contraction of the quadriceps
applies a significant pull on the ACL.

Clinical Relevance

FORCES IN THE ACL DURING CONTRACTION OF THE
QUADRICEPS FEMORIS MUSCLE: Biomechanical models
and cadaver studies demonstrate that quadriceps activity
increases the pull on the ACL [5,51,60]. These findings create
a challenge for the clinician. Chapter 42 reveals that

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

799

Figure 43.9: The quadriceps femoris force (Q) can be decomposed
into compressive (Q c ) and shear (Q s ) forces.

quadriceps muscle activity is an important stabilizing
force; particularly in the presence of ligamentous injury.
Therefore, muscle strengthening is an important component
of rehabilitation following injury. Yet a torn or reconstructed
ACL may incur further disruption if subjected to excessive
loads. Vigorous quadriceps activity, especially with the knee
extended, produces large and potentially damaging forces
on the ACL [12]. Studies demonstrate that closed-chain exer¬
cises generate smaller loads on the ACL than open-chain
exercises [60,71]. Consequently, patients undergoing rehabil¬
itation following injury to the ACL are instructed in quadri¬
ceps strengthening exercises using closed-chain exercises, or
they use a restraining device to limit the amount of anterior
glide of the tibia during open-chain exercise (Fig. 43.10).

Co-contraction of Muscles across
the Knee

In the biomechanical analyses presented thus far in this chap¬
ter, only the quadriceps femoris muscle is active. However,
Chapter 42 indicates that in many activities of daily living, the
hamstrings muscles contract with the quadriceps femoris
muscle. Indeed, such co-contraction is often used to protect
the ACL from excessive pull of the quadriceps femoris. The
hamstrings exert a posterior shear force on the tibia during
contraction and actually lessen the force on the ACL particu¬
larly when the knee is flexed [1,5,13,37,51]. (Fig. 43.11).

Clinical Relevance

CLOSED-CHAIN EXERCISES FOR INDIVIDUALS WITH
ACL DEFICIENCIES: Closed-chain exercises such as leg
presses and step-up or step-down exercises elicit significant
co-contractions of the quadriceps and hamstring muscles.
Consequently, they are a routine part of a rehabilitation
program for the ACL [44]. However, co-contraction of

muscles produces a significant increase in the compressive
component of the joint reaction force [28,70]. Clinicians
must remain aware of both the benefits and risks of various
exercise regimens to prescribe a program that optimizes
the benefits while minimizing the detrimental effects.

Forces and Stresses at
the Patellofemoral Joint

The extraordinarily thick articular cartilage found on the
patella suggests that the patella is subjected to very large joint
forces. The primary source of the large joint reaction forces at
the patellofemoral joint is the large muscle force of the
quadriceps femoris generated in so many activities in daily
life. The quadriceps pulls proximally on the tibia by pulling on
the patella and on the patellar tendon. From the perspective
of the patella, the quadriceps pulls proximally on the patella
while the patellar ligament pulls distally on the patella
(Fig. 43.12). If the patella functions as a pulley, as is fre¬
quently described, the magnitude of the proximal pull on the
patella equals the magnitude of the distal pull. Although there
is now evidence demonstrating that these magnitudes are not
equal, this assumption is a justifiable simplification frequently
used to estimate the force on the patella at the patellofemoral
joint [9,25,38].

Figure 43.10: A subject exercising the quadriceps femoris on
an isokinetic exercise device can use an antishear attachment
to protect the ACL.

800

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 43.11: During co-contraction of the
quadriceps and hamstrings, the pull of the
hamstrings (H) applies a posterior shear force
(H s ) that protects the ACL from the shear force
of the quadriceps (Q s ). Q is the force of the
quadriceps muscle and H c and Q c are the com¬
pressive components of the hamstring and
quadriceps muscle force, respectively.

Examining the Forces Box 43.4 provides a simplified
calculation of the forces on the patella generated when
holding the knee extended to 30° while holding a 10-lb
weight at the ankle. The results of the calculations esti¬
mate loads of approximately 83% BW (515 N) on the
patellofemoral joint. Not surprisingly, the patellofemoral
joint reaction forces are larger in activities that generate
larger quadriceps femoris forces. Estimated compressive
forces on the patella range from over 800 N (180 lb) to
approximately body weight in level walking [7,43] to over
5000 N (1125 lb) in running [18] and in dancers’ jump
landings [52].

The directions of pull of the quadriceps and patellar ten¬
don also are important in determining the load on the patella.
The more flexed the knee, the more the patella is pulled into

Figure 43.12: It is a justifiable simplification to assume that the
pull of the quadriceps tendon {F Q ) and the patellar tendon (F r )
are equal, because the patella acts somewhat like a pulley for
the quadriceps complex.

the femur. Conversely, the more extended the knee, the more
the patella is pulled parallel to the femur (Fig. 43.13). This
knowledge can be applied to understand the effects of various
knee-strengthening protocols.

Clinical Relevance

PATELLOFEMORAL JOINT FORCES IN THREE
DIFFERENT EXERCISES: The forces in the quadriceps
during knee extension with a free weight , using a cam
system and during a closed-chain exercise are reported
earlier in this chapter. The patellofemoral joint forces also
vary in these exercises (Fig. 4344). With a free weight , the
patellofemoral joint forces are small in knee flexion when
the quadriceps force is small. However , in this exercise ,
the patellofemoral joint reaction force also is small in
knee extension , despite a large quadriceps force , because
the patella is pulled parallel to the femur , producing very
little compression. In resisted knee extension using a cam
system , a relatively constant quadriceps force is produced ,
even at 90° of flexion. Therefore , the patellar joint force
reflects the angle of knee flexion , large with knee flexion
and steadily decreasing with knee extension. Finally , in
closed-chain knee extension , the quadriceps femoris force
increases with knee flexion , and the patella is pulled more
into the femur as the knee is flexed. Therefore , the patello¬
femoral joint force increases as knee flexion increases in
the closed-chain exercise [64]. Clinicians must be mindful
of these relationships when designing exercise regimens
for knee strengthening.

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

801

Although joint reaction forces are important to consider in
designing an exercise program, the stresses on a joint are also
important to consider. This is particularly true at the
patellofemoral joint, where the contact surfaces change dra¬
matically through the range of knee flexion. As noted in
Chapter 41, there is little contact between the patella and
femur when the knee is completely extended and only the
inferior portion of the patella contacts the femur in early knee
flexion. The area of contact increases as the knee flexes to
about 90° [21,38]. The change in contact area on the patella
has dramatic effects on the stress applied to the patellofemoral
joint. When the knee is completely extended in the free-
weight exercise, the quadriceps muscle force is large.
However, if there is no contact between patella and femur,
there is no stress on the patella. By 15 to 30° of knee flexion
there is contact between the patella and femur but over a
small area. In the free-weight exercise, the muscle force
remains high, and consequently, the patellofemoral stress is
quite high. In comparison, the closed-chain exercise actually
generates smaller patellofemoral joint stresses with the knee

slightly flexed, because the force of the quadriceps femoris
muscle is smaller [56]. The reverse is true with the knee flexed
to 90°. The patellofemoral stresses are higher in the closed-
chain exercises than in the free-weight exercise with the knee
flexed to 90° because of the differences in quadriceps muscle
force. As a result, closed-chain exercises in slight knee flexion
frequently are recommended to strengthen the quadriceps
femoris muscle while avoiding large stresses to the
patellofemoral joint [14,44]. Table 43.2 presents a comparison
of the muscle forces and patellofemoral joint forces and
stresses developed in the four extensor-strengthening exer¬
cises discussed throughout this chapter.

Patellofemoral joint stresses of approximately 3 MPa are
reported in normal walking and up to approximately 6 MPa in
stair ascent and descent [7,8]. Some individuals with
patellofemoral joint pain exhibit decreased patellofemoral joint
contact area, which may contribute to increased stress and
patellofemoral joint pain. Understanding the relationship
between joint stresses and function allows the clinician to con¬
sider treatment alternatives to reduce stress and increase function.

802

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 43.13: A. When the knee is flexed, the forces of the
extensor mechanism (F Q and F r ) pull the patella into the femur.
B. When the knee is extended, the forces of the extensor
mechanism pull the patella almost parallel to the femur.

Figure 43.14: The mode of exercise affects the joint reaction
forces on the patella between 0 and 90° of flexion. During
knee extension against a free weight, the reaction force
peaks in midrange of knee flexion; it peaks at 90° of knee
flexion in a closed-chain exercise.

Clinical Relevance

PATELLAR BRACING OR TAPING TO REDUCE
LATERAL TRACKING: The use of taping or bracing to
reduce pateiiofemorai joint pain is a common treatment
approach. The premise of such treatment is that the tape or
brace applies a medial force on the patella to decrease its
lateral tracking. Studies consistently report decreased ante¬
rior knee pain and improved function with such treatments
[2]. Yet studies provide little or no evidence for patellar
realignment. Powers et al. demonstrates that patellar brac¬
ing while not repositioning the patella increases the contact
area between patellar and femoral articular surfaces [46].
These data suggest that patellar taping or bracing may be
effective, not because it repositions the patella but because it
reduces pateiiofemorai joint stress .

TABLE 43.2: Comparison of the Mechanics of Quadriceps-Strengthening Exercises between 0 and 90°
of Knee Flexion

Free Weight

Resistance

Pulley-System

Resistance

Isokinetic

Resistance

Closed-Chain

Resistance

Knee position with maximum muscle force

0°

Almost constant

Midrange

90°

Knee position with minimum muscle force

90°

Almost constant

0°

Knee position with maximum PFJ force

Midrange

90°

Knee position with minimum PFJ force

0°

Knee position with maximum PFJ stress

Early to midrange flexion

90°

Midrange flexion
or 90 oa

Knee position with minimum PFJ stress

0°

Midrange

0° or midrange

0°

Comments

Quadriceps
femoris pull
from midrange
to 0° extension
increases load
on ACL

Quadriceps femoris pull
from midrange to 0°
extension increases load
on ACL

Quadriceps
femoris pull
from midrange
to 0° extension
increases load
on ACL

Co-contraction
of extensor and
flexors helps
protect ACL

PFJ, pateiiofemorai joint; ACL, anterior cruciate ligament,
investigators differ in the joint position of maximum PFJ stress [20].

Chapter 43 I ANALYSIS OF THE FORCES ON THE KNEE DURING ACTIVITY

803

SUMMARY

This chapter uses simple two-dimensional analysis to demon¬
strate the forces sustained by the quadriceps femoris muscle
during exercise and activity. These data are then used to
estimate the loads on the tibiofemoral joint and the ACL in
similar activities. Finally the force in the quadriceps femoris
muscle also is used to approximate the forces sustained by the
patellofemoral joint. The examples provided demonstrate that
these structures withstand very large loads. The quadriceps
generates loads approximately equal to body weight during
low resistance, open-chain exercises and loads several times
body weight in maximum resistance exercises. Tibiofemoral
joint reaction forces range from approximately 100% BW to
more than 1200% BW during jogging. Patellofemoral joint
reaction forces over 5000 N (1125 lb) are reported in running
and jumping activities.

This chapter also examines the loads on the tibiofemoral
and patellofemoral joints in terms of the stress (F/area).
Factors influencing the stress on these joints include magni¬
tude of the external loads, joint alignment, and joint position.
Since the joints of the knee are commonly affected by OA, an
appreciation of the loads and stresses to which the structures
of the knee are subjected can help the clinician modify inter¬
ventions to minimize joint stress.

Throughout this chapter commonly prescribed exercises
for quadriceps femoris strengthening are used to demon¬
strate the concepts of muscle force and joint loads. These data
directly influence the clinical decisions necessary when
designing a rehabilitation program for an individual with knee
pathology. However, in a larger sense, these exercises illus¬
trate how biomechanical analysis of joints informs the prac¬
tice of rehabilitation. This same approach is useful in studying
the mechanics and pathomechanics of the foot and ankle,
which are presented in the following unit.

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805

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22 : 1 - 10 .

UNIT 8

ANKLE AND FOOT UNIT

T he foot and ankle represent the final component of the lower extremity and function together to make habi¬
tual bipedal stance and locomotion possible. Just as the wrist and hand are the working unit of the upper
extremity, the ankle and foot complex greatly enhances the functional capacity of the lower extremity.
Although there are a great number of similarities between the wrist and hand of the upper extremity and the ankle
and foot of the lower extremity, the peculiar functional demands of persistent weight bearing induce unique charac¬
teristics in the ankle/foot complex. In addition, because the ankle and foot function much of the time in contact with
the ground, they complete a closed chain with the rest of the lower extremity. Consequently, the ankle/foot complex
has a substantial effect on the knee and even on the hip and spine.

The ankle/foot complex must be stable enough to bear the weight of the rest of the body and also must participate in
the advancement of the body over the fixed foot during locomotion. As a result, the muscles of the leg and foot play
an essential role in stabilizing the ankle/foot complex during loading but also in propelling and controlling the
advancement of the body over the foot during locomotion. As the ankle and foot participate in locomotion they sus¬
tain very large loads that may contribute to some of the clinical complaints reported by patients.

The purposes of this three-chapter unit on the ankle and foot complex are to

■ Discuss the structure of the bones and joints of the ankle and foot and how these features contribute to the role of
weight bearing and propulsion

■ Discuss the role of muscles in the mechanics and pathomechanics of the ankle and foot

■ Analyze the forces to which the ankle and foot are subjected, particularly during weight-bearing activities

806

CHAPTER

Structure and Function of the Bones
and Noncontractile Elements of the
Ankle and Foot Complex

CHAPTER CONTENTS

BONES OF THE ANKLE AND FOOT.808

Shaft and Distal Tibia.808

Alignment of the Tibia.809

Fibula.810

Tarsal Bones.810

Bones of the Digits.813

Structural Organization of the Foot.814

JOINTS AND SUPPORTING STRUCTURES OF THE LEG AND FOOT .814

Joints and Supporting Structures between the Tibia and Fibula .815

Joints of the Foot.817

Motion of the Whole Foot.828

Closed-Chain Motion of the Foot .829

FOOT ALIGNMENT.830

Arches of the Foot .830

Subtalar Neutral Position.831

SUMMARY.832

4 lthough the bones of the foot bear some resemblance to those of the hand, their unique features have a
substantial impact on the mobility and stability of the ankle, as well as the weight-bearing capacity of the
entire complex. The supporting structures within the ankle and foot also bear some similarities to those of
the wrist and hand, and the differences between these two anatomical units reflect the differences in functional
demands.

The purpose of this chapter is to discuss the bones and joints of the ankle/foot complex and how these influence the
function of the lower extremity. Specifically, the objectives of the current chapter are to

■ Discuss the functionally relevant structural features of the bones of the ankle and foot
■ Describe the architecture and supporting structures of the joints of the ankle and foot
■ Review the motions available at the individual joints of the ankle and foot

■ Describe how the joints of the foot function together to produce total foot motion in the open and
closed chain

■ Describe the normal alignment of the foot and ankle

■ Present the normative data on range of motion (ROM) available at the ankle and foot

807

808

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

BONES OF THE ANKLE AND FOOT

The ankle/foot complex comprises the distal tibia and
fibula, the seven tarsal bones, and the digits, consist¬
ing of five metatarsals and fourteen phalanges
(Fig. 44.1). Their unique characteristics contribute substan¬
tially to the functional capabilities of the ankle/foot complex.

Shaft and Distal Tibia

TIBIAL SHAFT

The proximal tibia is described in Chapter 41 in the knee unit.
The shaft of the tibia continues from the tibial plateaus and
tibial tuberosity (Fig. 44.2). The anterior border of the tibia
extends from the tibial tuberosity distally to the anterior aspect
of the medial malleolus. It is superficial and easily palpated
until its distal end. Most individuals who are capable of upright
walking recognize the anterior border of the tibia as the “shin”
that bumps painfully into chair legs or other obstacles. The

Figure 44.1: The ankle/foot complex consists of the tibia, fibula,
seven tarsal bones, five metatarsal bones, and fourteen phalanges.

Gerdy's

tubercle

Figure 44.2: The tibia and fibula possess several palpable land¬
marks and together form the cavity, or mortise, for the talus.

medial surface of the tibia also is palpable the length of the
tibia. The posterior surface, from the interosseous border lat¬
erally to the medial border, contains the soleal line that runs
obliquely from the articular surface of the head of the fibula
medially to the medial border of the tibia, approximately one-
third of the length of the tibia from its proximal end.

DISTAL TIBIA

The shaft of the tibia ends distally in an inferiorly and medi¬
ally projecting mass, the medial malleolus, which is readily
palpated. The lateral surface of the medial malleolus provides
an articular surface for the medial aspect of the talus. It is ver¬
tically aligned and almost flat, so it bears little weight. This
articular surface on the tibia is continuous with the distal tib¬
ial surface, which also offers an articular surface for the talus.
The articular surface of the distal tibia, known as the plafond,
is saddle shaped, concave in an anterior-posterior direction

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

809

Figure 44.3: Tri-malleolar fracture. A tri-
maleollar fracture includes fractures of
the medial and lateral malleoli (A) and
a fracture of the posterior surface of
the distal tibia, the third maleollus (B).
(Reprinted with permission from
Greenspan A. Orthopedic Imaging: A
Practical Approach, 4 th ed. Philadelphia,
Lippincott Williams & Wilkins 2004.)

and convex in a medial-lateral direction. This surface bears
approximately 90% of the load through the ankle [14,28].

The lateral aspect of the distal tibia provides the articular
surface for the distal fibula. The posterior surface of the dis¬
tal tibia continues from the articular surface for the fibula to
the medial malleolus and is marked by a groove for the ten¬
don of the posterior tibialis, which also marks the medial
malleolus. The posterior margin of the distal tibia is some¬
times referred to as the third malleolus because it projects
distally beyond the superior surface of the talus and con¬
tributes to the stability of the ankle joint [145,174].

Clinical Relevance

TRI-MALLEOLAR FRACTURE: A tri-malleolar fracture
consists of fractures of the medial and lateral malleoli
and the posterior margin of the tibia; the third malleolus
(Fig. 44.3). Fracture of the posterior portion of the tibia can
include a portion of the articular surface of the distal tibia.
As in any fracture, involvement of the articular surface
increases the complexity of the fracture and its morbidity.

Alignment of the Tibia

In adults, the distal portion of the tibia is laterally rotated in
the transverse plane with respect to the proximal end of the
tibia, creating a normal lateral, or external, tibial torsion
[87,154,169] (Fig. 44.4). Lateral torsion of the tibia moves the
medial malleolus anteriorly and consequently influences the
position of the foot with respect to the leg, affecting posture
and gait. Tibial torsion is measured in a variety of ways, includ¬
ing by the angle between a line through the tibial plateaus and

a line through the medial and lateral malleoli [30,87,169]. Like
femoral torsion, tibial torsion changes throughout develop¬
ment, beginning in slight lateral torsion or even medial torsion
at birth and gradually progressing to 20-^40° of lateral torsion
by adulthood [112,152,154,156,169,197].

Clinical Relevance

TORSIONAL DEFORMITIES OF THE TIBIA: Medial tor¬
sion of the tibia is the second most common cause of an in-
toeing posture , following only excessive femoral anteversion
[34]. (Torsional deformities of the femur are discussed in

Axis through malleoli

Figure 44.4: Average tibial torsion in adults without pathology,
indicated by an angle between a line through the tibial plateaus
and the medial and lateral malleoli, ranges from 20 to 40° of lat¬
eral torsion.

810

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Chapter 38.) Excessive lateral or external tibiaI torsion defor¬
mities are associated with increased Q angles and recurrent
patellar dislocations [15,30]. Skeletal malalignments in the
lower extremity can contribute to abnormal loading patterns
anywhere in the lower extremity , and clinicians should con¬
sider tibiaI torsion when assessing skeletal alignment of the
lower extremity.

Fibula

The fibula is a long, thin bone extending from just distal to the
knee to the ankle joint and contains a head, shaft, and lateral
malleolus (Fig. 44.2). The fibula provides muscle attachment
in the leg and also participates in the ankle articulation allow¬
ing complex movements of the foot.

HEAD OF THE FIBULA

The head of the fibula is slightly enlarged, with a medial artic¬
ular facet for the tibia. The apex of the fibular head projects
proximally and, with the head, is readily palpable distal to the
knee joint. The head of the fibula provides attachment for the
biceps femoris tendon of the hamstrings and the lateral col¬
lateral ligament of the knee and thus plays a role at the knee.
The peroneal nerve lies close to the posterior aspect of the
fibular head and can be compressed against the fibula by
restrictive structures such as a tight cast.

SHAFT OF THE FIBULA

The shaft of the fibula comprises three surfaces, an anterior
one that gives rise to the extensor muscles of the foot, a lateral
surface providing attachment for the peroneal muscles, and
the largest surface, the posterior surface, where the flexor
muscles gain attachments. The shaft is palpable distally only
for a few centimeters as it blends with the lateral malleolus.

LATERAL MALLEOLUS

The fibula ends in an expansion projecting distally and poste¬
riorly. Its medial surface provides an articular surface for the
talus and is convex from superior to inferior. The plane of the
articular surface of the lateral malleolus is oriented laterally
and inferiorly so that some of the load through the ankle can
be shared by the fibula [74,155,172]. The lateral malleolus is
easily palpated anteriorly, posteriorly, laterally, and distally.

Clinical Relevance

FRACTURES OF THE DISTAL TIBIA AND FIBULA: The

distal tibia and fibula are second only to the distal radius in
their frequency of fractures. Collectively known as Pott's
fractures ; they frequently result from a sprained ankle pro¬
ducing an avulsion fracture in which the stretched liga¬

ment or tendon applies a tensile force on the bone causing
it to fail. Fractures of the distal tibia and fibula also result
from shear forces that slide the talus along on the surface of
the tibia or fibula.

Tarsal Bones

The foot is joined to the leg by a complex organization of
bones that allow considerable mobility while still ensuring
adequate stability for weight bearing and ambulation. The
tarsal bones show considerably more variation among them¬
selves than do the carpal bones of the hand.

TALUS

The talus joins the foot to the leg, which helps account for its
irregular shape. It is an unusual bone lacking any direct mus¬
cle attachments [150,191]. Articular cartilage covers more
than half its surface with articular facets on its superior, infe¬
rior, medial, lateral, and anterior surfaces. Consequently,
movement of the talus is governed by the forces applied to it
by proximal bony attachments, the tibia and fibula, and its dis¬
tal articulation with the calcaneus.

The talus consists of a large, proximal body and a distal
head, with a neck joining the two parts (Fig. 44.5). The body
of the talus articulates with the tibia superiorly and medially,

Figure 44.5: The talus and calcaneus compose the hindfoot and
possess reciprocal facets for their articulation to each other.

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

811

with the fibula laterally, and with the calcaneus inferiorly. The
superior, or dorsal, surface of the body, also known as the
talar dome, or trochlea, is trochlear in shape, convex in an
anterior-posterior direction, and concave in a medial-lateral
direction to fit congruently with the distal surface of the tibia.
The anterior aspect of this superior surface is slightly wider
than the posterior aspect [142,171,191]. In addition, the lat¬
eral ridge, or condyle, of the trochlea is slightly larger than the
medial ridge, or condyle [73,171]. This asymmetry in the
medial and lateral aspects of the articular surface helps to
explain the motion of the ankle, which occurs in an oblique
plane close to the sagittal plane.

The medial and lateral surfaces of the talar body are con¬
tinuous with the superior surface and provide articular sur¬
faces for the medial and lateral malleoli, respectively. These
surfaces roughly parallel the articular surfaces of the respec¬
tive facets on the malleoli [171]. The inferior, or plantar, sur¬
face of the body of the talus has a large posterior facet for
articulation with the posterior facet on the superior surface of
the calcaneus. The body of the talus is grooved posteriorly
and medially by the tendon of the flexor hallucis longus.

The head of the talus is a smoothly curved, convex surface
projecting distally for articulation with the navicular. The head
is almost entirely covered with articular cartilage. Three facets
mark the articular surface on the plantar aspect of the talar
head. The posterior and largest of the three provides articula¬
tion with the sustentaculum tali of the calcaneus. Another
facet lying anterior and lateral to the posterior facet also artic¬
ulates with the calcaneus. The third facet, positioned medial¬
ly, rests on the calcaneonavicular ligament.

The neck of the talus joins the head to the body of the
talus. Both the neck and head project inferiorly and medially
from the body of the talus, contributing to the contour of the
medial longitudinal arch of the foot. The neck is roughened
on its dorsal and plantar surfaces by ligamentous attachments.
The sulcus tali is a prominent groove on the medial side of the
plantar surface that, together with the calcaneus, forms the
sinus tarsi.

The head and neck of the talus are readily palpated. The
medial aspect of the head can be palpated just proximal to the
tubercle of the navicular, particularly with the foot pronated
[77]. The clinician finds the neck of the talus medially
between the tendons of the anterior and posterior tibialis ten¬
dons and laterally just medial to the sinus tarsi [178].

CALCANEUS

The calcaneus, the largest of the tarsal bones, serves impor¬
tant functions in the foot. As the “heel bone,” it sustains large
impact forces at heel contact in locomotion; it provides a long
moment arm for the tendo calcaneus (Achilles tendon), thus
sustaining large tensile forces; and it transmits the weight of
the body from the hindfoot to the forefoot.

The calcaneus can be divided into three segments: poste¬
rior, middle, and anterior. A large posterior facet that articu¬
lates with the posterior facet on the talus covers the superior
surface of the middle segment. The superior surface of the

anterior segment possesses two facets, a middle and an ante¬
rior facet, which frequently communicate with one another.
The middle facet covers the superior surface of a palpable
shelf, the sustentaculum tali, which projects from the medial
surface of the calcaneus and supports the head of the talus.
The small, anterior facet also supports the talar head. A deep
groove, the sulcus calcanei, separates the posterior and mid¬
dle facets on the superior surface of the calcaneus. This
groove combines with the corresponding groove on the plan¬
tar surface of the talus to form the sinus tarsi.

Clinical Relevance

SINUS TARSI: The sinus tarsi is a depression palpated read¬
ily on the lateral aspect of the dorsum of the foot. The neck of
the talus and the anterior talofibular ligaments are palpated
within the sinus tarsi. Tenderness within the sinus tarsi may
indicate an injury to either of these structures. The sinus tarsi
also contains a venous plexus that is frequently torn in a
sprained ankleproducing the almost instantaneous golf
ball-sized swelling that appears after an acute sprain.

The posterior third of the calcaneus serves to lengthen the
moment arm of the Achilles tendon that attaches to the pos¬
terior surface of the bone. The distal aspect of the posterior
surface continues onto the plantar surface and is the only por¬
tion of the calcaneus that contacts the ground during weight
bearing (Fig. 44.6). The plantar surface is marked by a cal¬
caneal tuberosity where intrinsic muscles and the plantar
aponeurosis attach. The lateral and medial surfaces of the
posterior third of the calcaneus are palpable and assist the cli¬
nician in identifying the alignment of the foot. The anterior
surface of the calcaneus contains a slightly curved, saddle-
shaped facet for articulation with the cuboid.

The calcaneus possesses a thin shell of cortical bone that
encloses a sparse but highly organized array of trabecular

Figure 44.6: In weight bearing, the calcaneus is aligned so that
only its posterior aspect contacts the ground and directly sustains
ground reaction forces.

812

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

bone [55]. The relatively sparse cancellous bone within the
calcaneus leaves space that is filled by blood. This high fluid
content helps the calcaneus to function as a hydrodynamic
shock absorber during impact [49,55].

Clinical Relevance

CALCANEAL FRACTURES: The calcaneus is the most com¬
monly fractured tarsal bone [55], which can produce signifi¬
cant impairments and functional disabilities [16,95,138,181].
Calcaneal fractures typically result from high-impact loading
such as in a motor vehicle accident or from a fall onto the
heels from a large height [16,196]. They frequently are
intraarticular fractures and occur by large compression loads
between the talus and calcaneus (Fig. 44.7). These fractures
are difficult to treat and often lead to significant disability.

NAVICULAR

The navicular is a crescent-shaped bone with a concave
posterior surface that is congruent with the head of the talus
(Fig. 44.8). Three relatively flattened facets for the three

Figure 44.7: Calcaneal fracture. The lateral radiograph shows a
fracture of the calcaneus in which the posterior facet is com¬
pressed into the body of the calcaneus. (Reprinted with permis¬
sion from Chew FS, Maldjian C, LEffler SG. Musculoskeletal
Imaqinq: A Teachinq File. Philadelphia Lippincott Williams &
Wilkins 1999)

Navicular

Medial cuneiform
Intermediate cuneiform
Lateral cuneiform

A. Dorsal

Calcaneus

B. Lateral

Calcaneus

cuneiform

navicular

C. Medial

Figure 44.8: The bones of the midfoot
include the navicular, cuboid, and three
cuneiform bones. A. Dorsal view. B. Lateral
view. C. Medial view.

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

813

cuneiform bones cover the navicular’s convex anterior surface.
The medial surface of the navicular ends in a prominent
tuberosity that is a useful landmark for clinicians, lying
approximately 2 or 3 cm distal to the medial malleolus and
anterior to the sustentaculum tali of the calcaneus.
Supination of the foot facilitates palpation of the tubercle of
the navicular [77]. The lateral surface of the navicular may
be nonarticular or may bear a small facet for articulation
with the cuboid. The dorsal and plantar surfaces are rough¬
ened for ligamentous attachments.

CUBOID

The cuboid is named for its six-sided shape. Its posterior sur¬
face is slightly curved for the saddle-shaped facet of the cal¬
caneus, and facets for the bases of the fourth and fifth
metatarsal bones flatten its anterior surface. Medially, it bears
a flattened facet for the lateral cuneiform and perhaps a small
facet for the navicular. The peroneus longus forms a groove
on the lateral and plantar surfaces, which meet at the tuberos¬
ity of the cuboid occasionally palpable on the plantar surface
of the foot.

THREE CUNEIFORM BONES

The three cuneiform bones help form the transverse arch of
the foot. The medial cuneiform, the largest of the three, is
slightly kidney-shaped and wider on its plantar aspect than its
dorsal surface. The middle (intermediate) and lateral
cuneiform bones are wedge-shaped, with the apex facing in a
plantar direction. The wedge shape of the middle and lateral
cuneiforms allows these bones to function as keystones to
assist in stabilizing the transverse arch of the foot. The
cuneiform bones possess articular facets on their proximal
and distal surfaces for articulation with the navicular bone
proximally and with the medial three metatarsal bones distally.
The medial and lateral cuneiform bones extend distally far¬
ther than the middle, forming a socket into which the base of
the second metatarsal fits. The cuneiform bones bear facets
on their medial and lateral surfaces for articulation with
one another and with the cuboid. The dorsal surfaces of the
cuneiform bones are palpable through the dorsal skin of the
foot but bear no readily identifiable landmarks.

Bones of the Digits

The bones of the digits are very similar to the bones of the fin¬
gers, consisting of five metatarsal bones and fourteen pha¬
langes (Fig. 44.9).

METATARSAL BONES

The metatarsal bones, like their counterparts in the hand, are
miniature long bones consisting of a base, shaft, and head.
The metatarsals are generally similar to one another, with a
few distinguishing characteristics that influence the mechan¬
ics and pathomechanics of the foot and toes. The metatarsal
of the great toe is shorter than the second or third metatarsals

Figure 44.9: The bones of the digits are the metatarsal and pha¬
langeal bones of the five toes. A. Dorsal view. B. Lateral view of
the bones of the fifth toe.

and is the thickest of all the metatarsal bones. Applying
Wolffs law, which states that a bone’s structure responds to its
function (Chapter 3), the robust circumference of the great
toe’s metatarsal suggests that the bone is specialized to sustain
large loads, such as those generated in bipedal ambulation.
The ground reaction force on the foot progresses through the
medial side of the foot during gait, applying large forces on
the great toe.

The second metatarsal is the thinnest and longest of all the
metatarsal bones although it projects distally approximately
the same distance as the first and third metatarsals. The
metatarsal to the second toe extends farther proximally and is
securely wedged in by the three cuneiform bones and by the
first and third metatarsal bones. The metatarsal of the fifth
toe also projects proximally and forms a palpable tuberosity
that provides attachment for the peroneus brevis tendon.

Clinical Relevance

METATARSAL LENGTH: In the normal foot the metatarsal
heads of the medial three toes lie approximately in the same
frontal plane. Some individuals have an unusually short first

814

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

metatarsal bone or ; conversely , a long second metatarsal
bone. This produces uneven stress on the distal ends of the
metatarsals; particularly as the body rolls over the foot dur¬
ing walking and running. The increased stress may produce
pain and disability as the individual has difficulty rolling
evenly over the metatarsals.

The bases of all five metatarsal bones are similar to each
other. Unlike its counterpart in the thumb that has a saddle-
shaped base, the metatarsal of the great toe has flattened
facets on its base similar to the bases of the other metatarsals.
These facets provide articular surfaces for the cuneiform and
cuboid bones and for adjacent metatarsal bones.

The heads of the metatarsals of the lateral four toes are
quite similar to each other and to the metacarpals of the fin¬
gers. They are biconvex, with an articular surface that is con¬
tinuous on the plantar, distal, and dorsal surfaces. The head of
the first metatarsal is larger than those of the other
metatarsals and is grooved medially and laterally on the plan¬
tar surface by sesamoid bones that improve the mechanical
advantage of muscles of the great toe and protect the surface
of the metatarsal head. The metatarsal heads are readily pal¬
pated on the plantar surface of the foot, where they form the
“ball” of the foot. In normal upright standing, all five
metatarsal heads contact the floor [56].

PHALANGES

The phalanges are very similar to, albeit shorter than, the pha¬
langes of the fingers. There are three in each of the lateral four
(lesser) toes and two in the great toe. Each has a proximal base
and a distal head. The bases of the proximal phalanges are
biconcave to accommodate the heads of the metatarsals. The
bases of the middle and distal phalanges possess a central
ridge to fit the trochlear-shaped heads of the proximal and
middle phalanges.

Structural Organization of the Foot

The foot can be described by functional units—typically
the hindfoot, midfoot, and forefoot —although some
authors include the midfoot in the forefoot. The hindfoot
consists of the talus and calcaneus, and the remaining
tarsal bones compose the midfoot [129]. The forefoot is
composed of the metatarsals and phalanges. The digits
also can be described in motion segments, known as rays
(Fig. 44.10). The first ray includes the metatarsal of the
great toe and the medial cuneiform bone [44]. The sec¬
ond and third rays contain the second and third
metatarsals, respectively, and their proximal cuneiform
bones. The fourth and fifth rays consist only of the
fourth and fifth metatarsal, respectively.

Figure 44.10: The functional units of the foot consist of the first
ray, including the metatarsal of the great toe and the medial
cuneiform; the second ray, composed of the second metatarsal and
the middle cuneiform; the third ray, composed of the third
metatarsal and the lateral cuneiform; and the fourth and fifth rays,
consisting of only the fourth and fifth metatarsals, respectively.

JOINTS AND SUPPORTING STRUCTURES
OF THE LEG AND FOOT

The movement of the foot with respect to the leg is the sum of
motions among the tibia, the fibula, the tarsal bones, and the
metatarsals. To appreciate the contributions of each joint, it is
important to understand the movements of which the foot is
capable. Terminology describing foot motion is both confusing
and inconsistent, although consistency within the clinical and
biomechanical literature is beginning to emerge. Figure 44.11
presents the axes and motions that operationally define the
motions of the foot. Dorsiflexion and plantarflexion occur
in the sagittal plane about a medial-lateral axis. Eversion and
inversion occur in the frontal plane about the long axis of the
foot, which lies within the second metatarsal of the foot.
Abduction and adduction occur in the transverse plane
about a longitudinal axis through the leg, typically described as
parallel to the long axis of the tibia [21,85].

Although the motions of the foot and ankle are defined
operationally by motions within the cardinal planes of the
body, the actual motions of the ankle and foot occur about

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

815

Figure 44.11: Dorsiflexion and plantarflexion occur about a medial-
lateral axis; eversion and inversion occur about a long axis
through the foot; and abduction and adduction occur about a
long axis through the tibia.

axes that lie oblique to the cardinal planes. Consequently,
these joints exhibit motions that occur outside the cardinal
planes but pass through all three cardinal planes. As a result,
motions of the foot are described as triplanar motions
[143,193].

Since the motions of the foot are triplanar, they can be
described as the sum of individual motions in all three planes,
even if those motions cannot occur independently. The most
typical motions exhibited by joints of the ankle and foot com¬
bine either dorsiflexion, eversion, and abduction or plan¬
tarflexion, inversion, and adduction. These motions are known
as pronation and supination, respectively [85] (Table 44.1).
Clinicians are cautioned that inconsistency in terminology
within the literature creates considerable confusion. Although

TABLE 44.1: Terminology Convention for Triplanar
Motion of the Ankle and Foot

Pronation

Supination

Sagittal plane component

Dorsiflexion

Plantarflexion

Frontal plane component

Eversion

Inversion

Transverse plane component

Abduction

Adduction

the terminology described here is becoming the norm, some
authors continue to use other conventions. Table 44.2 provides
variations in terminology that are found in the literature.

Joints and Supporting Structures
between the Tibia and Fibula

The tibia and fibula are joined proximally and distally at the
proximal and distal tibiofibular joints (Fig. 44.12).

PROXIMAL TIBIOFIBULAR JOINT

The proximal tibiofibular joint is a gliding, synovial joint sup¬
ported by a synovial joint capsule and reinforced by anterior
and posterior ligaments to the head of the fibula. The
interosseous membrane supports both the proximal and dis¬
tal tibiofibular joints. Although in close proximity to the knee
joint, the joint functions primarily with the distal tibiofibular
joint to accommodate the rotation of the tibia during knee
motion and the triplanar motion of the foot [74,137]. Loss of
the proximal tibiofibular joint through fibular head resection
typically results in little residual dysfunction. Consequently,
the fibular head is regarded as a good harvest site for articu¬
lar cartilage to use in osteochondral autografts [33].
Osteochondral autografts are used to repair small localized
defects in articular cartilage.

DISTAL TIBIOFIBULAR JOINT

The distal tibiofibular joint is a fibrous joint, or syndesmo¬
sis. The primary support of the distal tibiofibular joint is the
interosseous ligament that is an extension of the interosseous
membrane. It also is supported by the anterior and posterior
tibiofibular ligaments as well as by the interosseous mem¬
brane. The medial collateral ligament of the ankle provides
additional support to the joint. The distal tibiofibular joint
provides essential stability to the ankle joint. Any instability
at the distal tibiofibular joint may lead to chronic ankle dys¬
function [182].

TABLE 44.2: Variation in Terminology Describing Ankle and Foot Pronation

This Textbook

Inman [70]

Alternative Terminology
found in the Literature

Sagittal plane component

Dorsiflexion

Frontal plane component

Eversion

Eversion or pronation (used interchangeably)

Pronation [96]

Transverse plane component

Abduction

External rotation [90,96,194]

816

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Clinical Relevance

HIGH ANKLE SPRAINS: Sprains of the distal tibiofibular
joint are known as "high ankle sprains." They occur typically
in athletes participating in running sports that require quick
cuts and turns; including American football soccer ; and
skiing. Unlike typical inversion ankle sprains; high ankle
sprains more often occur with ankle/foot abduction, ever¬
sion, and/or dorsiflexion, that is, with pronation. High
ankle sprains, although less common than inversion ankle
sprains, typically require considerably more recovery time.

Figure 44.12: The proximal and distal tibiofibular joints are sup¬
ported by anterior and posterior ligaments at both joints, by the
interosseous membrane, and distally by a synovial capsule and
interosseous ligament. Both joints allow translation and rotation
about a long axis through the fibula. The distal tibiofibular joint
allows translation superiorly and interiorly, anteriorly and poste¬
riorly, and medially and laterally. In addition, the fibula rotates
laterally about its own long axis.

MOTION OF THE TIBIOFIBULAR JOINTS

The tibiofibular joints function together just as the radioulnar
joints and temporomandibular joints do. The motion available
between the tibia and fibula is quite limited, allowing slight
rotation of the fibula about a longitudinal axis as well as slight
proximal-distal and medial-lateral translations [74,137,155].
The importance of motion at the tibiofibular joints has been
debated, leading to differing treatment approaches to the treat¬
ment of distal fibular fractures. Surgical screws that cross the
distal tibiofibular joint ensure an anatomically accurate reduc¬
tion of the fracture but limit the mobility of the fibrous joint.

One aspect of the controversy surrounding tibiofibular
motion focuses on the width of the superior surface of the
talus. As noted earlier, the anterior aspect of the superior sur¬
face is slightly wider than the posterior surface. Ankle dorsi-
flexion moves the anterior aspect of the talar articular surface
into the cavity formed by the distal tibia and fibula. Ankle
plantarflexion inserts the thinner posterior aspect into the
cavity. The varying widths of the talus in contact with the dis¬
tal tibia and fibula have suggested that the tibia and fibula
must separate during dorsiflexion and has led to the assump¬
tion that restricted tibiofibular joint motion limits dorsiflexion
ROM. Extensive anatomical analysis suggests that the mortise
spreads only slightly, if at all, during ankle dorsiflexion but
that the fibula does rotate laterally slightly during dorsiflexion

[26.74.122] . Studies report minimal or no decrease in dorsi¬
flexion ROM in ankles with immobile distal tibiofibular joints

[11.122] . Yet some patients with no motion available at the
distal tibiofibular joint report pain and functional limitations.
A study using cadaver specimens demonstrates a decrease in
contact area between the talus and tibia when the distal
tibiofibular joint is surgically immobilized [128]. Perhaps
some of the complaints reported by patients with restricted
tibiofibular joint mobility arise as a result of increased joint
stress (force/area) at the ankle joint.

Clinical Relevance

MOBILIZATION OF THE DISTAL TIBIOFIBULAR JOINT:

The mobility of the tibiofibular joint can become restricted
during immobilization of the ankle, even with no direct
ankle pathology. Gentle remobilization of the distal
tibiofibular joint is a frequently applied intervention to
relieve pain and improve function. Individuals whose activ¬
ities demand large ankle and foot mobility may require
increased tibiofibular joint mobility. But even in individuals
who are moderately sedentary, increased mobility of the
distal tibiofibular joint may increase functional ability by
restoring the normal contact area between the tibia and
talus, thereby decreasing joint stress and increasing com¬
fort during weight-bearing activities. Well-controlled out¬
come studies are needed to verify the clinical value of such
intervention.

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

817

Joints of the Foot

The movement of the foot on the leg reflects motion at the
ankle, intertarsal joints, and tarsometatarsal joints and is
affected by the motion of the toes. Weight-bearing activities
require motion of the foot on the leg or the leg on the foot
and thus involve virtually all of the joints of the foot. An
understanding of the motion of the foot requires an under¬
standing of each joint individually but also the interaction that
occurs among the joints of the foot.

STRUCTURE AND SUPPORTING ELEMENTS
OF THE ANKLE JOINT

The ankle joint consists of articulations between the talus and
the tibia and fibula, which, bound together at the distal
tibiofibular joint, form a cavity, or mortise, for the talus. The
primary articular surface is on the superior surface of the talus
and on the distal surface of the tibia. The anterior joint line is
palpated 1 or 2 cm proximal and anterior to the tip of the
medial malleolus [57].

The participating articular surfaces of the ankle possess
very similar, although not identical, curvatures [91,171]. This
congruity helps stabilize the ankle, contributes to a relatively
simple, hingelike motion, and increases the contact area of
the joint to decrease the joint stress [173]. Data from cadaver
experiments suggest that the articular surfaces themselves
contribute the primary stabilizing force for inversion and
eversion when the ankle is weight bearing [173,179].

Despite the congruent joint surfaces of the ankle, the con¬
tact area on the talus changes and moves with joint loads and
joint position [14,29,74]. With the ankle in the neutral posi¬
tion, loading occurs on the superior aspect of the talus. The
area of contact increases from approximately 10% to approx¬
imately 15% with loads of 490 N (110 lb) and 980 N (220 lb),
respectively, as the articular cartilage deforms more with
larger loads [14]. The area of contact moves anteriorly in dor-
siflexion and posteriorly in plantarflexion. Similarly, inversion
moves contact to the medial aspect of the talus and to the tib-
ial facet, while eversion moves the contact to the lateral
aspect of the talus and to the fibula [14,29]. The articular car¬
tilage of the ankle is thinner and stiffer than the cartilage
found at the knee and hip [158,159]. The ankles inherent
congruency may diminish the need for thick articular carti¬
lage, and the cartilage s stiffness may help protect the ankle
from degenerative joint disease.

Clinical Relevance

OSTEOARTHRITIS OF THE ANKLE: Despite its role in
weight bearing , the ankle joint rarely develops osteoarthritis
spontaneously [65,195]. On the other hand , once there is
trauma that alters the joint alignment joint degenerative
changes almost always follow [145,195]. Changes in the

relative alignment of the talus; tibia; and fibula produce
large changes in the contact areas and stresses between the
articular surfaces during weight bearing; probably contribut¬
ing to the development of degenerative changes.

A synovial capsule and collateral ligaments provide non-
contractile support to the ankle joint. The joint capsule is
characterized by numerous pleats anteriorly and posteriorly
that fold and unfold to allow the relatively free motion of dor-
siflexion and plantarflexion [57,142] (Fig. 44.13). The medial
and lateral collateral ligaments reinforce the capsule. Both
the medial and lateral collateral ligaments consist of three
major bands, with additional smaller and more variable com¬
ponents as well [113,114] (Table 44.3).

The medial collateral, also known as the deltoid, ligament
is larger than the lateral ligament and contains deep and

Figure 44.13: A. The lateral collateral ligament reinforces the
capsule laterally and consists of the anterior talofibular, calcane-
ofibular, and posterior talofibular ligaments. B. The deltoid liga¬
ment consisting of the superficial tibiospring and tibionavicular
ligaments and the deep posterior tibiotalar ligament reinforces
the ankle joint capsule medially. Both collateral ligaments may
contain additional fibers not shown here.

818

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

TABLE 44.3: Components of the Collateral Ligaments of the Ankle [113,114]

Medial Collateral (Deltoid) Ligament

Lateral Collateral Ligament

Primary ligamentous bands

Tibiospring (superficial)

Anterior talofibular

Tibionavicular (superficial)

Calcaneofibular

Posterior tibiotalar (deep)

Posterior talofibular

Additional variable fibrous bands

Posterior tibiotalar (superficial)

Lateral talocalcaneal

Tibiocalcaneal (superficial)

Posterior intermalleolar

Anterior tibiotalar (deep)

superficial portions. Of the primary bands of the medial col¬
lateral ligament, the deep (posterior tibiotalar) segment runs
from tibia to talus, providing direct support to the tibiotalar
joint, while the superficial fibers (tibiospring and tibionavicu¬
lar) extend from the tibia to the navicular and calcaneus,
affecting the subtalar articulation as well as the ankle. The lat¬
eral collateral ligament also consists of major bands that sup¬
port the ankle joint directly (the anterior and posterior
talofibular ligaments) and a band that crosses both the ankle
and subtalar joints (the calcaneofibular ligament).

The collateral ligaments, along with the articular surfaces,
help stabilize the ankle and subtalar joints and guide the
motion of the ankle [90]. Like other collateral ligaments at the
knee and elbow, the precise role that each ligament plays is
complex, and controversies remain. However, the ligaments
appear to function primarily to limit extremes of movement
[73,179]. The deltoid ligament helps support the medial side
of the ankle and subtalar joint against laterally directed forces
on the foot (valgus stresses), while the lateral collateral lig¬
ament protects these joints from medially directed forces
(varus stresses) on the foot. The specific contributions made
by each ligamentous segment depends on the position of the
ankle [18,101,139,172].

Stability of the ankle joint often is described in terms of the
anterior, posterior, medial, and lateral translation, or shift, of
the talus within the mortise and by the amount of medial or
lateral talar tilt about an anterior-posterior axis, which occurs
when force is applied (Fig. 44.14). The deltoid ligament is

Figure 44.14: Stability of the ankle joint is assessed by examining
(A) the talar tilt, which is medial or lateral rotation of the talus
about an anterior-posterior axis, and (B) talar shift, which is the
translation of the talus in a medial or lateral direction.

positioned to limit lateral tilt and lateral shift of the talus. Some
studies report that lateral talar tilt increases significantly when
the entire deltoid ligament is cut [52,172,173], while others
report a small or inconsistent increase [29]. The lateral malle¬
olus and lateral supporting structures also appear to provide
important limits to lateral talar shift by acting as a buttress
against the movement [26,52]. The lateral collateral ligament,
especially the anterior talofibular and calcaneofibular liga¬
ments, prevent excessive medial tilt of the talus [6,8,38].

Anterior glide of the talus is limited by the lateral malleo¬
lus and lateral collateral ligaments and by the deltoid liga¬
ment, although the lateral supporting structures appear to be
primary [22,80]. Posterior glide of the lateral malleolus is lim¬
ited primarily by the posterior talofibular and calcaneofibular
ligaments [53]. Plantar- and dorsiflexion alter the tension
within the individual components of the collateral ligaments.
Anterior glide of the talus is greatest with the ankle close to
neutral and is more restricted when the ankle is either dorsi-
flexed or plantarflexed [22,23]. Plantarflexion stretches the
anterior talofibular component of the lateral collateral liga¬
ment and the anterior tibiotalar and tibionavicular portions of
the deltoid ligament; dorsiflexion relaxes these same liga¬
ments [5,18,101,125,139,168]. Most investigators report that
dorsiflexion stretches the calcaneofibular ligament while
plantarflexion puts it on slack [5,18,125,139,172]. Others,
however, suggest that the ligament maintains an almost con¬
stant length through the range of plantar- and dorsiflexion
and thus provides lateral stability to the ankle regardless of
position, helping to guide the ankle motion through the entire
ROM [90,101,168]. Clinicians assessing the stability of the
ankle and the integrity of surrounding ligaments are advised
to maintain a consistent ankle position when performing any
mobility tests to ensure reliable test results.

ANKLE JOINT MOTION

The ankle joint basically functions as a hinge joint rotating
about an axis that lies close to the malleoli [73,163]. However,
several biomechanical studies determine that the precise axis
of rotation varies throughout the ankles ROM [14,100,
147,161]. These studies demonstrate that slight translation
accompanies the ankles rotation. The tibia translates anteri¬
orly during dorsiflexion and posteriorly during plantarflexion.
Such translation helps to explain the change in contact area
between the tibia and talus that occurs during plantar- and
dorsiflexion [14]. Two-dimensional analysis reveals that trans¬
lation of the tibia produces a change in the instant center of
rotation (ICR) of the ankle joint so that the ICR moves

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

819

posteriorly with plantarflexion, anteriorly with dorsiflexion,
medially with inversion, and laterally with eversion. (Chapter
7 discusses ICR in more detail.)

Clinical Relevance

MANUAL THERAPY OF THE ANKLE JOINT: Gentle
mobilization consisting of anterior and posterior glides of the
talus using manually applied pressure form a common inter¬
vention for treatment of a stiff ankle. This treatment is
consistent with the goal of restoring the normally occurring
translation during ankle dorsiflexion and plantarflexion. A ran¬
domized clinical trial of anterior-posterior joint mobilizations
of the ankle demonstrates better outcomes with mobilization
intervention than with treatment without mobilization [47].

Ankle dorsiflexion and plantarflexion also are accompanied
by talar rotation and fibular glide and rotation [63,90,97,
110,172]. Studies suggest that the talus and fibula both rotate
laterally with respect to the tibia as the ankle dorsiflexes. This
movement is consistent with the shape of the talus. The lat¬
eral condyle of the talus is slightly larger than the medial
condyle, producing lateral rotation of the talus during its pos¬
terior rotation during dorsiflexion. These data demonstrate
that although the ankle joint is considered a classic hinge
joint, its motion is considerably more complex. In addition,
the talus reportedly rocks medially and laterally within the
mortise, contributing one third of the supination and prona¬
tion of the hindfoot [62,175].

The preceding description of ankle motion reveals the
ankle exhibits three-dimensional motion and six degrees of
freedom, with rotations about and translations along medial-
lateral, anterior-posterior, and longitudinal axes [162,192].
This complexity of joint motion may contribute to the limited
success that engineers and surgeons have had in developing
successful total joint arthroplasties of the ankle.

Despite the translations and rotations of the talus and the
variability of the axes reported during ankle motion, for clini¬
cal measurements of ankle ROM, the use of an axis that pass¬
es through the two malleoli appears to be a valid simplification
to measure motion. This single axis of the ankle is oblique,
passing from medial to lateral in a posterior and inferior direc¬
tion [73] (Fig. 44.15). The obliquity of the ankle joint axis pro¬
duces ankle motion that takes place in a plane perpendicular
to the joint axis rather than in any of the cardinal planes of the
body [73,78,193]. When the foot rotates in an upward direc¬
tion about this axis, the obliquity causes the foot to move
slightly laterally with respect to the leg and to rotate the plan¬
tar surface of the foot laterally. Using the terminology
described earlier for leg-foot motions, the ankle dorsiflexes,
abducts, and everts; in other words, the foot pronates. It is use¬
ful to note that the foot could achieve this same position by

Figure 44.15: To measure the ROM of the ankle, a simplified axis
is assumed that runs laterally, interiorly and posteriorly from the
medial to the lateral malleolus.

using a ball-and-socket joint that allows rotation about three
separate axes. However, a ball-and-socket joint requires a
more complex musculoligamentous system for support and
control. Although ankle motion combines dorsiflexion, abduc¬
tion, and eversion or plantarflexion, adduction, and inversion,
the ankle s motion occurs very close to the sagittal plane, and
consequently, the ankle s motion consists mostly of dorsiflexion
and plantarflexion [161,192].

Range of Motion of the Ankle

Passive and active ROMs reported in the literature are found
in Table 44.4. These values show considerable variability,
attributable at least in part to the magnitude of the force
pushing the joint to end range [117]. The largest dorsiflexion
range is reported in a study in which the subjects stood and
used full body weight to reach end range [141]. Other studies
use only manual pressure to achieve the end range. Data sug¬
gest that women exhibit greater ankle ROM than men

[3.124.184] . Reports disagree regarding the effects of age on
ankle ROM in adults, but comparing data across studies sug¬
gests that ROM decreases with increased age in adults

[10.124.147.184] . Children exhibit more ankle ROM than
adults [10,50,124].

STRUCTURE AND SUPPORTING ELEMENTS
OF THE SUBTALAR JOINT

The subtalar joint is defined functionally as the joint formed by
all three articulating facets of the calcaneus and the matching
facets on the talus [129,150,183], although anatomy textbooks
often refer only to the articulation between the posterior

820

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

TABLE 44.4: Reported Passive and Active Ankle ROM

Dorsiflexion with Knee Flexed (°)

Plantarflexion (°)

American Academy of Orthopaedic Surgeons [48] a

Gerhardt and Rippstein [41P

Roass and Andersson [141 ] c

15 ± 5.8

40 ± 7.5

Astrom and Arvidson [3] d

43 ± 7

49 ± 7

Walker et al. [184] e

10 ± 5

34 ± 8

Boone and Azen [10f

12.2 ± 4.1

54.3 ± 5.9

a Passive ROM.
b Active ROM.

c Mean and standard deviation of passive ROM in 190 ankle joints in men aged 30 to 40 years.

d Mean and standard deviation of 121 adults with a mean age of 35 years. Measured with the subjects in weight bearing.

e Mean and standard deviation of active ROM in 30 men and 30 women aged 75 to 84 years.

f Mean and standard deviation of active ROM in 56 males aged 20 to 54 years.

facets of the calcaneus and talus as the subtalar joint, includ¬
ing the anterior and middle facets in the talocalcaneonavicular
joint [142,191]. This textbook uses the functional definition of
the subtalar joint including all articulations between the calca¬
neus and talus.

The subtalar joint is critical to bipedal ambulation and acts
to translate the motion of the tibia to the foot or, conversely,
to translate the rotation of the foot to the tibia. This transla¬
tion allows humans to walk smoothly over uneven surfaces
and to pivot on one foot, rapidly changing the direction of
progression (Fig. 44.16). To accomplish this role, the subtalar
joint must allow apparently complex motion while remaining
stable during weight bearing.

The larger posterior articulation between the talus and cal¬
caneus is saddle-shaped, allowing slight three-dimensional

Figure 44.16: Motion of the subtalar joint allows an individual to
pivot or accommodate to uneven ground.

motion, while the anterior facets are flatter, allowing gliding
motion. The bony configuration of the joint provides inherent
stability to the joint that is amplified by strong reinforcing lig¬
amentous structures. Two joint capsules provide initial sup¬
port to the subtalar joint, one surrounding the large posterior
facet, and another surrounding the anterior and middle facets
(Fig. 44.17). This latter capsule also surrounds the articulation
between the talus and navicular. Two thickenings reinforce
the posterior capsule, the medial and lateral talocalcaneal
ligaments [183]. The two capsules of the subtalar joint are
adjacent to one another in the sinus tarsi. These adjacent cap¬
sular surfaces blend together and are reinforced, forming the
thick interosseous ligament (Fig. 44.18). A cervical ligament
attaches to the calcaneus and talus at the lateral border of the
sinus tarsi. The interosseous ligament is an important support

Articular surface

for cuboid

Anterior facet

Figure 44.17: The subtalar joint contains two joint capsules, one
surrounding the posterior facets of the talus and calcaneus and
the other surrounding the anterior and middle facets of the talus
and calcaneus as well as the proximal articulating surface of the
navicular.

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

821

Cervical ligament

Figure 44.18: The interosseous ligament of the subtalar joint
is formed by a thickening of the adjacent walls of the sub¬
talar joint capsules and lies deep in the sinus tarsi. More
superficially in the sinus tarsi lies the cervical ligament of the
subtalar joint.

of the subtalar joint, and its effectiveness is unaltered by ankle
or subtalar joint position [101,170]. It appears to restrict
supination more than pronation [89]. The cervical ligament
also provides important support, preventing excessive supina¬
tion [170].

The collateral ligaments of the ankle also contribute impor¬
tant support to the subtalar joint, preventing excessive motion.
Cadaver measurements reveal stretch of the posterior tibiota-
lar, tibiocalcaneal, and tibionavicular components of the del¬
toid ligament with eversion of the foot [101] (Fig. 44.19). The
lateral collateral ligament limits inversion of the ankle and
subtalar joints [18,139,170]. The calcaneofibular component
of the lateral collateral ligament provides strong limits to
inversion throughout plantar- and dorsiflexion of the ankle,
while the anterior talofibular ligament limits inversion most
effectively when the ankle is plantarflexed [58,106]. Tests of
the strength of the lateral collateral ligament reveal that the
anterior talofibular component exhibits the smallest load to
failure, with estimates of average peak loads to failure ranging

Figure 44.19: A. Eversion of the ankle and hindfoot is limited by
the deltoid ligament. B. Inversion is resisted by the lateral collat¬
eral ligament and cervical ligament. The interosseous ligament
resists both eversion and inversion.

from 140 to 297 N (31.5-67 lb) [4,39]. Estimates of loads in
the calcaneofibular ligament range from 205 to 598 N
(46-134 lb).

Clinical Relevance

INVERSION ANKLE SPRAINS: Most ankle sprains occur
with the foot in inversion , straining the lateral structures of
the foot and ankle. The most common ligament injured in
an inversion sprain is the anterior talofibular ligament ,
stretched in combined ankle plantarflexion and subtalar
supination. Common mechanisms for such an injury are
landing on someone else's foot when jumping for a basket¬
ball or volleyball or tripping while descending stairs
(Fig. 44.20). In each instance ’ the foot typically is forced
rapidly and forcefully into plantarflexion and inversion ,
rupturing the weakest of all the ankle and subtalar joint
ligaments , the anterior talofibular ligament.

Motion of the Subtalar Joint Complex

Although the motion of the subtalar joint has been studied for
well over half a century, there continues to be debate about
the nature of its motion. Many authors describe it as a hinge
joint whose axis lies oblique to both the foot and the leg
[148,149,175]. Yet some biomechanical studies suggest that
motion at the subtalar joint occurs about multiple axes
[92,161,162,192].

822

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 44.20: Vigorous plantarflexion and inversion that occurs
by landing in a hole or tripping on a step is a classic mechanism
to produce an inversion sprain.

Clinical Relevance

SUBTALAR JOINT-A HINGE OR MULTI AXIAL JOINT?:

Clinicians may ponder the importance of determining the
precise number of axes about which the subtalar joint
rotates. The fact that engineers appear to be the avid inves¬
tigators of the question may suggest that the question is
esoteric and clinically irrelevant. Yet many patients with
rheumatoid arthritis have significant erosive damage to the
subtalar joint , making weight-bearing activities painful and
difficult. One treatment to improve function and decrease
pain is fusion of the subtalar joint , but a better understand¬
ing of the true biomechanical nature of the subtalar joint
may lead engineers and clinicians to develop joint arthro¬
plasties that preserve the function of the foot while eliminat¬
ing the pain of a diseased subtalar joint.

Despite the controversy regarding the precise axes of
motion at the subtalar joint, there is remarkable agreement
about the location and direction of the primary axis of the
subtalar joint. This axis lies within, and approximately parallel
to, the body of the calcaneus, projecting almost 45° from pos¬
terior to anterior and almost 25° medial to the long axis of the
foot [92,149] (Fig. 44.21). Investigators also agree that there
is considerable variability in these numbers among individu¬
als without pathology.

Figure 44.21: The mean standard deviation and range of orienta¬
tion of the subtalar joint axis are shown in the sagittal plane (A)
and in the transverse plane (B). The data reveal large intersubject
variability in the axis orientation. (Reprinted with permission from
Stiehl JB, ed. Inman's Joints of the Ankle. 2nd ed. Baltimore:
Williams & Wilkins, 1991.)

Regardless of whether the subtalar joint rotates about one
or multiple axes, it, like the ankle, rotates about axes that dif¬
fer from the orthogonal axes of the foot. Thus the subtalar
joint motion passes through the cardinal planes of the body
and is triplanar. Hinge motion of the subtalar joint is classi¬
cally compared to a mitered hinge (Fig. 44.22). The motion
of the subtalar joint consists of pronation and supination, with
unique contributions from the three components of each
motion. The specific orientation of the axes of the subtalar
joint defines the actual contributions of each motion. When
the primary axis lies closer to the long axis of the foot, inver¬
sion and eversion constitute the primary components of sub¬
talar motion, but when the axis is closer to the long axis of the
leg, the adduction and abduction contributions increase [121]
(Fig. 44.23). Studies show that the subtalar joint contributes
most of the inversion-eversion and adduction-abduction
motion of the hindfoot, while contributing minimally to plan¬
tarflexion and dorsiflexion [92,97-99,107,162].

Range of Motion of the Subtalar Joint

Table 44.5 presents the reported ROMs found in the litera¬
ture. ROMs for the subtalar joint typically are reported in
terms of the frontal plane components of supination and

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

823

Figure 44.22: The mitered-hinge model helps demonstrate the
role of the subtalar joint in transferring motion of the foot to
the leg or the motion of the leg to the foot.

pronation—namely, inversion and eversion—which are
determined by the angle formed by a line drawn along the
long axis of the leg and a line through the posterior aspect of
the calcaneus [48,126] (Fig. 44.24). The reported values of
subtalar ROMs exhibit considerable variability, and studies
report poor intertester reliability [12,131,164,180]. One study
demonstrates that the inversion ROM measurement depends

Long axis of leg
Abduction Adduction

Figure 44.23: When the axis of the subtalar joint lies closer to the
long axis of the foot, motion at the subtalar joint (STJ) consists
mostly of inversion and eversion. When the axis lies closer to the
long axis of the leg, the abduction-adduction component increases.

TABLE 44.5: Reported Ranges of Motion
of the Subtalar Joint (in degrees)

Inversion

Eversion

Gerhardt and Rippstein [41]

McPoil and Cornwall [108] a

18.7 ± 5.2

12.2 ± 4.0

Walker et al. [184p

30 ± 10

12 ± 6

Milgrom et al [111 ] c

32 ± 7.4

21.4 ± 5.4

3.9 ± 4.1

3.4 ± 3.1

Roass and Andersson [141 ] d

27.6 ± 4.6

27.7 ± 6.9

a Mean and standard deviation of ROM in 9 men and 18 women; mean age, 26.1
± 4.8 years.

fa Mean and standard deviation of active ROM in 30 men and 30 women aged 75
to 84 years.

cMean and standard deviation of passive ROM in 272 males aged 18 to 20 years.
c/Mean and standard deviation of passive ROM in 190 right subtalar joints in men
aged 30 to 40 years.

strongly upon the ankle position maintained during the meas¬
urement [111]. Clinicians are cautioned that ROM measures
of subtalar motion may have limited clinical use unless
acceptable reliability is established.

TRANSVERSE TARSAL JOINT

The transverse tarsal joint consists of the talonavicular and
calcaneocuboid joints. It is also known in some clinical litera¬
ture as Chopart’s joint.

Talonavicular Joint

The talonavicular joint consists of the well-curved talar head
articulating with the reciprocally concave posterior surface of
the navicular. The capsule that encloses the anterior articu¬
lation between the talus and calcaneus also supports the
talonavicular articulation. Additional supports include the talo¬
navicular ligament that crosses the dorsal surface of the joint,
the dorsal calcaneonavicular ligament (the medial branch of
the bifurcate ligament), and, most importantly, the plantar
calcaneonavicular, or spring, ligament (Fig. 44.25). The spring

Figure 44.24: Measurement of subtalar joint ROM is performed
by measuring the frontal plane component of pronation and
supination (eversion and inversion).

824

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

calcaneonavicular)

Figure 44.25: The primary supporting structure of the talonavicu¬
lar joint is the spring ligament.

ligament contains a fibrocartilaginous facet for the head of the
talus and acts as a sling for the talar head [25].

The substantial curvature of the talus and navicular
allows considerable mobility at the talonavicular joint. Like
most of the joints of the foot, motion at this joint is triplanar,
consisting of pronation and supination. Fewer studies exist
examining the specific motion of the talonavicular joint, but
available data reveal that it is quite mobile and contributes
significantly to leg-foot motion. Studies suggest that it con¬
tributes significantly to plantarflexion of the foot on the leg
[85,97,127]. Lundberg et al. report that approximately 12%
of the first 30° degrees of plantarflexion is attributable to
talonavicular joint motion [97]. The talonavicular joint also
exhibits substantial abduction or adduction and eversion or
inversion during pronation and supination [85,98,107,127].
The mobility of the talonavicular joint is similar to that of
the subtalar joint [97].

Calcaneocuboid Joint

The calcaneocuboid joint is a saddle joint supported by a
synovial joint capsule and by several reinforcing ligaments
(Fig. 44.26). Dorsally, the joint receives support from the
bifurcate ligament composed of the dorsal calcaneonavicular
and dorsal calcaneocuboid ligaments. On its plantar surface,
the joint is supported by two strong ligaments, the short and

Bifurcate ligament :

Calcaneonavicular

Figure 44.26: The supporting structures of the calcaneocuboid
joint consist of the bifurcate ligament and the long and short
plantar ligaments.

long plantar ligaments. The short plantar ligament (also
known as the plantar calcaneocuboid ligament) lies deep to
the long plantar ligament and travels from the anterior aspect
of the calcaneus to the plantar surface of the cuboid. It is a
very strong ligament that is an important support to the lateral
longitudinal arch of the foot [191]. The long plantar ligament
extends from the plantar surface of the calcaneus to the plan¬
tar surface of the cuboid and to the bases of the second
through fourth or fifth metatarsal bones, although its distal
attachment is variable [186]. This ligament also provides sub¬
stantial support for the lateral longitudinal arch.

Discrete motion of the calcaneocuboid joint is less well
studied than the other joints of the tarsus. Data from 10
cadaver specimens reveal significantly less motion at the cal¬
caneocuboid joint than at the talonavicular joint, with the lat¬
ter exhibiting more than twice the amount of pronation and
supination and three times the amount of dorsiflexion and
plantarflexion [127].

Motion of the Transverse Tarsal Joint

Some investigators report the motion of the transverse tarsal
joint as a whole [46,105,143]. Biomechanical analyses suggest
that the navicular and cuboid move as a unit [107]. When
considered as a whole, two axes of motion are described, a
longitudinal axis that is similar to the primary axis of the sub¬
talar joint and an oblique axis that is more similar to the axis
of the ankle [46,105] (Fig. 44.27). Regardless of whether one
considers the transverse tarsal joint as two separate joints or
as a single unit, data consistently demonstrate that the mid-
foot contributes to pronation and supination of the
LBjk foot- By considering the axes of the transverse tarsal
XX joint as a whole, the clinician is reminded that the
joints of the midfoot amplify the motion of the ankle and
hindfoot. Motion about the longitudinal axis of the transverse

Figure 44.27: Motion about the theoretical long axis of the trans¬
verse tarsal joint contributes more eversion and inversion of the
foot, while motion about the theoretical oblique axis of the
transverse tarsal joint contributes mostly dorsiflexion and plan¬
tarflexion of the foot.

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

825

tarsal joint contributes to the inversion and eversion motion
of the subtalar joint. Motion about the oblique axis adds to the
plantarflexion and dorsiflexion provided by the ankle joint.

Clinical Relevance

TRANSVERSE TARSAL JOINT MOTION: Together the
talonavicular and calcaneocuboid joints are quite mobile
and amplify the motion of the ankle and subtalar joints.
Dorsiflexion is a component of pronation and consequently,
pronation of the transverse tarsal joint can provide additional
"functional" dorsiflexion ROM in an individual with a flexible
midfoot (Fig. 44.28). However, the use of large midfoot motion
during activities that require dorsiflexion ROM such as squat¬
ting, jumping, walking, or running may lead to excessive stress
to structures on the medial side of the foot such as the posteri¬
or tibialis tendon, or to abnormal loads to the knee.

DISTAL INTERTARSAL JOINTS

The distal intertarsal joints include those between the navicu¬
lar and the cuneiform bones, between the cuboid and lateral

cuneiform, and among the cuneiform bones themselves.
These articulations are supported by joint capsules that fre¬
quently communicate with one another and by dorsal and
plantar ligaments that run between adjacent bones. Although
poorly studied, the motion appears to be limited to only a few
degrees but contributes to the pronation and supination of
the rest of the foot [96,127].

Clinical Relevance

TARSAL COALITION: Tarsal coalition is an abnormal
connection between tarsal bones, leading to decreased
mobility between the affected bones. The connections may
be bony, cartilaginous, or fibrous and may be partial or
complete. Because the joints among the tarsal bones pro¬
vide important amplification of the motion at the ankle
and subtalar joints, any loss of tarsal motion may lead to
excessive motion elsewhere, including the ankle and hind-
foot. A retrospective analysis of 223 acute ankle sprains
suggests that tarsal coalitions may be a risk factor for
ankle sprains [166].

A B

Figure 44.28: Functional dorsiflexion. The ankle is the primary source of dorsiflexion ROM while the joints of the hindfoot and midfoot
contribute slight additional dorsiflexion ROM (A). If the midfoot is too flexible, its excessive pronation provides substantial dorsiflexion
ROM (B).

826

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

TARSOMETATARSAL AND INTERMETATARSAL
JOINTS OF THE TOES

The tarsometatarsal joints of the toes, also known as the
Lisfranc’s joint, are gliding joints with limited mobility
[148,191]. The articulations are supported by joint capsules
that typically form three separate joint spaces, one enclosing
the articulation between the medial cuneiform and metatarsal
of the great toe, another enclosing the second and third
metatarsals with the middle and lateral cuneiform bones, and
the lateral joint space encircling the cuboid and the fourth
and fifth metatarsal bones. The synovial spaces also expand to
include the intermetatarsal joints of the metatarsals within
each joint capsule. Dorsal and plantar ligaments, the latter of
which are thick and strong to support the arches of the foot,
reinforce the joint capsules. The cuneometatarsal ligaments
of the great and second toes are particularly strong and
appear to provide the primary support to these joints [115].

The mobility of the tarsometatarsal joints varies across the
toes. There is considerable variability in the reported mobility
of the tarsometatarsal joint of the great toe. Studies of the
mobility in the sagittal plane report 0-4° of total excursion
[37,127,136,185]. Total linear excursion in the sagittal plane
of approximately 15 mm also is reported [44]. Reports suggest
that increased dorsal mobility of the first tarsometatarsal joint
accompanies hallux valgus deformities of the great toe and
may be associated with forefoot deformities [42,43,72].
Although some suggest that the first tarsometatarsal joint
allows motion in the frontal and transverse planes as well,
attempts to measure this motion are limited, and the available
data suggest that the joint exhibits less than 5° of frontal plane
motion [82,127,146,185]. Other motions appear to be negligi¬
ble [185]. The motions of the tarsometatarsal joint of the
great toe are small in the transverse and frontal planes, but
they combine with the great toe s sagittal plane motion differ¬
ently from the triplanar motions at the ankle, subtalar joint,
and midfoot. The first ray combines plantarflexion with
abduction and eversion and, therefore, does not contribute to
pronation or supination [62,72].

Motion at the second tarsometatarsal joint is even more
limited than at the first. Limited motion here results from
the tightly wedged base of the second metatarsal among the
cuneiforms and first metatarsal. Mobility increases from
the third to the fifth tarsometatarsal joints as the bases
become progressively less tightly wedged in and the articu¬
lar surfaces become progressively more curved [127]. The
relative immobility of the medial side of the foot provided
by the bony surfaces and ligamentous support produces the
necessary stability to the foot during propulsive activities
such as walking, running, and jumping.

METATARSOPHALANGEAL JOINTS OF THE TOES

The architecture of the metatarsophalangeal joints of the toes
is remarkably similar to that of the metacarpophalangeal joints
of the fingers. The joints are biaxial, supported by a joint cap¬
sule, collateral ligaments, and a fibrous plantar plate covering

Figure 44.29: Supporting structures of the metatarsophalangeal
joints of the toes include the capsule, collateral ligaments, and
the plantar plate.

the plantar surface of the joints. Like the joint capsules in the
fingers, the capsules of the toes are reinforced dorsally by the
extensor tendons and by collateral ligaments that extend from
the dorsal aspects of the medial and lateral surfaces of the
metatarsal heads toward the plantar aspects of the medial and
lateral sides of the proximal phalanges (Fig. 44.29).

The plantar plate serves a similar purpose in the toes as in
the fingers, protecting the articular surface of the metatarsal
heads. The weight-bearing function of the foot makes the
plantar plate particularly important in the toes. The plates are
attached to the metatarsal heads and the bases of the pha¬
langes and are pulled distally with hyperextension of the toes
to protect the distal aspect of the articular surface. Their func¬
tion is critical to protecting the metatarsal heads during
ambulation, when the body rolls over the stance foot, pushing
the toes into hyperextension while the foot participates in
propelling the body forward (Fig. 44.30).

Clinical Relevance

CLAW AND HAMMER TOE DEFORMITIES OF THE

TOES: Claw toe deformities of the toes are similar to claw
deformities in the fingers, characterized by hyperextension of
the metatarsophalangeal joints of the toes with flexion of
the toes (Fig. 44.31). The hyperextension of the metatar¬
sophalangeal joints pull the plantar plate distally , leaving
the weight-bearing head of the metatarsal bone unprotected
and producing pain as the patient bears weight on the
exposed metatarsal heads.

Two sesamoid bones lie imbedded in the tendon of the
flexor hallucis brevis and are attached to the plantar plate of
the metatarsophalangeal joint of the great toe. These bones
provide additional protection to the metatarsal head and
increase the angle of application of muscles to the great toe
[140]. Hyperextension injuries to the metatarsophalangeal
joint of the great toe, also known as turf toe, can produce
fractures of the sesamoid bones, ruptures of the plantar plate,
or tears in the capsular and collateral ligaments, causing sig¬
nificant pain and dysfunction [135].

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

827

Figure 44.30: Hyperextension of the toes that occurs normally
late in the stance phase of gait pulls the plantar plate distally
over the distal ends of the metatarsal heads.

Figure 44.31: Claw toe deformities include hyperextension of the
metatarsophalangeal joints and flexion of the interphalangeal
joints pulling the plantar plate distally, allowing weight bearing
on the exposed metatarsal heads.

Motion of the Metatarsophalangeal Joints

The metatarsophalangeal joints of the toes are biaxial joints
but exhibit motion primarily in the sagittal plane. Studies of
the kinematics of these joints focus on the motion at the great
toe. Flexion and extension occur about a moving joint axis,
indicating that the motions include rotation and translation
[148]. It is likely that the remaining metatarsophalangeal
joints also combine rotation and translation during flexion and
extension. The great toe displays deviation in the transverse
plane with slight rotation motion in the frontal plane in the
familiar hallux valgus deformity in which the proximal pha¬
lanx deviates laterally on the metatarsal head [103]. Some
individuals are able to spread the toes, exhibiting abduction at
the metatarsophalangeal joints of the toes, but such excursion
apparently is not studied, and its clinical significance is
unknown.

Studies examine the available sagittal plane mobility of the
metatarsophalangeal joint. Reports of mean hyperextension
(dorsiflexion) excursion at the great toe s metatarsophalangeal
joint range from 55 to 96° [13,51,66,120,160]. Seventeen to
34° of flexion (plantarflexion) are also reported [13,160].
Cadaver studies of the flexion and extension mobility of the
second through fifth metatarsophalangeal joints report
60-100° of hyperextension ROM, decreasing from the second
to the fifth toe [94,119]. These studies report flexion from
15-35° at these joints. Toe hyperextension mobility bears con¬
siderable clinical significance, since reports of the hyperex¬
tension used during locomotion as the body rolls over the foot
vary from 40 to 90° [9,104,120].

Clinical Relevance

HALLUX RIGIDUS: Limited hyperextension mobility of the
metatarsophalangeal joint of the great toe, known as hal¬
lux rigidus, can produce pain and significant functional
limitations and disability. Inability to hyperextend the great
toe alters normal walking and running patterns; since these
activities require the ability to roll over the toes; hyperex¬
tending the great toe to at least 40°. Hallux rigidus usually
results from degenerative joint disease at the metatarsopha¬
langeal joint and progresses insidiously, leading to progres¬
sive pain and disability. Conservative management includes
shoe modifications to protect the toe, but severe cases often
require surgery.

INTERPHALANGEAL JOINTS OF THE TOES

The interphalangeal joints of the toes are simple hinge joints
like those in the fingers. They are supported by a joint cap¬
sule, collateral ligaments, and a plantar plate. The plantar
plate serves a purpose similar to that of the palmar plate at the
metacarpophalangeal joints, protection of the underlying
articular surface. Their motions are poorly studied, but the
proximal interphalangeal joints exhibit less than 90° of

828

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

flexion, with little or no extension [119,191]. Flexion mobility
decreases from the second to the fifth toe. The distal inter-
phalangeal joints of the lateral four toes and perhaps the
interphalangeal joint of the great toe exhibit some hyperex¬
tension mobility, but normative data are not available.

Motion of the Whole Foot

The preceding discussion reveals that most of the joints of the
foot contribute to the same triplanar motions of the foot,
pronation and supination. Consequently, when the joints move
in the same direction, the total motion of the foot is increased.
Conversely, individuals with restricted motion at one joint may
develop increased mobility at a nearby joint to maintain over¬
all mobility of the foot. In addition, pronation and supination
of the foot affect the mechanical properties of the whole foot
[45,121,165]. Pronation of the hindfoot increases the passive
mobility of the sagittal plane motion of the forefoot, and
supination of the hindfoot decreases that mobility [7].
Pronation of the foot after ground contact in gait allows the
foot to accommodate to the walking surface. Supination of all
of the joints of the foot makes the foot more rigid and
occurs later in the stance phase of gait to help stabilize
the foot as the body rolls over it [116,157,193].

Clinical Relevance

DANCING EN POINTE: A ballerina dancing en pointe
appears to be able to plantarflex the ankle 90° (Fig. 44.32).
Since such motion is not physiologically possible at the
ankle, it is clear that mobility throughout the rest of the inter-
tarsal and metatarsal joints contributes sufficient plantarflex-
ion to ailow the toes of the foot to be in line with the long
axis of the leg; allowing the dancer to balance on the toes.

PLANTAR FASCIA

The individual ligaments described to this point support spe¬
cific joints. The foot also contains a structure that functions to
support the entire foot. The plantar fascia of the foot provides
essential stabilization to the skin on the plantar surface and
helps maintain the shape of the whole foot. It exhibits a
complex web of attachments extending from the calcaneal
tuberosity to the skin, metatarsal and phalangeal bones, and
intervening ligaments [191]. Its thick, deep portion, known as
the plantar aponeurosis, possesses remarkable tensile
strength (1000-1500 N, 225-337 lb), almost twice the strength
of the strongest ligament in the foot, the deltoid ligament [86].

The plantar aponeurosis spans the arches of the foot and
plays a critical role in supporting them. Weight bearing lowers
the medial longitudinal arch and stretches the plantar aponeu¬
rosis [151]. Studies of full and partial transection of the
aponeurosis consistently demonstrate a decrease in arch height
when the aponeurosis is compromised [2,19,118,157,176].

Figure 44.32: What appears to be 90° of plantarflexion allowing
a dancer to dance en pointe actually requires plantarflexion from
the ankle and throughout the foot.

Plantar fascia releases also produce large increases in strains in
other ligaments of the foot, particularly the spring and long
plantar ligaments [19,24]. Although plantar fascia releases may
be indicated to relieve chronic foot pain, the resultant changes
in other structures of the foot suggest that all possible conser¬
vative measures should be tried before resorting to surgery.

The extensive attachments of the plantar fascia also suggest
a special dynamic role in locomotion. Plantarflexion of the
ankle rotates the calcaneus toward the ground, lowering the
arch and stretching the aponeurosis (Fig. 44.33). Similarly,
hyperextension of the toes applies tension to the aponeurosis
by pulling on its distal end. During the push-off phase of gait,
an individual rolls over the foot, hyperextending the metatar¬
sophalangeal joints of the toes and simultaneously plantarflex-
ing the ankle. Thus the aponeurosis is pulled taut to stabilize
the foot as it bears the load of the body moving over the stance
foot toward the opposite foot [20,32,61].

Clinical Relevance

PLANTAR FASCIITIS—A CASE REPORT: A 40-year-old
woman entered physical therapy complaining of medial
arch pain. The woman reported that she was training for a
5-km (3.2 miles) race and had noticed a gradual onset of
pain in the bottom of her foot, particularly upon rising from
bed in the morning. Physical examination revealed a patient

(< continued )

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

829

(Continued)

with a normally aligned foot and a rather high medial arch.
The patient also exhibited limited dorsiflexion ROM on the
painful side. The patient noted that she had become rather
careless about her ankle stretching routine despite training
more vigorously , concentrating on increased speed. The
therapist surmised that the patient's faster running required
increased dorsiflexion ROM at the ankle and increased
hyperextension ROM of the metatarsophalangeal joints of
the toes. However ; the patient lacked adequate ankle ROM ,
so the joints of the hindfoot and midfoot compensated by
providing more dorsiflexion range. The increased dorsiflex¬
ion motion in the midfoot stretched the plantar fascia that
was also being stretched by increased toe hyperextension
and by higher impact loads on the foot with increased run¬
ning speed. The cumulative effect of these factors produced
an inflammatory response in the aponeurosis, plantar
fasciitis . Treatment included rest and vigorous stretching.
Over time the patient was able to resume training without
pain as long as she stretched regularly.

Closed-Chain Motion of the Foot

Discussion of the foot thus far has focused on the movement of
the foot with respect to the leg, in which the foot functions in
an open chain. Yet the foot often functions while fixed to the
ground, with the superincumbent body moving over it, func¬
tioning in a closed chain. Many complaints of pain and dys¬
function in the foot, ankle, knee, or hip arise from closed-chain
activities such as running, jumping, or dancing. It is essential

Figure 44.33: The plantar aponeurosis is stretched by plantarflex-
ion of the calcaneus and by hyperextension of the toes.

for the clinician to understand the mechanics of foot
function whether the foot is fixed to the ground or
moving above it.

The role of the subtalar joint in transforming movements
of the foot to the leg or vice versa becomes critical in closed-
chain activities. When the foot is fixed to the ground, the foot
pronates and supinates by allowing the proximal segments to
move on the distal segments. Thus pronation of the subtalar
joint occurs by the tibia and talus moving on the calcaneus.
Pronation with the foot fixed on the ground produces medial
rotation of the tibia, which carries the talus medially within
the mortise [62,69,129]. As the talus moves medially, the cal¬
caneus everts and pulls the cuboid and navicular into abduc¬
tion and eversion.

Thus the motion of the hindfoot is coupled to the motion
of the leg and the forefoot. The extent to which foot motion
is directly coupled with tibial motion remains unclear. In
walking as the foot pronates the tibia medially rotates and as
the foot supinates the tibia laterally rotates. Yet neither the
magnitude nor timing of foot motion directly parallels tibial
motion [132]. Studies suggest that some of the motion of the
foot is absorbed within the foot rather than transmitted
directly to the tibia. Running appears to increase the correla¬
tions between foot and tibial motions, suggesting more direct
coupling of the motion between the foot and the leg [27,132].

As noted earlier in this chapter, pronation of the foot tends
to make the foot more flexible [45,121,165]. In addition,
medial rotation of the tibia accompanies flexion of the knee
(Chapter 41), so pronation of the foot tends to facilitate knee
flexion [54,147]. Consequently, pronation of the foot during
weight bearing may help the lower extremity accommodate to
the ground by enhancing its flexibility and shock absorption
capabilities. Foot pronation is the normal response of the foot
at contact during locomotion [68]. In contrast, supination of
the subtalar joint with the foot on the ground produces lateral
rotation of the tibia with inversion of the calcaneus, cuboid,
and navicular; it tends to extend the knee, making the foot
and rest of the lower extremity more rigid [54,69,147].
Supination normally occurs during locomotion in midstance
as the body is moving over the foot and push-off begins [68].

A common clinical perception is that inadequate or exces¬
sive pronation or supination may contribute to complaints of
foot, knee, hip and even back pain by interfering with the cou¬
pling between the foot and the rest of the lower extremity dur¬
ing weight bearing [121]. However studies investigating the
relationship between excessive pronation and anterior knee
pain report little association [60,93,133]. Like low back pain,
anterior knee pain is likely to be associated with multiple,
interdependent mechanical factors that together help explain
the presence or absence of pain. Such factors may include the
coupling of foot and leg motion, foot alignment, knee and
patellofemoral joint alignment, strength and flexibility at the
foot and knee, and even body weight and height. A better
understanding of the interactions among these factors and
others will help clinicians address the underlying mechanical
flaws when treating lower extremity dysfunctions.

830

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

FOOT ALIGNMENT

Because the foot functions primarily in a closed chain while
bearing significant loads, foot alignment is implicated in
many disorders of the lower extremity. To understand the
potential impact of foot alignment on lower extremity func¬
tion, the clinician requires an understanding of the normal
alignment of the foot and the factors that influence that
alignment.

Arches of the Foot

The articulated foot exhibits three distinct arches, a medial
and a lateral longitudinal arch and the transverse arch
(Fig. 44.34). The medial longitudinal arch includes the calca¬
neus, talus, navicular, medial cuneiform, and first metatarsal
bone. The lateral longitudinal arch consists of the calcaneus,
cuboid, and fifth metatarsal bones. The transverse arch is
formed by the cuboid and cuneiform bones and continues at
the bases of the metatarsals. The transverse arch disappears
in the normal foot at the heads of the metatarsal bones so that
all five metatarsal heads contact the ground in normal weight
bearing [56].

The arches serve several purposes: they protect the nerves,
blood vessels, and muscles on the plantar surface of the foot
from compression during weight bearing; they help the foot
to absorb shock during impact with the ground; and they help

Figure 44.34: The foot possesses three arches: (A) lateral and (B)
medial longitudinal arches and (C) a transverse arch.

store mechanical energy then release it to improve the effi¬
ciency of locomotion [78,144]. The arches of the foot develop
in parallel with gait during childhood and continue to form
until a child is 8 or 10 years of age [35,59]. Integrity of the
arches depends primarily on ligamentous support with assis¬
tance from bony alignment and additional support from
extrinsic muscles of the foot [67,102]. The middle and lateral
cuneiform bones are shaped and positioned to play the role of
keystone in the transverse arch. Their wedge shape, wider
dorsally than on their plantar surface, helps prevent their
descent through the arch [78].

The plantar fascia, long and short plantar ligaments, the
spring ligament, the collateral ligaments of the ankle, and
the interosseous ligament of the subtalar joint all contribute
important soft tissue support to the arches of the foot.
Sectioning these ligaments in cadaver specimens produces
altered joint kinematics of the joints that form the arches, and
sequential sectioning progressively lowers the arches
[67,81,83,84,176]. These studies demonstrate that support of
the arches depends on several structures, with no single struc¬
ture providing the primary support.

Arch height is measured directly by determining the
height of the navicular or the dorsum of the foot from the
ground during weight-bearing [188]. The arch index pro¬
vides an indirect measure of the integrity of the arch by exam¬
ining the amount of contact with the ground made by the
midfoot [17]. A flat foot, or pes planus, refers to a dimin¬
ished medial longitudinal arch, and a pes cavus indicates an
abnormally high medial longitudinal arch. Both arch abnor¬
malities appear to predispose individuals to specific muscu¬
loskeletal injuries [189,190].

Clinical Relevance

RUNNING INJURIES: Arch abnormalities in runners are
associated with different injury patterns [189]. In a study of
40 runners with histories of running injuries , the runners
with high arches reported more ankle and bony injuries ,
while those with low arches reported more knee and soft tis¬
sue injuries. Screening athletes for arch abnormalities may
lead to better preventive measures including instructing indi¬
viduals in appropriate footwear, the use of shoe modifica¬
tions, and athlete education.

Loading causes complex movements of the joints of the
foot leading to a decrease in the arch height. The primary
motion is pronation of the foot, with dorsiflexion, eversion,
and abduction of the navicular and calcaneus on the talus
(pronation) [18,82,151]. In addition, both the calcaneus and
first metatarsal become more horizontally aligned, contribut¬
ing to the flattening of the arch [82,180,187]. Consistent with
the pronation of the calcaneus and navicular, the tibia and
talus rotate medially [63,64]. This normal response to loading

Chapter 44 I STRUCTURE AND FUNCTION OF THE BONES AND NONCONTRACTILE ELEMENTS OF THE ANKLE AND FOOT

831

Figure 44.35: The medial alignment of the talus on the calcaneus
produces a pronation moment (MP pr ) on the hindfoot due to the
ground reaction force (G) and the weight of the body (W).

occurs because the ground reaction force on the calcaneus
lies lateral to the axis of rotation of the subtalar joint, produc¬
ing a pronation moment on the foot [129] (Fig 44.35). An
excessively flattened arch produces increased excursions
within the foot and tibia and can even lead to subluxations of
joints of the foot [1,36,83].

Pes planus and pes cavus describe the qualitative shape of
the arch, and arch height and arch index quantify the magni¬
tude of the arch. However, none of these descriptions pro¬
vides insight into the mechanisms that lead to the abnormal
arch. Arch deformities result from disruptions of the support¬
ing ligaments as well as from muscle weakness or tightness
leading to direct impairments of the arch. In some individu¬
als, the foot position is the dynamic compensation for other
bony malalignments.

Subtalar Neutral Position

The concept of the subtalar neutral position is central to
understanding postural compensations in the foot. It is oper¬
ationally defined as the position of the subtalar joint that is
neither pronated nor supinated [143]. Subtalar neutral posi¬
tion appears to maximize the area of contact between the
talus and calcaneus, and movements away from the subtalar
neutral position into pronation or supination decrease the
contact areas [14,29].

Subtalar neutral position is determined with the subject
either weight-bearing or non-weight-bearing by palpating the
medial and lateral aspects of the talar head to identify the

position in which the talus articulates symmetrically with the
navicular. The position is measured by using the same refer¬
ence lines used to measure subtalar ROM (Fig. 44.22). The
angle made by a line bisecting the leg and another bisecting
the posterior aspect of the calcaneus quantifies the subtalar
neutral position [130,134]. Medial deviation of the calcaneus
with respect to the leg constitutes a varus deformity, while
valgus indicates a lateral deviation of the hindfoot on the leg.
Varus and valgus also apply to forefoot alignment, although
the forefoot position is referenced to the hindfoot [40].

Although a commonly used clinical assessment, the reliabil¬
ity of subtalar neutral measurements remains in question, with
some studies demonstrating satisfactory intertester and intrat¬
ester reliability [76,108,134,180], and other studies reporting
unacceptable levels of reliability [31,130]. Recognizing the lim¬
its in reliability, reported values of subtalar neutral position in
individuals without foot pathology vary from 1-2° of varus
[3,108] to less than 1° of valgus [131].

Clinical Relevance

FOOT ORTHOSES FOR TREATMENT OF A VARUS
HINDFOOT DEFORMITY: In upright stance with the tibias
approximately vertical a medially aligned hindfoot or hind¬
foot with a varus deformity , must pronate excessively to
contact the ground fully (Fig. 44.36). The greater the varus
deformity , the more pronation is required to contact the
ground. Consequently , an individual with a varus hindfoot
may pronate excessively or through a prolonged period

( continued )

Varus Pronated

Figure 44.36: A varus hindfoot deformity in standing requires
excessive pronation for the foot to come into full contact with
the ground.

832

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

(Continued)

during the gait cycle [177]. To prevent the excessive prona¬
tion, some clinicians use orthotic devices that build up, or
post , the medial aspect of the foot to provide a mechanical
block to pronation [71] (Fig. 44.37). Conversely, supination
provides compensation for a valgus deformity of the hind-
foot or forefoot. Lateral posting attempts to limit the com¬
pensatory supination.

Orthotic devices to control the foot's compensatory
movements resulting from foot deformities are common
interventions for foot and knee pain [28,88]. The use of
foot orthoses to treat knee pain is based on an under¬
standing of the natural coupling between foot pronation
or supination and tibiaI rotation and knee motion in
closed-chain movement. Medial wedge orthoses are report¬
ed to decrease pain in individuals with patellofemoral joint
pain [75]. Yet the common clinical belief that excessive
pronation is a risk factor for anterior knee pain lacks
strong scientific evidence [60,93,133]. In contrast, gait stud¬
ies demonstrate that medial wedges exhibit a small but
significant ability to control medial rotation of the tibia
that occurs with excessive pronation [109,123,167].
Biomechanical studies also suggest that medial or lateral
wedges may alter the loading patterns at the knee [153].

In light of the continued debate regarding the reliability of
subtalar neutral measures and the limited understanding
of the precise coupling among the motions of the foot and
leg, clinicians must exercise caution in interpreting foot
alignment data and seek additional evidence for the effec¬
tiveness of orthotic therapy.

Figure 44.37: A foot orthosis with medial posting raises the
ground to the foot, allowing full contact between the foot and
the ground without excessive pronation.

SUMMARY

This chapter describes the structure of the bones and joints
and how that structure influences the motions of the individ¬
ual joints of the ankle and foot. Most of the joints of the foot
and ankle are hinge or gliding joints, but their alignment pro¬
duces triplanar motion known as pronation and supination.
With the exception of the tarsometatarsal joint of the great
toe, the joints of the ankle, hindfoot, and midfoot can pronate
together, making the foot more flexible, or supinate together
to increase the rigidity of the foot. These movements are
essential to the normal loading and unloading of the foot dur¬
ing locomotion. Hyperextension mobility at the metatar¬
sophalangeal joints of the toes also is essential in gait to allow
the body to roll over the foot. This chapter also describes the
movements that occur when the foot functions in a closed
chain, pronation of the foot producing medial rotation of the
tibia and flexion of the knee and supination producing the
opposite.

Several ligaments provide essential support to the joints of
the ankle and foot. The ankle joint capsule and collateral lig¬
aments are particularly important in supporting the ankle and
subtalar joints, the latter also being supported by the
interosseous ligament. The spring ligament and long and
short plantar ligaments are important supporting structures
for the midfoot. The plantar plates provide important protec¬
tion to the articular surfaces of the toes, particularly the
metatarsophalangeal joints.

The normal shape and alignment of the foot also are
described. Specifically, the tibia normally exhibits lateral tor¬
sion, and the hindfoot is very close to 0° subtalar neutral posi¬
tion. The foot maintains a medial and lateral longitudinal and
a transverse arch. Although the primary support of the arch¬
es is ligamentous, muscles provide dynamic support to the
arches of the foot. The muscles of the ankle and foot are pre¬
sented in the following chapter.

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CHAPTER

Mechanics and Pathomechanics
of Muscle Activity at the Ankle
and Foot

CHAPTER CONTENTS

DORSIFLEXORS OF THE ANKLE.839

Anterior Tibialis.839

Extensor Hallucis Longus .842

Extensor Digitorum Longus .842

Peroneus Tertius .843

SUPERFICIAL MUSCLES OF THE POSTERIOR COMPARTMENT.844

Achilles Tendon.844

Gastrocnemius.845

Soleus.846

Plantaris.848

DEEP MUSCLES OF THE POSTERIOR COMPARTMENT .849

Posterior Tibialis .850

Flexor Digitorum Longus .851

Flexor Hallucis Longus .852

MUSCLES OF THE LATERAL COMPARTMENT OF THE LEG.853

Peroneus Longus.853

Peroneus Brevis.854

INTRINSIC MUSCLES OF THE FOOT.855

First Muscular Layer in the Foot.855

Second Muscular Layer in the Foot.857

Third Muscular Layer in the Foot.858

Fourth Muscular Layer in the Foot .859

Extensor Digitorum Brevis .860

Group Effects of the Intrinsic Muscles of the Foot.860

COMPARISONS OF GROUP MUSCLE STRENGTH.860

SUMMARY .862

T he preceding chapter discusses the bony components of the ankle and foot and describes the architectural

organization of the foot. That chapter also identifies the importance of the ligamentous structures in support¬
ing the foot at rest. While the muscles of the leg and ankle play only a limited role in supporting the static
foot, they are essential for the proper function of the foot in its most important role, locomotion. The current chapter
presents the function of the muscles in the leg and foot and the effects of their impairments.

838

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

839

Specifically, the purposes of this chapter are to

■ Examine the actions of the individual muscles of the ankle and foot

■ Consider the effects of impaired strength and extensibility of these muscles

■ Briefly discuss the roles these muscles play during locomotor activities

■ Compare the strengths of the different muscle groups measured in individuals without impairments

The muscles of the ankle and foot include both extrinsic and intrinsic muscles as found in the wrist and hand. The
extrinsic muscles are conveniently organized into an anterior group that dorsiflexes the ankle and contributes to
extension of the toes, a posterior group that contributes to plantarflexion of the ankle and flexion of the toes, and a
lateral group that pronates the foot. Most of these muscles cross several joints of the foot, and an understanding of
each muscle's action and function requires careful attention to the muscle's action at each joint.

Individual roles for each intrinsic muscle of the foot are less clear. This chapter briefly presents the actions of individual
intrinsic muscles and discusses the current understanding of the function of the whole group.

Terminology to describe the actions of the muscles of the ankle and foot is confusing. The last chapter presents the
notion of triplanar motion and defines pronation and supination as the combined motions of dorsiflexion, eversion,
and abduction or plantarflexion, inversion, and adduction, respectively. Yet the literature typically describes the actions
of the muscles of the leg and foot in terms of single-plane movements such as dorsiflexion or inversion [35,76,95]. To
avoid continual reinterpretation of the literature, this chapter adheres to the traditional terminology. The reader is
reminded that a single-plane motion is only one component of the overall triplanar movement. For example, the
peroneus brevis everts the foot and thus can also be described as a pronator of the foot.

Finally, one of the primary functional responsibilities of the muscles of the leg and foot is to control the foot and facilitate
the movement of the body over the foot during locomotion, and no discussion of the mechanics of these muscles is com¬
plete without reference to their role in gait. The mechanics of normal locomotion are described in detail in Chapter 48.
Therefore, the role that these muscles play during locomotion is described only briefly in the current chapter.

DORSIFLEXORS OF THE ANKLE

The dorsiflexor muscles of the ankle are found in the ante¬
rior compartment of the leg and include the anterior tibialis,
the extensor hallucis longus, the extensor digitorum longus,
and the peroneus tertius (Fig 45.1). All lie anterior to the
axis of the ankle joint and thus dorsiflex the ankle [56,95].
Their roles at the other joints of the foot depend on their
location with respect to each joint. The dorsiflexor group
provides two important functions during locomotion:
During the swing phase when the foot is off the ground, the
dorsiflexor muscles help lift the foot and toes off the ground
to provide adequate ground clearance. The second function
occurs at, and immediately after, ground contact, when the
dorsiflexors oppose the plantarflexion moment imparted to
the foot by the ground reaction force and control the
descent of the foot onto the ground (Fig. 45.2). During
swing, the muscles contract concentrically and isometrically;
following contact, the contraction is primarily eccentric.

All of the dorsiflexor muscles are stabilized at the ankle by
the extensor retinaculum, which prevents the tendons from
pulling away from the ankle joint, or bowstringing, as the

ankle dorsiflexes. (See Chapter 17 for more information
about bowstringing.)

Anterior Tibialis

The anterior tibialis is the largest dorsiflexor muscle, with a
physiological cross-sectional area that is as much as twice the
physiological cross-sectional area of the rest of the dorsiflexor
muscles combined [11,94] (Muscle Attachment Box 45.1).

ACTIONS

MUSCLE ACTION: ANTERIOR TIBIALIS

Action Evidence

Ankle dorsiflexion Supporting

Inversion of foot Supporting

The role of the anterior tibialis in dorsiflexion of the ankle is
undisputed. Reports of its maximum dorsiflexion moment arm
range from approximately 30 to 70 mm [10,38,45,49,59,85]
(Fig. 45.3). In contrast, less agreement exists regarding its abil¬
ity to invert the foot. Some suggest that the muscle lies so close
to the axis of the subtalar joint, that its effect on that joint is

840

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.1: The dorsiflexor muscles of the foot include the ante¬
rior tibialis, the extensor hallucis longus, the extensor digitorum
longus, and the peroneus tertius.

negligible [54]. Others report a small inversion moment on the
subtalar joint [15]. A comprehensive study using cadaver spec¬
imens and live subjects investigated the contribution of the
anterior tibialis during inversion of the whole foot, not just
inversion at the subtalar joint [3]. This study demonstrates that
while the anterior tibialis never contracts alone to invert the
foot, it actively contracts during inversion of the foot in most
individuals. Other investigators also note that the activity of
the anterior tibialis in inversion is variable [7]. A biomechani¬
cal analysis to measure its inversion moment arm estimates
lengths of approximately 10 mm or less, considerably smaller
than its dorsiflexion moment arm and the inversion moment
arm of the posterior tibialis and flexor hallucis longus, which
may explain its variable activity during inversion [45]. Just as
the anterior tibialis contracts eccentrically to control plan-
tarflexion of the ankle at heel strike in gait, its ability to invert

Figure 45.2: The dorsiflexor muscles must generate an extension
(dorsiflexion) moment (M DRFX ) to balance the flexion (plantarflex-
ion) moment (M PLFX ) applied by the ground reaction force (G).

MUSCLE ATTACHMENT BOX 45.1

ATTACHMENTS AND INNERVATION
OF THE ANTERIOR TIBIALIS

Proximal attachment: Lateral condyle and proximal
half to two thirds of the lateral surface of the tibial
shaft, anterior surface of the interosseous mem¬
brane, deep fascia, and the intermuscular septum
between it and the extensor digitorum longus

Distal attachment: Medial and inferior surfaces of
the medial cuneiform and the adjoining part of the
base of the first metatarsal bone

Innervation: Deep peroneal nerve (L4, L5)

Palpation: The muscle is readily palpated proximally
at its attachment on the proximal tibia and along its
distal tendon.

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

841

Figure 45.3: The tibialis anterior exhibits a large dorsiflexion
moment arm (A) f but has a much smaller moment arm for
inversion (B).

the foot may help control the pronation that occurs normally
just after heel strike. Regardless of the extent of the tibialis
anteriors participation in inversion, it is important to recognize
that the anterior tibialis is able to produce some combination
of dorsiflexion and inversion (i.e., pronation and supination)
because it is a multijointed muscle, producing dorsiflexion
(pronation) at the ankle and inversion (supination) at the sub¬
talar and transverse tarsal joints.

The anterior tibialis’s contribution to active inversion
explains its ability to provide dynamic support to the medial
longitudinal arch. As noted in the previous chapter, the pri¬
mary support to the arches of the feet during quiet stance is
ligamentous. Yet individuals with flattened arches exhibit
increased activity of the anterior tibialis as well as other mus¬
cles of the leg during dynamic functions [7]. This increased
activity may indicate an attempt to increase the stability of
the foot.

Figure 45.4: Drop foot. Significant weakness of the anterior tib¬
ialis can lead to a "drop foot" during the swing phase of gait,
when the muscle is needed to lift the foot for toe clearance.

EFFECTS OF WEAKNESS

Although there are other dorsiflexor muscles, its size and
mechanical advantage make the anterior tibialis the strongest
of the dorsiflexors. With the ankle positioned in neutral, elec¬
trical stimulation of the anterior tibialis produced 42% of the
total dorsiflexion torque produced by a maximum voluntary
contraction of all of the dorsiflexors [57]. Thus weakness of the
anterior tibialis severely weakens, but does not eliminate,
active ankle dorsiflexion. Loss of the anterior tibialis alone
impairs the ability to control the foot after heel contact during
normal locomotion. Inability to control the foot may cause the
individual to slap the ground with the foot immediately after
contact, often producing an audible foot slap. Weakness of
the anterior tibialis in conjunction with weakness of the other
dorsiflexor muscles may lead to an inability to lift the foot away
from the ground during the swing phase of gait. Inadequate
dorsiflexion during swing produces a foot drop in which the
foot dangles toward the ground as the limb advances, making
ground clearance difficult (Fig. 45.4). In addition, isolated
weakness of the anterior tibialis leaves the peroneus
longus, its antagonist, unopposed, producing plan-
tarflexion of the first metatarsal [45].

EFFECTS OF TIGHTNESS

Tightness of the anterior tibialis develops in the absence of
adequate plantarflexion strength. The forefoot is pulled medi¬
ally, accentuating the medial longitudinal arch and producing

a cavus foot.

842

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 45.2

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR HALLUCIS LONGUS

Proximal attachment: Middle half of the medial sur¬
face of the fibula and the adjacent anterior surface
of the interosseous membrane

Distal attachment: Dorsal aspect of the base of the
distal phalanx of the hallux

Innervation: Deep peroneal nerve (L5, SI)

Palpation: The tendon is easily palpated on the dor¬
sum of the foot and ankle, but the muscle belly lies
deep to the anterior tibialis and extensor digitorum
longus and cannot be palpated directly.

Extensor Hallucis Longus

The extensor hallucis longus has a smaller physiological cross-
sectional area than either the anterior tibialis or the extensor
digitorum longus [11,94] (Muscle Attachment Box 45.2).

ACTIONS

MUSCLE ACTION: EXTENSOR HALLUCIS LONGUS

Action

Evidence

Extension of the metatarso¬

Supporting

phalangeal and interphalangeal

joints of great toe

Ankle dorsiflexion

Supporting

Inversion of foot

Inadequate

The role of the extensor hallucis at the great toe is clear. The
extensor hallucis longus provides the only active extension
force to the interphalangeal joint and the primary active
extension force to the metatarsophalangeal joint. The exten¬
sor hallucis longus has only a slightly smaller moment arm
for dorsiflexion at the ankle than the anterior tibialis and,
consequently, also contributes to active ankle dorsiflexion.
In contrast, although some authors report that the extensor
hallucis longus contributes to inversion of the foot [43], it
crosses very close to the axis of the subtalar joint, and its
contribution to the subtalar joint is unclear [56]. A study of
cadaver specimens and live subjects demonstrates a slight
ability to supinate the whole foot and reveals electromyo¬
graphic activity of the extensor hallucis longus during some
supination activities of the whole foot, such as lifting the
medial side of the foot from the ground when standing [3].

EFFECTS OF WEAKNESS

Weakness of the extensor hallucis longus weakens extension
at the metatarsophalangeal and interphalangeal joints of the
great toe. Since it is the only extensor at the interphalangeal

joint, weakness in interphalangeal joint extension is diagnos¬
tic for extensor hallucis longus weakness.

During normal locomotion, an individual contacts the
ground with the heel of the foot first. The ground reaction
force applies a plantarflexion moment to the whole foot,
which is resisted by all of the dorsiflexors. Weakness of the
extensor hallucis longus diminishes an individuals ability to
control the descent of the medial portion of the foot, partic¬
ularly the great toe. Patients with weakness of the extensor
hallucis longus also report that the toe tends to fold under
the foot when they are pulling on socks or shoes and can
cause tripping.

EFFECTS OF TIGHTNESS

Tightness of the extensor hallucis longus pulls the metatar¬
sophalangeal joint of the great toe into extension, which, as in
the fingers and thumb, tends to produce flexion at the inter¬
phalangeal joint as the flexor hallucis longus is stretched, and a
claw toe deformity emerges. Hyperextension of the great toe
pulls the plantar plate distally, exposing the metatarsal head to
excessive loads and producing pain. Similarly, hyperextension
of the metatarsophalangeal joint pulls the interphalangeal joint
into the toe box of a shoe, causing pain and calluses, or corns,
on the dorsal surface of the interphalangeal joint.

Clinical Relevance

CLAW DEFORMITIES OF THE TOES: Claw toe deformities
in afoot with sensation are quite painful. Claw deformities
in a foot without sensation put the individual at risk of skin
breakdown as the result of increased pressure under the
metatarsal heads and between the dorsal surfaces of the
toes and the shoe.

Extensor Digitorum Longus

The extensor digitorum longus has a physiological cross-
sectional area greater than that of the extensor hallucis longus
but may be only half the area of the anterior tibialis [11, 94].
(.Muscle Attachment Box 45.3).

ACTIONS

MUSCLE ACTION: EXTENSOR DIGITORUM LONGUS

Action

Evidence

Extension of the metatarso¬

Supporting

phalangeal joints of lateral

four toes

Extension of PIP and DIP

Supporting

joints of lateral four toes

Ankle dorsiflexion

Supporting

Eversion of foot

Supporting

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

843

MUSCLE ATTACHMENT BOX 45.3

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR DIGITORUM LONGUS

Proximal attachment: Lateral surface of the lateral
condyle of the tibia, proximal two thirds to three
quarters of the medial surface of the fibula, deep
fascia, and adjacent anterior surface of the
interosseous membrane

Distal attachment: Extensor hood mechanism on the
dorsum of the metatarsophalangeal joints and prox¬
imal phalanges of the four lateral toes. A central
slip inserts to the base of the middle phalanx, and
two collateral strips insert to the base of the distal
phalanx.

Innervation: Deep peroneal nerve (L5, SI)

Palpation: The tendons are readily palpated along
the dorsum of the foot and across the lateral four
metatarsophalangeal joints.

Reports agree that the extensor digitorum longus is the
primary extensor of the metatarsophalangeal joints of the
lateral four toes and, with the intrinsic muscles of the foot,
contributes to extension of the proximal and distal interpha-
langeal joints of these toes [43,68,76,95]. Similarly, its partic¬
ipation in ankle dorsiflexion is well accepted [43,68,76,95].
The extensor digitorum longus possesses a dorsiflexion
moment arm similar to those of the anterior tibialis and exten¬
sor hallucis longus.

Several authors report that the extensor digitorum longus
participates in eversion of the foot [30,43,84], and its location
lateral to the axis of the subtalar joint measured in cadavers
supports this view [56]. Data from live subjects reveal a con¬
sistent role in active eversion [3].

EFFECTS OF WEAKNESS

As the primary extensor to the metatarsophalangeal joints of
the lateral toes, weakness of the extensor digitorum longus
decreases an individuals ability to lift the toes from the
ground during the swing phase of gait and, like the extensor
hallucis longus, to control the descent of the toes onto the
ground as the heel contacts the ground.

EFFECTS OF TIGHTNESS

Tightness of the extensor digitorum longus produces effects
on the lateral toes similar to those produced by tightness of
the extensor hallucis longus on the great toe. The metatar¬
sophalangeal joints are hyperextended, and typically, the
proximal and distal interphalangeal joints flex as the result of
the stretch on the flexor digitorum longus. As with extensor

MUSCLE ATTACHMENT BOX 45.4

ATTACHMENTS AND INNERVATION
OF THE PERONEUS TERTIUS

Proximal attachment: Distal one third or more of
the medial surface of the fibula and adjoining sur¬
face of the interosseous membrane (This muscle is a
partially separated portion of the extensor digito¬
rum longus.)

Distal attachment: Medial part of the dorsal surface
of the base of the fifth metatarsal bone

Innervation: Deep peroneal nerve (L5, SI)

Palpation: If present, the tendon is palpated on the
dorsolateral surface of the foot as it inserts into the
base of the fifth metatarsal bone.

hallucis longus tightness, the resulting claw toe deformities
are painful and functionally limiting.

Peroneus Tertius

The peroneus tertius is part of the extensor digitorum longus
and is absent in about 5% of the population (Muscle
Attachment Box 45.4). When present, it is visible on the lat¬
eral aspect of the dorsum of the foot. It is the smallest of the
dorsiflexor muscles [11].

ACTIONS

MUSCLE ACTION: PERONEUS TERTIUS

Action

Evidence

Ankle dorsiflexion

Supporting

Eversion of foot

Supporting

The role of the peroneus tertius in ankle dorsiflexion and foot
eversion are well accepted [76,84,95]. However, its size and
variable presence suggests that it plays only an accessory role
in these movements.

EFFECTS OF WEAKNESS

Weakness of the peroneus tertius occurs in conjunction with
weakness of the extensor digitorum longus and the other dor-
siflexor muscles. Consequences of isolated weakness, albeit
unlikely, are probably minimal.

EFFECTS OF TIGHTNESS

Like weakness, isolated tightness of the peroneus tertius is
unlikely, and the consequences of concomitant tightness of
the extensor digitorum longus are greater than any potential
consequences of peroneus tertius tightness.

844

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

SUPERFICIAL MUSCLES OF THE
POSTERIOR COMPARTMENT

The superficial muscles of the posterior compartment include
the gastrocnemius, soleus, and plantaris muscles (Fig. 45.5).
These three muscles form the bulk of the calf musculature
and give the calf its characteristic shape. Although many
other muscles contribute to the total plantarflexion torque
available at the ankle, estimates suggest that these three
muscles contribute 60-87% of the total plantarflexion
torque [13, 24, 66, 80].

The gastrocnemius and soleus insert jointly on the posteri¬
or surface of the calcaneus by way of the tendo calcaneus
(Achilles tendon) and together form the triceps surae. The
plantaris may also join the Achilles tendon. The functions of
these three muscles at the ankle and hindfoot are similar and
depend on their common attachment to the Achilles tendon.
To appreciate the behavior of the superficial muscles of the
posterior compartment, it is useful to examine the mechanics
of the Achilles tendon.

Figure 45.5: The superficial plantarflexor muscles include the gas¬
trocnemius, soleus, and the plantaris (not pictured).

Achilles Tendon

The Achilles tendon is the thickest and strongest tendon of
the body [95]. Its attachment onto the posterior surface of the
calcaneus gives the triceps surae muscles a large moment arm
and a significant mechanical advantage in plantarflexion [45,
51,78,85] (Fig. 45.6). Estimates of the plantarflexion moment
arm of the Achilles tendon vary from approximately 5 to 6 cm.

Reports of the effects of ankle position on Achilles tendon
length are conflicting. Some studies report that the plan¬
tarflexion moment arm of the Achilles tendon is maximum
when the ankle is in neutral [60,85]. However, others report
that the moment arm increases as the ankle plantarflexes
[24,45,78]. All reports agree that the Achilles tendon pos¬
sesses the largest moment arm of all the muscles that cross
the ankle. [45,85]. The Achilles tendon also possesses an
inversion moment arm, at least when the foot is in the neu¬
tral or pronated position. So both the grastrocnemius and
soleus potentially contribute to inversion of the hindfoot
[3,45,98].

The strength and stiffness of the Achilles tendon con¬
tribute to the overall stiffness of the ankle and increase the
efficiency of gait by enabling the tendon to store energy as it
is stretched like a spring during the stance phase of gait.
[18,79,87]. The Achilles tendon exhibits up to approximately
5-6% strain (percent change in length) during vigorous plan¬
tarflexion contractions and normal gait [48,52]. The elasticity

Figure 45.6: The Achilles tendon has a large moment arm for
plantarflexion (A) and a small moment arm for inversion (B).

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

845

of the Achilles tendon provides passive energy to activities
such as walking, running and jumping and plays an important
role in maximizing the efficiency of these activities [37,48,50].

Cadaver data suggest that the ultimate strength of the
Achilles tendon is approximately 4600 N (1034 lb) at low
loading rates and increases with loading rate [96]. Despite its,
size and strength, the Achilles tendon is the most frequently
ruptured tendon in the body, and the incidence of rupture is
increasing [23,96]. One reason for the high incidence of
injury may be related to the vascular supply to the tendon,
which is reduced in the tendons midsection, between its
attachments to the muscle bellies and to the calcaneus [12].

Clinical Relevance

ACHILLES TENDON RUPTURES: Achilles tendon ruptures
are more common in sedentary individuals who participate
in sporadic ; physically strenuous activity During his first term
as vice-president of the United States , Mr. Al Gore ruptured
his Achilles tendon playing tennis , a typical scenario.
Clinicians may help to reduce the incidence of such injuries
by actively instructing individuals to gradually increase their
level of physical activity and to avoid sudden bursts of
intense activity without appropriate preconditioning.

Gastrocnemius

The gastrocnemius is the superficial muscle of the calf, and its
two muscle bellies are easily identified on the posterior sur¬
face of the leg (Muscle Attachment Box 45.5).

MUSCLE ATTACHMENT BOX 45.5

ATTACHMENTS AND INNERVATION
OF THE GASTROCNEMIUS

Proximal attachment: Medial head: Upper and pos¬
terior part of the medial femoral condyle behind
the adductor tubercle and from a slightly raised
area on the popliteal surface of the femur above
the medial condyle

Lateral head: Upper and posterior part of the lateral
surface of the lateral femoral condyle and lower
part of the corresponding supracondylar line

Distal attachment: Posterior surface of the calcaneus
via the tendo calcaneus

Innervation: Tibial nerve (SI, S2)

Palpation: The gastrocnemius muscle bellies are pal¬
pated as two almost symmetrical muscle masses in
the proximal one half of the posterior leg.

ACTIONS

MUSCLE ACTION: GASTROCNEMIUS

Action

Evidence

Ankle plantarflexion

Supporting

Inversion of foot

Supporting

Knee flexion

Supporting

Investigators agree that the gastrocnemius plays a major role
in plantarflexion of the ankle. It functions with the soleus to
lift the weight of the body when rising onto the forefoot
[3,7,30,43]. It is active during the stance phase of gait to assist
in forward progression and to control the forward glide of
the body over the stance foot [67]. It also helps to stabilize the
ankle as the individual rolls over and off the foot in late stance
[35,82,83]. The inversion moment arm of the Achilles tendon
supports the role of the gastrocnemius in inversion.

The gastrocnemius crosses the knee and has a significant
moment arm for flexion of the knee. The moment arm
increases from almost zero when the knee is extended to
more than 3 cm when the knee is flexed beyond 90° [14,45]
(Fig. 45.7). This allows the gastrocnemius to generate a sub¬
stantial flexion moment at the knee, although the hamstrings
are the primary flexors of the knee.

Clinical Relevance

TESTING KNEE FLEXION STRENGTH: Because the gas¬
trocnemius is capable of producing knee flexion, it is impor¬
tant for the clinician to detect any substitutions made by the
gastrocnemius when testing the strength of the hamstrings
(Fig. 45.8). The clinician must ensure that the ankle remains
relaxed when testing the strength of the hamstring muscles
specifically.

EFFECTS OF WEAKNESS

The gastrocnemius provides substantial plantarflexion force,
and loss of the gastrocnemius produces a large decrease in
plantarflexion strength. Decreased plantarflexion strength
hampers an individuals ability to rise up on toes or
climb hills or ladders, and normal locomotion is
Vs/ impaired significantly [73].

EFFECTS OF TIGHTNESS

Tightness of the gastrocnemius may limit an individuals dorsi-
flexion range of motion (ROM), but because it crosses the
knee and the ankle, its effect on ankle ROM depends on knee
position. A clinician identifies tightness of the gastrocnemius
by examining dorsiflexion ROM with the patients knee
extended, putting the gastrocnemius on stretch, and with the
knee flexed, putting the muscle on slack (Fig. 45.9). Most indi¬
viduals exhibit less dorsiflexion ROM with the knee extended

846

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.7: The moment arm of the gas¬
trocnemius at the knee is smaller when
the knee is extended than when the knee
is flexed to 90°.

than with the knee flexed [75,80]. Reports of peak dorsiflexion
ROM with the knee extended range from 10 to 18° in individ¬
uals without pathology [62,64,75]. Moseley uses normative
data from 300 male and female subjects without pathology to
suggest that dorsiflexion of less than 4° with the knee extend¬
ed indicates hypomobility [62]. Normal upright standing pos¬
ture requires the ability to reach neutral dorsiflexion with the
knee extended. Extreme tightness of the gastrocnemius causes
the individual to stand on the forefoot without heel contact or
to stand with the knees flexed. Normal locomotion uses

Figure 45.8: Active plantarflexion during resisted knee flexion
suggests that the subject is using the gastrocnemius as a knee
flexor to enhance knee flexion strength.

approximately 5° of dorsiflexion with the knee extended, and
tightness of the gastrocnemius muscle may impair an individ-
uals ability to roll over the foot later in the stance phase of
locomotion [44,47,65]. Using a mechanical simulation of gas¬
trocnemius tightness, Matjacic et al suggest that gastrocne¬
mius tightness results in a significant increase in knee flexion
at initial contact and midstance [58].

Soleus

The soleus lies deep to the gastrocnemius and possesses the
largest physiological cross-sectional area of all of the muscles
of the leg (Muscle Attachment Box 45.6). Rs physiological
cross-sectional area is approximately twice that of the gas¬
trocnemius [11,94].

ACTIONS

MUSCLE ACTION: SOLEUS

Action

Evidence

Ankle plantarflexion

Supporting

Inversion of foot

Supporting

Like the gastrocnemius, the soleus is undoubtedly a plan-
tarflexor [7,30,43,76,95]. With its large cross-sectional area, it
is capable of large forces. The soleus is composed mostly of
type I muscle fibers, while the gastrocnemius consists of
approximately half type I and half type II fibers [40,61,63,80].
Electromyographic activity of the soleus is apparent in low-
resistance plantarflexion, while activity of the gastrocnemius
appears with increased resistance [28]. Similarly, high-velocity
contractions with the knee extended recruit the gastrocne¬
mius more than the soleus [81,88]. However, when the knee

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

847

Figure 45.9: A. With the knee flexed, the gastrocnemius is on
slack, and ankle dorsiflexion ROM is limited only by the soleus
and the joint capsule. B. With the knee extended, the gastrocne¬
mius is stretched, and ankle dorsiflexion ROM is reduced.

MUSCLE ATTACHMENT BOX 45.6

ATTACHMENTS AND INNERVATION
OF THE SOLEUS

Proximal attachment: Posterior surface of the head
and proximal 1/4 to 1/3 of the shaft of the fibula,
soleal line and middle 1/3 of the medial border of
the tibia, and a fibrous band between the tibia and
fibula

Distal attachment: Posterior surface of the calcaneus
via the tendo calcaneus

Innervation: Tibial nerve (SI, S2)

Palpation: The soleus is palpable just deep to the
medial and lateral borders of the gastrocnemius as
the muscle bellies of the gastrocnemius insert into
the Achilles tendon (Fig. 45.10).

is flexed, the soleus is recruited regardless of plantarflexion
velocity. Increasing the pedaling speed during cycling appears
to produce greater activation of the gastrocnemius with little
change in soleus recruitment [81].

These studies suggest that the soleus and gastrocnemius
play related but distinct roles in lower extremity function. The
soleus appears well suited to play a larger role in such phasic

Figure 45.10: The soleus is palpated along the distal borders of
the gastrocnemius bellies as they insert into the Achilles tendon.

848

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

activities as controlling upright posture, while the gastrocne¬
mius is critical in high-velocity and forceful activities such as
jumping [7,19]. Both the gastrocnemius and soleus muscles
are active during the stance phase of gait, although the activ¬
ity of the soleus begins earlier, and the activity of the gastroc¬
nemius lasts longer [35,82]. The soleus and gastrocnemius
both contribute to forward progression in stance, but the
soleus helps to decelerate the leg as the body glides forward
over the fixed foot during midstance [67]. Like the gastroc¬
nemius, the soleus with its insertion into the Achilles tendon
has a small inversion moment arm, suggesting that the soleus
can contribute to inversion of the hindfoot [3,45].

EFFECTS OF WEAKNESS

Weakness of the soleus produces a significant loss in plan-
tarflexion strength with resulting impairments in locomotion.
Weakness of the soleus impairs the ability to control the leg as
the body glides over the stance foot, and excessive ankle dor-
siflexion during stance may result. In addition, an individual
with weakness of the soleus has difficulty rolling onto the
forefoot later in stance and, consequently, may exhibit a late
heel rise [73] (Fig. 45.11).

EFFECTS OF TIGHTNESS

Tightness of the soleus also restricts dorsiflexion ROM; how¬
ever, unlike tightness of the gastrocnemius, the resulting
plantarflexion contracture is independent of knee position.

Figure 45.12: Tightness of the soleus muscle restricts the forward
progression of the tibia during stance. The forward progression
of the thigh and trunk over the fixed tibia produces an extension
moment ( M EXT ) on the knee joint.

Figure 45.11: Weakness of the plantarflexor muscles is manifest¬
ed in the latter half of the stance phase of gait by inadequate
roll off and often a late heel rise.

Despite the fact that the soleus does not cross the knee joint,
tightness of the soleus may produce important effects on the
knee. During the stance phase of gait, the tibia normally
glides over the fixed foot. Tightness of the soleus restricts for¬
ward glide of the tibia, even though momentum may contin¬
ue the forward progression of the thigh and trunk. Forward
movement of the thigh and trunk on a tibia that is unable to
move forward produces an extension moment on the knee
and a tendency to hyperextend the knee (Fig. 45.12) [29,58].
Similarly, in quiet standing an individual normally stands with
the ankles close to neutral plantar- and dorsiflexion. An indi¬
vidual with soleus tightness is unable to achieve the neutral
position and tends to lean backward. To stand upright, the
individual must move the body’s center of gravity anteriorly
over the base of support. Forward movement of the center of
mass may be achieved by hip flexion but also may occur with
hyperextension of the knee, known as genu recurva-
tum (Fig. 45.13). Thus tightness of the soleus is a risk
factor for genu recurvatum.

Plantaris

The plantaris is a small muscle that lies between the gastroc¬
nemius and soleus muscles (Muscle Attachment Box 45.7).
Cadaver studies suggest that it is absent in 5-10% of the
population [91,95], although examination of 40 individuals

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

849

Figure 45.13: Tightness of the soleus may contribute to a genu
recurvatum (hyperextension) deformity of the knee by holding
the ankle in plantarflexion, tending to cause the individual to
lean backward. To keep the center of mass over the base of sup¬
port, an individual leans forward. The forward lean may occur at
the hips or at the knee. Forward lean at the knee produces genu
recurvatum.

MUSCLE ATTACHMENT BOX 45.7

ATTACHMENTS AND INNERVATION
OF THE PLANTARIS

Proximal attachment: Lower part of the lateral
supracondylar ridge, adjacent part of the popliteal
surface of the femur, and oblique popliteal
ligament

Distal attachment: Posterior surface of the calcaneus
via the tendo calcaneus

Innervation: Tibial nerve (SI, S2)

Palpation: Not palpable

Although the plantaris crosses the knee and ankle in line with-
the medial gastrocnemius, its size and variable presence sug¬
gests that it provides no unique function at the ankle and foot.

EFFECTS OF WEAKNESS AND TIGHTNESS

Impairments of the plantaris cannot be identified clinically.

Clinical Relevance

"TENNIS LEG": An injury known as tennis leg is character¬
ized by the sudden , acute onset of pain in the posteromedial
aspect of the upper calf usually following sudden and rapid
weight bearing on the leg, such as a fall off a curb or a
lunge for a tennis ball. Historically , these complaints were
attributed to an isolated tear of the plantaris muscle. The
inability to perform a clinical test to assess the integrity of
the muscle prevented verification of the injury. More recently r ,
the complaints have been associated with a tear of the
medial head of the gastrocnemius. Only one known case
report verifies the occurrence of an isolated tear of the plan¬
taris muscleconfirmed at surgery [26].

undergoing surgical repair of a ruptured Achilles tendon
revealed absence of the plantaris in 24 (60%) of the sub¬
jects [36].

ACTIONS

MUSCLE ACTION: PLANTARIS

Action

Evidence

Ankle plantarflexion

Inadequate

Inversion of foot

Inadequate

Knee flexion

Inadequate

DEEP MUSCLES OF THE POSTERIOR
COMPARTMENT

The deep muscles of the posterior compartment of the leg
include the posterior tibialis, flexor digitorum longus, and
flexor hallucis longus (Fig. 45.14). These muscles wrap
around the medial aspect of the ankle and foot, where they
are easily palpated. The tendons of the posterior tibialis, flex¬
or digitorum longus, and flexor hallucis longus are contained
with the neurovascular bundle in the tarsal tunnel formed
by the deltoid ligament and the flexor retinaculum [41].
Entrapment of the nerve or tendons is reported as they enter
or exit the tarsal tunnel.

850

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.14: The deep muscles of the posterior compartment of
the leg include the posterior tibialis, the flexor digitorum longus,
and the flexor hallucis longus.

Posterior Tibialis

The posterior tibialis is the deepest of the deep muscles of the
posterior compartment of the leg [95] (Muscle Attachment
Box 45.8). Its physiological cross-sectional area is more than
the physiological cross-sectional area of the other two deep
muscles combined [11,94].

ACTIONS

MUSCLE ACTION: POSTERIOR TIBIALIS

Action

Evidence

Inversion of foot

Supporting

Ankle plantarflexion

Supporting

The posterior tibialis has an
subtalar joint of almost 3 cm,

inversion moment arm at the
almost three times that of the

MUSCLE ATTACHMENT BOX 45.8

ATTACHMENTS AND INNERVATION
OF THE POSTERIOR TIBIALIS

Proximal attachment: Medial part: Posterior surface
of the interosseous membrane and lateral area on
the posterior surface of the tibia between soleal
line above and the junction of the middle and
lower thirds of the shaft below

Lateral part: Upper two thirds of posterior fibular
surface, deep transverse fascia, and intermuscular
septa

Distal attachment: Navicular tuberosity and plantar
surface of medial cuneiform with tendinous bands
to tip and distal margin of the sustentaculum tali
[76], all tarsal bones except the talus, and bases of
the middle three metatarsals

Innervation: Tibial nerve (L4, L5)

Palpation: The tendon of the posterior tibialis is pal¬
pated along the posterior border of the medial
malleolus. The muscle belly can be palpated just
posterior to the medial surface of the shaft of the
tibia.

anterior tibialis [45]. Its size and large moment arm make it
the primary inverter of the subtalar joint, and electromyo¬
graphic data support this role [3]. Similarly, its extensive
attachment on the other tarsal bones contributes to its effec¬
tiveness in inverting the whole foot [55]. In contrast, its plan-
tarflexion moment arm at the ankle is approximately 1 cm and
approaches zero when the ankle is plantarflexed [85]. The
posterior tibialis with the other deep muscles of the posterior
compartment produces some plantarflexion torque, but its
primary action is inversion [66].

The posterior tibialis also appears to contribute to
the dynamic support of the medial longitudinal arch [16,42].
The preceding chapter reports that the ligaments of the foot
are the primary support of the arches of the foot during static
posture. However, muscles provide additional support to the
foot during activity such as locomotion. The posterior tibialis
helps control the descent of the arch during loading and con¬
tributes to the restoration of the arch later in stance [69,82].

EFFECTS OF WEAKNESS

Weakness of the posterior tibialis impairs inversion strength,
producing at least a 50% reduction in strength [42]. The pos¬
terior tibialis is an important stabilizer of the forefoot, and
weakness impairs an individuals ability to rise up on the toes,
even with intact plantarflexor muscles, because the foot is
unstable. Weakness also produces an imbalance with the

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

851

everter muscles, and the foot tends to evert and abduct; that
is, it tends to pronate [25,55,89]. Patients with posterior tibi-
lalis tendon dysfunction (PTTD) exhibit increased pronation
at the hindfoot and forefoot, reflecting the muscle s extensive
role in supporting most of the foot [89]. PTTD is a primary
cause of acquired flat feet and alters the normal movement of
the tarsal bones during weight bearing and gait [31,69].
Factors associated with increased risk of PTTD are obesity,
aging, hypertension, diabetes, and vascular insufficiency with¬
in the tendon [31]. A preexisting flat foot deformity also
appears to be a risk factor for a rupture of the posterior tib¬
ialis. [16]. Arai et al. report increased resistance to glide of the
posterior tibialis around the medial malleolus in specimens
with flat feet [4]. The increased frictional force on the poste¬
rior tibialis tendon may contribute to the tendon s increased
risk of rupture in individuals with flat feet.

MUSCLE ATTACHMENT BOX 45.9

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITORUM LONGUS

Proximal attachment: Medial part of the posterior
surface of the tibia inferior to the soleal line and
the fascia covering tibialis posterior

Distal attachment: Plantar surface of the base of the
distal phalanx of the lateral four digits

Innervation: Tibial nerve (L5 # S1 f S2)

Palpation: The tendon of the flexor digitorum
longus is palpated just posterior to the posterior
tibialis tendon at the medial malleolus.

Clinical Relevance

POSTERIOR TIBIALIS RUPTURE: Spontaneous rupture
of the posterior tibialis tendon produces pain and significant
functional limitations. It frequently occurs after a prolonged
episode of chronic tendinitis. Its association with preexisting
foot deformities; obesity , aging , and hypertension suggests
that treatments to control the flat foot such as orthotic
devices to limit pronation may help to reduce the stress on
the posterior tibialis tendon and may help prevent rupture.
Outcome studies are needed to determine the efficacy of
such interventions.

EFFECTS OF TIGHTNESS

Tightness of the posterior tibialis pulls the foot into inversion
and adduction of the forefoot and may include slight plan-
tarflexion, producing a varus or an equinovarus deformity
of the foot. Such deformities are often found in individuals
with spasticity of the posterior tibialis or with an imbalance
between the posterior tibialis and the everters of the foot [77].

Flexor Digitorum Longus

The flexor digitorum longus has a cross-sectional area similar
to that of the flexor hallucis longus, and considerably smaller
than that of the posterior tibialis [11,94] (Muscle Attachment
Box 45.9). It is palpable just posterior to the posterior tibialis
tendon as it wraps around the medial malleolus.

ACTIONS

MUSCLE ACTION: FLEXOR DIGITORUM LONGUS

Action

Evidence

Flexion of the metatarsophalangeal,

PIP and DIP joints of the lateral four toes

Supporting

Ankle plantarflexion

Supporting

Inversion of foot

Supporting

The flexor digitorum longus clearly flexes the joints of the
toes and is the sole muscle able to flex the distal interpha-
langeal joints of the toes.

Clinical Relevance

MANUAL MUSCLE TESTING OF THE FLEXOR
DIGITORUM LONGUS: An isolated manual muscle test to
assess the strength of the flexor digitorum longus requires
manual resistance to flexion of the distal interphalangeal joints
of the toes. Because this muscle is the only muscle capable of
flexing these joints , weakness in flexion of the distal interpha¬
langeal joint confirms flexor digitorum longus weakness.

Although flexion of the toes is the open-chain activity of the
flexor digitorum longus, in the closed-chain activity of locomo¬
tion, the muscle functions to stabilize the toes and foot against
the ground reaction force that tends to extend, or dorsiflex, the
toes and midfoot as the body rolls over the foot (Fig. 45.15).

Assessment of plantarflexion moment arms at the ankle
reveals that the flexor digitorum longus has a larger plan¬
tarflexion moment arm than the posterior tibialis. However,
its small size limits its potential to produce plantarflexion.
Although the data are limited, the flexor digitorum longus
appears to have a substantial inversion moment arm and par¬
ticipates consistently with the posterior tibialis during inver¬
sion of the foot [3].

EFFECTS OF WEAKNESS

Weakness of the flexor digitorum longus produces weakness
in toe flexion, most clearly identified at the distal interpha¬
langeal joints. Functionally, weakness of the flexor digitorum
longus produces difficulty in stabilizing the foot and toes dur¬
ing stance and is manifested by delayed or limited heel rise as
the body rolls over the foot.

852

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.15: The ground reaction force (G) applies an extension
moment (M EXT ) on the toes as the body rolls off the stance foot.

EFFECTS OF TIGHTNESS

Tightness of the flexor digitorum longus impairs extension
ROM of the toes. It can occur with tightness of the extensor
digitorum longus and contributes to the claw toe deformities
described earlier.

Flexor Hallucis Longus

The flexor hallucis longus lies deeper and posterior to the ten¬
dons of the posterior tibialis and flexor digitorum longus
(.Muscle Attachment Box 45.10).

ACTIONS

MUSCLE ACTION: FLEXOR HALLUCIS LONGUS

Action

Evidence

Flexion of the metatarsophalangeal,

Supporting

interphalangeal joints of the great toe

Ankle plantarflexion

Supporting

Inversion of foot

Supporting

MUSCLE ATTACHMENT BOX 45.1

ATTACHMENTS AND INNERVATION
OF THE FLEXOR HALLUCIS LONGUS

Proximal attachment: Distal two thirds of the poste¬
rior surface of the fibula, the adjacent interosseous
membrane, and the fascia covering the tibialis
posterior

Distal attachment: Plantar aspect of the base of the
distal phalanx of the hallux

Innervation: Tibial nerve (L5, SI, S2)

Palpation: The tendon of the flexor hallucis longus is
palpated posterior and slightly distal to the medial
malleolus.

hallucis longus and identifies weakness in that muscle. The
flexor hallucis longus possesses a larger plantarflexion
moment arm than the posterior tibialis or flexor digitorum
longus and contributes plantarflexion torque to the ankle
[45,85,95]. Although the soleus and gastrocnemius are the
primary plantarflexors of the ankle, some individuals appear
to recruit the flexor hallucis longus as an important plan-
tarflexor as well [17]. Individuals who have sustained an
Achilles tendon rupture recruit the flexor hallucis longus
more than the soleus during submaximal contractions of
both the injured and uninjured ankles. Whether this recruit¬
ment pattern is the result of the injury or a motor pattern
that predisposes an individual to injury is unclear. But these
data reinforce the role of the flexor hallucis longus as an
ankle plantarflexor.

Although some studies report that it is similar in size to the
flexor digitorum longus [11,94], others suggest that the flexor
hallucis longus is larger and stronger than the flexor digito¬
rum longus [66,93]. In contrast, the flexor hallucis longus has
the smallest inversion moment arm of the three muscles and
contributes variably to inversion of the foot [3,45].

EFFECTS OF WEAKNESS

Weakness of the flexor hallucis longus weakens flexion of the
great toe and probably contributes to decreased plantar flex¬
ion strength. Weakness may also contribute to slight inversion
weakness. A case report of a strain or partial rupture in a
42-year-old man documents pain and weakness in toe flexion
and plantarflexion [33].

EFFECTS OF TIGHTNESS

The flexor hallucis longus is the primary flexor of the great
toe and is the only muscle to flex the interphalangeal joint of
the great toe. Like the flexor digitorum longus, manual mus¬
cle testing at the interphalangeal joint isolates the flexor

Tightness of the flexor hallucis longus limits extension of the
joints of the toes particularly when the ankle is
dorsiflexed. Plantarflexing the ankle puts the muscle
on slack and allows more toe extension [33].

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

853

Tightness of the flexor hallucis longus also is implicated in a
claw deformity of the great toe.

Tightness of the flexor hallucis longus may also contribute
to foot pain in the medial longitudinal arch. Runners occa¬
sionally develop pain along the flexor hallucis longus tendon
as the result of repeatedly stretching the contracting muscle
during the push off phase of running.

Clinical Relevance

RUNNING STRETCHES: Most runners are familiar with
the need to stretch the plantarflexor muscles before and
after running. However the flexor hallucis longus is fre¬
quently ignored during those stretches. Runners need to
learn to include stretching the big toe into hyperextension
at the MTP joint to help avoid irritation of the flexor hallu¬
cis longus tendon. Stretches must stabilize the ankle and
metatarsal of the great toe in neutral in order to provide
adequate stretch to the flexor hallucis longus tendon.

MUSCLES OF THE LATERAL
COMPARTMENT OF THE LEG

The peroneus longus and brevis lie on the lateral aspect of
the leg and are palpable as they curve around the lateral
malleolus (Fig. 45.16). Although not the only everters of
the foot, they appear to be the primary everters, con¬
tributing an estimated 65% of the total work capacity of
the everters [13].

Peroneus Longus

Figure 45.16: The muscles of the lateral compartment consist
of the peroneus longus and brevis.

frequently is ignored [56,90] (Fig. 45.17). This function is
apparent during gait, as the peroneus longus is active in mid-
stance to stabilize the forefoot as the body moves over the
foot [9,22,35,82].

The peroneus longus is palpable through much of its length
along the lateral aspect of the leg (Muscle Attachment
Box 45.11).

ACTIONS

MUSCLE ACTION: PERONEUS LONGUS

Action

Evidence

Eversion of foot

Supporting

Ankle plantarflexion

Supporting

Plantarflexion of first ray

Supporting

The peroneus longus clearly everts the foot, exhibiting an
eversion moment arm of 1-3 cm and a larger physiological
cross-sectional area than the peroneus brevis [11,45,94].
Cadaver studies reveal that it possesses a plantarflexion
moment arm, although considerably smaller than that of the
Achilles tendon [45,85]. In vivo, it appears to play only a sec¬
ondary role in plantarflexion of the ankle [3,13].

The peroneus longus plays an important role in stabilizing
the forefoot by plantarflexing the first ray, although this role

MUSCLE ATTACHMENT BOX 45.11

ATTACHMENTS AND INNERVATION
OF THE PERONEUS LONGUS

Proximal attachment: Fibers from the lateral
condyle of the tibia, head and proximal two thirds
of the lateral surface of the fibula, and anterior and
posterior crural intermuscular septa

Distal attachment: Lateral side of the base of the
first metatarsal bone and medial cuneiform by two
slips and occasionally a third slip to the base of the
second metatarsal bone [95]

Innervation: Superficial peroneal nerve (L5, SI)

Palpation: The peroneus longus muscle belly is palpated
just distal to the head of the fibula. The tendon is
palpated just posterior to the lateral malleolus.

854

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.17: The peroneus longus pulls on the medial cuneiform
and first metatarsal bone, producing plantarflexion of the first ray.

EFFECTS OF WEAKNESS

Weakness of the peroneus longus contributes to weakness in
eversion of the foot. Consequently, the inverters, particularly
the posterior tibialis, pull the foot into inversion or inversion
with plantarflexion, and a varus, or equinovarus, deformity
results [6,77].

EFFECTS OF TIGHTNESS

The peroneus longus is a multijointed muscle, affecting the
ankle, hindfoot, and forefoot, but tightness in the peroneus
longus is most apparent distally. Although tightness may
limit inversion ROM of the subtalar joint, tightness is mani¬
fested primarily by a plantarflexed first ray [56]. In weight
bearing the plantarflexed first ray may produce excessive
loading on the metatarsal head of the great toe, which can
lead to pain and large callus formation under the first
metatarsal head (Fig. 45.18). Weight bearing in upright
stance with a plantarflexed first ray also produces a supina¬
tion moment on the foot (Fig. 45.19). Consequently, indi¬
viduals with a plantarflexed first ray as a result of tightness
in the peroneus longus tendon may exhibit a supinated foot
in stance [1,56]. Thus a person may stand with the foot
supinated as the result of either tightness or weakness of the
peroneus longus.

Peroneus Brevis

The peroneus brevis lies anterior to the peroneus longus
(.Muscle Attachment Box 45.12).

Figure 45.18: Plantarflexed first ray. This individual with Charcot
Marie Tooth disorder has severely plantarflexed first rays result¬
ing from significant muscle weakness and imbalance with tight¬
ness of the peroneus longus bilaterally. Note the large calluses
on the metatarsal heads of the great toes.

Figure 45.19: A. With the first ray held in plantarflexion by tight¬
ness of the peroneus longus, the ground reaction force during
stance produces a supination moment on the foot. B. If supina¬
tion ROM is available, the individual with tightness of the per¬
oneus longus is likely to stand in supination.

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

855

MUSCLE ATTACHMENT BOX 45.12

ATTACHMENTS AND INNERVATION
OF THE PERONEUS BREVIS

Proximal attachment: Distal two thirds of the lateral
surface of the fibula and the anterior and posterior
crural intermuscular septa

Distal attachment: Lateral tubercle on the base of
the fifth metatarsal bone

Innervation: Superficial peroneal nerve (L5 # SI)

Palpation: The peroneus brevis tendon is palpated
as it emerges from posterior to the lateral malleo¬
lus and travels toward the base of the fifth
metatarsal. The muscle belly may be palpated
proximal to the malleolus and posterior to the per¬
oneus longus tendon.

ACTIONS

MUSCLE ACTION: PERONEUS BREVIS

Action

Evidence

Eversion of foot

Supporting

Ankle plantarflexion

Supporting

The peroneus brevis is unquestionably an everter of the foot,
affecting the subtalar and transtarsal joints of the foot
[55,76,95]. It has a moment arm similar to and perhaps slightly
larger than that of the peroneus longus. Like the peroneus
longus, the peroneus brevis also possesses a small but meas¬
urable moment arm for plantarflexion [45,85].

Clinical Relevance

INVERSION SPRAINS OF THE ANKLE: Most ankle
sprains occur as the result of a sudden , forceful medial
twist of the ankle , producing an inversion sprain . This
movement applies a vigorous tensile force to the ligaments
and tendons on the lateral aspect of the ankle , including
the peroneus brevis. These ligaments and tendons , in turn ,
apply tensile forces to their attachments. Chapter 3 notes
that bone sustains larger compressive forces than tensile
forces before failure. When the ultimate failure strength of
the tendon or ligament exceeds the ultimate strength of the
bone; the tendon or ligament pulls a piece of bone from
the rest of the bone, producing an avulsion fracture,
instead of a torn tendon or ligament. An inversion sprain
frequently produces an avulsion fracture of the fifth
metatarsal at its tuberosity by pulling the peroneus brevis
from its distal attachment.

EFFECTS OF WEAKNESS

Weakness of the peroneus brevis decreases eversion strength
and contributes to an imbalance between the inverter and
everter muscles. Consequently, weakness of the peroneus
brevis increases the relative contribution of the inverters and
leads to a varus hindfoot deformity [25,56,77].

EFFECTS OF TIGHTNESS

Although rare, tightness of the peroneus brevis may con¬
tribute to valgus deformities of the foot. However, other fac¬
tors such as weakness of the posterior tibialis or overactivity of
the extensor digitorum longus also are important contributors
to valgus deformities of the foot.

INTRINSIC MUSCLES OF THE FOOT

The intrinsic muscles of the foot exhibit many similarities with
the intrinsic muscles of the hand. However, although the data
are limited, the intrinsic muscles of the foot appear to function
as a single large group, at least during weight-bearing activi¬
ties. Therefore, this chapter briefly presents the reported
actions of the individual intrinsic muscles and then discusses
the group function. The intrinsic muscles are organized into
four layers and are described here by layer, starting with the
most superficial.

First Muscular Layer in the Foot

The first muscular layer is just deep to the plantar aponeuro¬
sis described in Chapter 44. This layer contains the abductor
hallucis, flexor digitorum brevis, and the abductor digiti min¬
imi (Fig. 45.20).

ABDUCTOR HALLUCIS

MUSCLE ACTION: ABDUCTOR HALLUCIS

Action

Evidence

Abduction of metatarsophalangeal

Insufficient

joint of great toe

Flexion of metatarsophalangeal

Insufficient

joint of great toe

The reported action of the abductor hallucis is flexion and
abduction of the metatarsophalangeal joint of the great toe
(.Muscle Attachment Box 45.13). Unlike its counterpart in the
thumb, the abductor hallucis has no attachment into an
extensor hood mechanism and, therefore, has no direct action
on the interphalangeal joint.

856

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Figure 45.20: The first layer of intrinsic muscles includes the abduc¬
tor hallucis, flexor digitorum brevis, and abductor digiti minimi.

FLEXOR DIGITORUM BREVIS

MUSCLE ACTION: FLEXOR DIGITORUM BREVIS

Action

Evidence

Flexion of metatarsophalangeal
joints of lateral four toes

Insufficient

Flexion of proximal interphalangeal
joints of lateral four toes

Insufficient

The insertion of the flexor digitorum brevis is almost identical
to that of its homologue in the hand, the flexor digitorum

MUSCLE ATTACHMENT BOX 45.14

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITORUM BREVIS

Proximal attachment: Medial process of the cal¬
canean tuberosity, central part of the plantar
aponeurosis, and intramuscular septa

Distal attachment: By two tendons (having been
perforated by the long flexor tendons) to the mid¬
dle phalanx of the four lateral digits

Innervation: Medial plantar nerve (SI, S2)

Palpation: Not palpable

superficialis (Muscle Attachment Box 45.14). Its apparent
action is similar, flexion of the metatarsophalangeal and prox¬
imal interphalangeal joints.

ABDUCTOR DIGITI MINIMI

MUSCLE ACTION: ABDUCTOR DIGITI MINIMI

Action

Evidence

Abduction of metatarso¬
phalangeal joint of little toe

Insufficient

Flexion of metatarso¬
phalangeal joint of little toe

Insufficient

Like the abductor hallucis, the abductor digiti minimi lacks an
attachment to an extensor hood mechanism (Muscle
Attachment Box 45.15). Consequently, its reported actions
are limited to flexion and abduction of the metatarsopha¬
langeal joint of the little toe.

MUSCLE ATTACHMENT BOX 45.13

ATTACHMENTS AND INNERVATION
OF THE ABDUCTOR HALLUCIS

Proximal attachment: Flexor retinaculum, medial
process of the calcanean tuberosity, plantar aponeu¬
rosis, and intermuscular septum

Distal attachment: Medial side of the base of the
proximal phalanx of the hallux

Innervation: Medial plantar nerve (SI, S2)

Palpation: The abductor hallucis is palpated on the
medial aspect of the foot.

MUSCLE ATTACHMENT BOX 45.15

ATTACHMENTS AND INNERVATION
OF THE ABDUCTOR DIGITI MINIMI

Proximal attachment: Both processes of the cal¬
canean tuberosity, plantar surface of the bone
between them, lateral part of the plantar aponeu¬
rosis, and intermuscular septum

Distal attachment: Lateral side of the base of the
proximal phalanx of the fifth toe

Innervation: Lateral plantar nerve (SI, S2, S3)

Palpation: The abductor digiti minimi is palpated on
the lateral aspect of the foot.

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

857

MUSCLE ATTACHMENT BOX 45.16

ATTACHMENTS AND INNERVATION OF THE
FLEXOR DIGITORUM ACCESSORIUS

Proximal attachment: Medial head: Medial concave
surface of the calcaneus below the groove for the
tendon of flexor hallucis longus. Lateral head:
Calcaneus distal to the lateral process of the cal¬
canean tuberosity, long plantar ligament

Distal attachment: Lateral border of the tendon of
flexor digitorum longus

Innervation: Lateral plantar nerve (SI, S2, S3)

Palpation: Not palpable

Second Muscular Layer in the Foot

The second muscular layer of the foot contains the flexor dig¬
itorum accessorius and the lumbricals.

FLEXOR DIGITORUM ACCESSORIUS

MUSCLE ACTION: FLEXOR DIGITORUM ACCESSORIUS

Action Evidence

Flexion of proximal and distal Insufficient

interphalangeal joints of the lateral four toes

Figure 45.21: The pull of the flexor accessorius (F AC ) on the ten¬
dons of the flexor digitorum longus adds to the force (F fdl ) of
the flexor digitorum longus to produce a flexion force (F 5AG ) on
the toes in the sagittal plane.

The flexor digitorum accessorius is unique to the foot and is
a necessary development, coincident with the development
of bipedal ambulation (Muscle Attachment Box 45.16).
Contraction of the flexor digitorum longus pulls the toes
medially, since the muscle enters the foot from its medial
aspect. The pull of the flexor digitorum accessorius on the
tendons of the flexor digitorum longus redirects the force on
the toes, producing flexion of the toes in the sagittal plane
(Fig. 45.21). The pull of the flexor digitorum accessorius on
the flexor digitorum longus provides a vivid example of vec¬
tor addition.

LUMBRICALS

MUSCLE ACTION: LUMBRICALS

Action

Evidence

Flexion of metatarsophalangeal

Insufficient

joints of lateral four toes

Extension of interphalangeal

Insufficient

joints of lateral four toes

The lumbricals of the foot are almost identical to those in the
hand with the exception that they travel and attach to the medi¬
al sides of the toes, while the lumbrical muscles in the hand lie
on the lateral (radial) side of the fingers (Muscle Attachment
Box 45.17). The apparent actions also are similar: flexion of the
metatarsophalangeal joints and extension of the interpha¬
langeal joints by their pull on the extensor hood mechanism.

MUSCLE ATTACHMENT BOX 45.17

ATTACHMENTS AND INNERVATION
OF THE LUMBRICALS

Proximal attachment: Four small muscles that arise
from the tendons of the flexor digitorum longus
tendons. They each arise from the sides of two adja¬
cent tendons except for the first, which arises only
from the medial border of the first tendon.

Distal attachment: Proximal phalanges via tendinous
fibers inserting on the medial side of the dorsal
hood of the four lateral digits

Innervation: First lumbrical: Medial plantar nerve
(S1,S2)

Lateral three lumbricals: Lateral plantar nerve
(S2, S3) [76], deep branch of lateral plantar nerve
(S2, S3) [95]

Palpation: Not palpable

858

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

Flexor hallucis
brevis

Figure 45.22: The third layer of the intrinsic muscles includes the
flexor hallucis brevis, adductor hallucis, and flexor digiti minimi
brevis.

Third Muscular Layer in the Foot

The third muscular layer consists of the flexor hallucis bre¬
vis, the adductor hallucis, and the flexor digiti minimi brevis
(Fig. 45.22).

FLEXOR HALLUCIS BREVIS

MUSCLE ATTACHMENT BOX 45.18

ATTACHMENTS AND INNERVATION
OF THE FLEXOR HALLUCIS BREVIS

Proximal attachment: Medial part of the plantar
surface of the cuboid, adjacent part of the lateral
cuneiform, tendon of posterior tibialis, and the
medial intermuscular septum

Distal attachment: Base of the proximal phalanx of
the hallux

Innervation: Medial plantar nerve (SI, S2)
Palpation: Not palpable

muscle and protect the underlying metatarsal head. In normal
walking and running , an individual pivots over these two
bones as the body rolls over the foot. Pathology related to
these bones can contribute to severe foot pain. Such patholo¬
gy includes stress fractures and inflammation known as
sesamoiditis. Muscle imbalances , abnormal movement pat¬
terns, and overuse may contribute to the pathology.

ADDUCTOR HALLUCIS

MUSCLE ACTION: ADDUCTOR HALLUCIS

Action

Evidence

Adduction of metatarsophalangeal

Insufficient

joint of great toe

Flexion of metatarsophalangeal

Insufficient

joint of great toe

The adductor hallucis has two heads, the oblique and trans¬
verse [71] (Muscle Attachment Box 45.19). The oblique head
is several times larger than the transverse head [5].

MUSCLE ACTION: FLEXOR HALLUCIS BREVIS

Action

Evidence

Flexion of metatarsophalangeal
joint of great toe

Insufficient

The two tendons of the flexor hallucis brevis each contain a
sesamoid bone that increases the angle of pull of the muscle
as it inserts onto the proximal phalanx of the great toe (Muscle
Attachment Box 45.18). The muscle appears well positioned
to contribute to flexion of the metatarsophalangeal joint of
the great toe.

Clinical Relevance

SESAMOID BONES OF THE GREAT TOE: The sesamoid
bones of the great toe , imbedded in the tendon of the flexor
hallucis brevis , increase the mechanical advantage of the

MUSCLE ATTACHMENT BOX 45.19

ATTACHMENTS AND INNERVATION
OF THE ADDUCTOR HALLUCIS

Proximal attachment: Oblique head: Plantar surface
of the base of second, third, and fourth metatarsal
bones and fibrous sheath of the tendon of peroneus
longus. Transverse head: Plantar metatarsophalangeal
ligaments of the third, fourth, and fifth digits and the
deep transverse metatarsal ligaments between them

Distal attachment: Lateral sesamoid bone and lateral
side of the base of the proximal phalanx of the hallux

Innervation: Deep branch of the lateral plantar
nerve (S2, S3)

Palpation: Not palpable

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

859

MUSCLE ATTACHMENT BOX 45.2

ATTACHMENTS AND INNERVATION
OF THE FLEXOR DIGITI MINIMI BREVIS

Proximal attachment: Medial part of the plantar
surface of the base of the fifth metatarsal bone and
sheath of peroneus longus

Distal attachment: Lateral side of the base of the
proximal phalanx of the fifth digit and some deeper
fibers to the lateral part of the distal half of the
fifth metatarsal bone [95]

Innervation: Superficial branch of the lateral plantar
nerve (S2, S3)

Palpation: Not palpable

It reportedly adducts and flexes the metatarsophalangeal joint
of the great toe. Surgeons often release this muscle during
surgery to correct a hallux valgus deformity.

MUSCLE ACTION: FLEXOR DIGITI MINIMI BREVIS

Action

Evidence

Flexion of metatarsophalangeal
joint of little toe

Insufficient

The flexor digiti minimi reportedly produces flexion of the
metatarsophalangeal joint of the little toe (Muscle Attachment
Box 45.20).

Fourth Muscular Layer in the Foot

The fourth and deepest layer of the foot contains the plantar
and dorsal interossei (Fig. 45.23).

Figure 45.23: The plantar and dorsal interossei form the fourth
layer of the intrinsic muscles of the foot. The extensor digiti bre¬
vis is palpable on the dorsal surface of the foot. A. Plantar view
containing the plantar interossei. B. Dorsal view containing the
dorsal interossei and the extensor digitorum brevis.

MUSCLE ACTION: DORSAL INTEROSSEI

Action

Evidence

Abduction of metatarsophalangeal
joints of middle three toes

Insufficient

Flexion of metatarsophalangeal
joints of middle three toes

Insufficient

Extension of interphalangeal
joints of middle three toes

Insufficient

The four dorsal interossei are capable of producing metatar¬
sophalangeal joint abduction of the second toe in the medial

MUSCLE ACTION: PLANTAR INTEROSSEI

Action

Evidence

Addition of metatarsophalangeal
joints of lateral three toes

Insufficient

Flexion of metatarsophalangeal
joints of lateral three toes

Insufficient

Extension of interphalangeal
joints of lateral three toes

Insufficient

The three plantar interossei are very similar to their counter¬
parts in the hand, the palmar interossei (Muscle Attachment
Box 45.21). Lying on the medial side of the lateral three toes,
they adduct the metatarsophalangeal joints, pulling the toes
toward the reference toe, the second toe. Like the palmar
interossei, they contribute to metatarsophalangeal joint flex¬
ion and by their insertion into the extensor hood, contribute
to interphalangeal joint extension.

MUSCLE ATTACHMENT BOX 45.21

ATTACHMENTS AND INNERVATION
OF THE PLANTAR INTEROSSEI
(3 MUSCLES)

Proximal attachment: Base and medial side of the
third, fourth, and fifth metatarsal bones

Distal attachment: Medial side and base of the prox¬
imal phalanx of the same toe and the dorsal digital
expansion [95]

Innervation: Deep branch of the lateral plantar
nerve (S2, S3)

Palpation: Not palpable

860

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

MUSCLE ATTACHMENT BOX 45.22

ATTACHMENTS AND INNERVATION
OF THE DORSAL INTEROSSEI
(4 MUSCLES)

Proximal attachment: Sides of adjacent metatarsal
bones by two heads

Distal attachment: Bases of the proximal phalanx
and dorsal digital expansion. The first inserts into
the medial side of the second toe; the second
inserts to the lateral side of the second toe; the
third and fourth insert to the lateral side of the
third and fourth toes, respectively.

Innervation: Deep branch of the lateral plantar
nerve (S2, S3)

Palpation: The dorsal interossei are palpated on
the dorsum of the foot between adjacent
metatarsals.

MUSCLE ATTACHMENT BOX 45.23

ATTACHMENTS AND INNERVATION
OF THE EXTENSOR DIGITORUM BREVIS

Proximal attachment: Anterior superolateral surface
of the calcaneus, the interosseous talocalcaneal liga¬
ment, and inferior extensor retinaculum

Distal attachment: Dorsal aspect of the base of the
proximal phalanx of the hallux (sometimes referred
to as the extensor hallucis brevis) and lateral sides
of the tendons of the extensor digitorum longus to
the second, third, and fourth toes

Innervation: Lateral terminal branch of the deep
peroneal nerve (L5, SI) [95], (SI, S2) [76]

Palpation: The muscle belly of the extensor digito¬
rum brevis is the only muscle belly on the dorsum of
the foot and is palpated just distal to the ankle and
lateral to the tendons of the extensor digitorum
longus.

and lateral direction as well as abduction of the third and
fourth toes (Muscle Attachment Box 45.22). In addition, they
apparently can produce flexion of the metatarsophalangeal
joints and extension of the interphalangeal joints.

Extensor Digitorum Brevis

MUSCLE ACTION: EXTENSOR DIGITORUM BREVIS

Action

Evidence

Extension of interphalangeal
joints of medial four toes

Insufficient

Extension of interphalangeal
joints of middle three toes

Insufficient

The extensor digitorum brevis sends tendons to the medial
four toes (Muscle Attachment Box 45.23). The most medial
tendon extends to the great toe and is sometimes known as
the extensor hallucis brevis. This tendon crosses only the
metatarsophalangeal joint that it helps to extend. The other
three slips of the extensor digitorum brevis blend with the
tendons of the extensor digitorum longus and, therefore,
assist with extension of all three joints of these toes.

Group Effects of the Intrinsic Muscles
of the Foot

Although some individuals are able to actively abduct their
toes and even isolate the lumbricals and interossei by flex¬
ing the metatarsophalangeal joints while extending the
interphalangeal joints of the toes, most individuals are inca¬
pable of fine motor control of the toes. Studies of the intrin¬
sic muscles of the foot suggest that these muscles function

as a group during weight-bearing activities [7,53]. The clas¬
sic study by Mann and Inman, now over 40 years old,
remains the cornerstone of current understanding of the
role of the intrinsic muscles [53]. This study demonstrates
activity of the whole group during the stance phase of gait,
as the foot is supinating. This activity is interpreted as a
contribution to the stabilization of the foot as the body rolls
over it onto the opposite foot. Individuals with excessive
pronation exhibit increased activity of the intrinsic muscles
of the foot, apparently to provide additional support to a
foot that remains too flexible. Finally, this study and others
repeatedly demonstrate that the intrinsic muscles of the
foot are quiet during normal quiet standing, confirming the
notion that static foot alignment is supported primarily by
inert tissues [7,97].

Weakness of the intrinsic muscles contributes to a loss of
muscle balance in the foot and leads to an accentuated medial
longitudinal arch and claw toe deformities [1,25].

COMPARISONS OF GROUP MUSCLE
STRENGTH

An understanding of the relative strengths of the major mus¬
cle groups of the ankle and foot helps the clinician make clin¬
ical judgments of strength impairments in the leg. Most of the
studies of strength in the muscles of the leg focus on the
strength of plantarflexion and dorsiflexion. These studies
reveal, as expected, that plantarflexion is significantly stronger
than dorsiflexion [20,34,92]. These comparisons consistently
demonstrate that the plantarflexors produce at least three
times more peak torque than the dorsiflexors (Table 45.1).

Chapter 45 I MECHANICS AND PATHOMECHANICS OF MUSCLE ACTIVITY AT THE ANKLE AND FOOT

861

TABLE 45.1: Comparisons of Peak Plantar and Dorsiflexion Torques

Subjects

Peak Plantarflexion

Torque (Nm) a

Peak Dorsiflexion Torque

Gadeberg et al. [20]

6 women, 12 men
(age, 29-64 years)

117 ± 26

32 ± 8

Vandervoort and

11 men

171 ± 34

43.5 ± 6.5

McComas [92]

11 women
(age, 20-32 years)

113 ± 35

26.6 ± 4.5

a Measured with the knee flexed to between 70 and 80°.

Such a difference in peak torque is consistent with the large
difference in total muscle mass between the two groups,
which shows a three- to fourfold difference in cross-sectional
area [20]. The strength comparisons reported in Table 45.1
are based on plantarflexion torque measurements made with
the knee flexed. Studies suggest that peak plantarflexion
torque increases by 10-20% when measured with the knee
extended [46,80].

Several factors influence the peak plantar- and dorsiflex¬
ion torque produced. Plantar- and dorsiflexion torques are
greater in men than in women and both decrease steadily
with age [21,32,39,86,92]. Joint position also affects torque
measurements by altering muscle length and moment arm.
(Chapter 4 describes these effects in detail.) The length of
the muscle appears to be the greater influence on isometric
peak plantarflexion torque, which occurs with the ankle
positioned just short of maximum dorsiflexion [8,70,80]. In
contrast, peak dorsiflexion occurs with the ankle in approxi¬
mately 10° of plantarflexion [34,57]. Studies demonstrate
that the moment arms of the dorsiflexors are longest when
the ankle is at neutral or slightly dorsiflexed, and shortest
when the ankle is plantarflexed [45,49,78,85]. In contrast,
the length of the dorsiflexor muscles is shortest in dorsiflex¬
ion and longest in plantarflexion. The peak torque output of
the dorsiflexors appears to be affected by both muscle
length and angle of application, so that, like the biceps
brachii at the elbow and the quadriceps femoris at the knee,
peak dorsiflexion torque occurs with the ankle in a midposi¬
tion where neither angle of application nor muscle length is
optimal, but where their combined effect produces the
largest torque output.

Fewer studies compare the strength of the inverter and
everter muscles of the foot. Although they report no direct
comparison, Paris and Sullivan report peak forces of 75.22 ±
20.99 N (17 ± 4.7 lb) and 74.73 ± 21.09 N (16.8 ± 4.7 lb) for

inversion and eversion, respectively, when the ankle is in neu¬
tral dorsiflexion [72]. Other studies reporting concentric and
eccentric strengths find little or no difference between the
two groups. Studies report torques of 27 Nm and 24 Nm for
inversion and eversion, respectively at 60°/sec and 16 Nm for
both groups at 120°/sec [2,27]. After lateral ankle sprains,
eversion strength appears reduced compared with inversion
strength [2,74]. Perhaps a goal of rehabilitation following
ankle sprains should be to restore the equality of strength
between the everter and inverter muscles.

Table 45.2 presents the physiological cross-sectional areas
of the muscles that can invert and those that can evert the foot
reported by Brand for a single male subject [11]. If only the
posterior tibialis as the primary muscle of inversion is com¬
pared with only the primary everters, peroneus longus and
brevis, the latter have a slight advantage in size. However, the
moment arm of the posterior tibialis is larger than those of the
peroneus longus and brevis, and thus it is not surprising that
strength of these two groups is similar. Recruitment of
additional muscles to invert or evert the foot changes these
relationships.

Clinical Relevance

STRENGTH TESTING OF INVERSION AND EVERSION:

Data suggest that the strength of inversion and eversion of
the foot is similar. However ; if the subject is allowed to use
toe muscles during one or both motions; the group
strengths are altered. To monitor a patient's change in
strength , the clinician must use caution to ensure that the
same procedure is followed and that the same muscles par¬
ticipate during each test.

TABLE 45.2: Physiological Cross-Sectional Areas (PCAs) of Muscles That Invert and Evert the Foot

Inverters

PCA (cm 2 )

Everters

PCA (cm 2 )

Posterior tibialis

26.27

Peroneus longus

24.65

Flexor digitorum longus

6.4

Peroneus brevis

19.61

Flexor hallucis longus

18.52

Extensor digitorum
longus

7.46

Anterior tibialis

16.88

Peroneus tertius

4.14

862

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

SUMMARY

This chapter presents the muscles that move the ankle and
joints of the foot. The extrinsic muscles of the foot are organ¬
ized according to their function and data are presented to
explain their relative contributions to the motions of the ankle
and foot. It is demonstrated that while the anterior tibialis is
the strongest dorsiflexor muscle, other muscles contribute
significantly to dorsiflexion torque. Similarly, the posterior
tibialis and the peroneus longus and brevis are the primary
inverter and everters, respectively, but additional muscles
make important contributions to both motions. The gastroc¬
nemius and soleus together contribute most of the
plantarflexion moment, but other muscles provide some
plantarflexion as well. Strength comparisons reveal that plan¬
tarflexion is considerably stronger than dorsiflexion. Inversion
and eversion strengths are more similar to each other,
although contributions from muscles affecting the toes can
alter that comparison.

The role of these muscles in normal locomotion is pre¬
sented, and the effects of impairments on normal locomotion
discussed. Details of normal locomotion are presented in
Chapter 48. However, before proceeding to discussions of
posture and gait, it is useful to examine the loads to which the
foot is subjected during activities such as quiet standing, walk¬
ing, and running. Chapter 46 discusses the forces applied to
the foot by the surrounding muscles and limb segments dur¬
ing various activities of daily life.

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CHAPTER

Analysis of the Forces on the Ankle
and Foot during Activity

CHAPTER CONTENTS

TWO-DIMENSIONAL ANALYSIS OF THE FORCES IN THE FOOT.865

Two-Dimensional Analysis at the Ankle .865

Forces Applied to the Ankle and Tarsal Regions during Activity .867

Two-Dimensional Analysis of Forces on the Great Toe .867

Forces on the Great Toe during Gait.869

LOADS ON THE PLANTAR SURFACE OF THE FOOT DURING WEIGHT BEARING .869

SUMMARY .870

T he feet are the platform on which human beings stand and from which they propel themselves along the earth
during locomotion, in a game of basketball, while jumping over a stream, or during any number of activities
throughout the day. Central to all of these tasks is the ability of the foot to bear large loads. The purpose of
this chapter is to examine the loads sustained by the structures of the foot during weight-bearing activities and how
these loads contribute to complaints patients report in the clinic. Specifically the objectives of this chapter are to

■ Review the applications of a two-dimensional analysis to calculate loads on joints of the foot

■ Examine the loads sustained by the muscles and joints of the ankle and foot during function

■ Investigate the reported loads applied to the plantar surface of the foot during weight-bearing activities

TWO-DIMENSIONAL ANALYSIS
OF THE FORCES IN THE FOOT

Several studies estimate the loads exerted on the ankle and
joints of the foot during weight-bearing activities. Analyses at
the ankle and the great toe provide opportunities to review
the methods of two-dimensional analysis to estimate muscle
and joint reaction forces.

Two-Dimensional Analysis at the Ankle

A two-dimensional analysis of the forces on the ankle while
standing on tiptoes demonstrates the important role that the
calcaneus plays during upright stance (Examining the Forces

Box 46.1). The plantarflexor muscles provide the necessary
force to lift the body weight from the floor, and the calcaneus
provides a large moment arm for the plantarflexors, enhanc¬
ing their mechanical advantage. The ground reaction force
produces an external extension, or dorsiflexion, moment
(M exx ) °f 47.4 Nm that requires an internal, plantarflexion
moment of equal magnitude produced by the plantarflexor
muscles. Assuming that each foot bears half the body weight,
standing on tiptoes requires that the plantarflexor muscles in
each foot generate a force that is approximately 1.2 times
body weight. A smaller moment arm for the plantarflexor
muscles would require a larger force of contraction. It is
worth noting that as the individual rises higher onto the toes,
the moment arm of the ground reaction force decreases so

865

866

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

EXAMINING THE FORCES BOX 46.1

CALCULATION OF FORCES AT THE ANKLE
WHILE STANDING ON TIPTOES

SF X :

J x + T x = 0

Jx - - T x

where T x = T (cos 80°)
J x = -T (cos 80°)
J x = -137 N

The following dimensions are based on an individual
who is approximately 5 feet 10 inches tall (1.75 m)
and weighs 150 lb (668 N). The limb segment
parameters are extrapolated from the anthropometric
data of Braune and Fischer [3] and Winter [30].

The plantarflexion force is assumed to be provided
entirely by the triceps surae (T).

Length of foot (L): 0.2 m

Moment arm of triceps surae: 0.06 m [15 # 21 ]

Ground reaction force (G) is half body weight: 334 N
Moment arm of ground reaction force: 0.142 m

2M = 0

2F y :

J Y + T y + 334 N = 0
where T Y = T (sin 80°)

J Y = -334 N - 782 N
J Y = -1116 N

Using the Pythagorean theorem:

J Y

(G X 0.142 m) - (T X 0.06 m) = 0

J 2 = V + v

47.4 Nm = T X 0.06 m

J = 1124 N

T = 790 N

T = 1.2 BW

Calculate the joint reaction forces (J) on the talus.
Assume that the force of the triceps surae is applied
to the foot at an angle of 80° and the ground reaction
force is vertical.

J ~ 1.7 BW

Using trigonometry, the direction of J can be
determined:

sin 0 = J x /J

0 « 7° from the vertical

Chapter 46 I ANALYSIS OF THE FORCES ON THE ANKLE AND FOOT DURING ACTIVITY

867

Figure 46.1: Increasing plantarflexion of the ankle decreases the
dorsiflexion moment arm (x) of the ground reaction force (G).

that the required force from the plantarflexor muscles
decreases [13] (Fig. 46.1). Despite the advantage of the plan-
tarflexors and the reduced moment arm of the ground
reaction force, the joint reaction force on the ankle during
tiptoe stance is almost twice body weight. The large joint
reaction force reflects the difference in moment arms
of the plantarflexors and ground reaction force, the
latter still larger than the former.

Forces Applied to the Ankle and Tarsal
Regions during Activity

Activities that generate larger ground reaction forces, such as
standing on one foot, in which the ground reaction force
equals full body weight, or walking, when accelerations cause
the ground reaction force to rise above body weight, generate
even larger muscle and joint forces at the ankle. Several stud¬
ies examine the loads at the ankle while walking at normal
speeds. Estimated internal plantarflexion moments during
normal locomotion range from 83 to 117 Nm [20,23].

In vivo determination of tendo calcaneus (Achilles tendon)
forces during gait reveal average peak forces of 1430 N ± 500
N (321 ± 112 lb) [9]. A biomechanical model of the foot pre¬
dicts peak forces in the Achilles tendon that are almost four
times the weight of the body [10]. A report of the Achilles
tendon s load to failure at high loading rates reveals that the
loads sustained during walking fall well below the average
ultimate strength of 5579 ± 1143 N (1253 ± 257 lb) [31]. Yet

degenerative changes within the tendon reduce the ultimate
strength of the tendon so that the loads sustained during
high-speed movements such as lunging for a tennis ball or
tripping are certainly sufficient to exceed the tolerance of a
weakened Achilles tendon and produce a rupture.

Mathematical models that calculate joint reaction forces at
the ankle joint during gait suggest that the ankle sustains peak
compressive loads three to five times body weight [18,25,26].
Loads of more than 10 times body weight are reported dur¬
ing running [22]. Despite these very large loads applied to the
ankle with every step, the ankle appears rather immune to
degenerative changes unless they are precipitated by joint
trauma. Chapter 44 demonstrates that the talocrural articula¬
tion is quite congruent, which appears to help reduce joint
stresses. In addition, studies demonstrate that the contact
area between the talus and the tibia and fibula is greatest and
stress (force/area) is minimized with the ankle plantarflexed
[6,17]. Peak ankle joint forces during walking and running
occur with the ankle joint plantarflexed, and although the
ankle joint sustains large peak joint reaction forces, the stresses
appear to be small enough to avoid degeneration of the
articular surfaces.

Clinical Relevance

ANKLE OSTEOARTHRITIS: Osteoarthritis of the ankle
is uncommon and occurs typically as a result of ankle
injuries. Such degenerative changes are likely the result
of changes in stress applied to the joint surfaces.

Lloyd et al. demonstrate that talar shifts of 1 mL produce a
40% reduction in contact area at the tibiotalar joint [14].
Talar shifts may occur in the unstable ankle following ankle
sprains. These data suggest that there is a strong biome¬
chanical rationale for rehabilitation or surgical intervention
to restore ankle stability and joint congruity following severe
ankle injuries. Such an intervention may help reduce the
risk of degenerative joint disease later in life.

Studies also examine the forces sustained elsewhere in the
foot. Data collected from cadaver specimens during loading
of the foot and ankle suggest that the subtalar joint sustains
loads up to 2000 N (450 lb), or over four times body weight,
and stresses from 3 to 4 MPa during axial loading or simulated
gait [10,19,29]. Contact forces and stresses are reportedly
smaller but still several times body weight at the calca¬
neocuboid and talonavicular joints.

Two-Dimensional Analysis of Forces
on the Great Toe

Rising up on the forefoot also applies large loads to the bones
and joints of the toes and particularly to the metatarsopha¬
langeal joint of the great toe, since most individuals locate the
load under the great toe during the final stages of stance
[2,16,28,32]. Examining the Forces Box 46.2 presents a

868

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

EXAMINING THE FORCES BOX 46.2

CALCULATION OF THE JOINT REACTION
FORCE AT THE FIRST METATARSO¬
PHALANGEAL JOINT DURING GAIT

Calculate the joint reaction forces (J) on the base of
the proximal phalanx. Assume that the force of the
flexor hallucis longus is applied to the phalanx at an
angle of 10°.

GRF h + J x — F x = 0
Jx = F x - 40 N
where F x = F (cos 10°)

J x = 431 N - 40 N
J x = 391 N

The following dimensions are based on an individual
who is approximately 5 feet 3 inches tall (1.6 m) and
weighs 125 lb (556 N). The limb segment parameters
are extrapolated from the literature [3 # 15 # 21 # 30].

The plantarflexion force on the great toe is assumed
to be provided entirely by the flexor hallucis longus
(F). The ground reaction force has both vertical (GRF V )
and horizontal (GRF H ) components.

SF y :

GRF V + J Y - F y = 0
where F Y = F (sin 10°)

J Y = -240 N + 76.1 N
J Y = -164 N

Using the Pythagorean theorem:

Moment arm of flexor hallucis longus: 0.02 m
Vertical ground reaction force (GRF V ): 240 N
Moment arm of vertical ground reaction force:
0.033 m

Horizontal ground reaction force (GRF H ): 40 N
Moment arm of horizontal ground reaction force:
0.021 m

J 2 = V + V

2M = 0

(GRF V X 0.033 m) + (GRF H X 0.021 m)

- (F X 0.02 m) = 0

240 N X (0.033) + (40 N X 0.021 m) = F X 0.02 m
8.76 Nm = F X 0.02 m

Note that the internal moment (M INT ) = 8.76 Nm
F = 438 N

J = 424 N
J « 0.76 BW

Using trigonometry, the direction of J can be
determined:

cos 0 = J x /J

0 « 23° from the horizontal

F = 0.79 BW

Chapter 46 I ANALYSIS OF THE FORCES ON THE ANKLE AND FOOT DURING ACTIVITY

869

two-dimensional analysis of the loads on the base of the prox¬
imal phalanx of the great toe as the body rolls over the foot
during locomotion. During the late stages of stance as the
body passes over the stance foot, the stance foot applies a
backward and downward force on the ground, and the
ground reaction force is upward (GRF y ) and forward
(GRF h ). The ground reaction force tends to extend, or dorsi-
flex, the metatarsophalangeal joint of the great toe. The
extension moment applied by the ground reaction force is
balanced by a flexion moment produced by the flexor muscle
force (F). Using estimates of the ground reaction force from
the literature, the flexor muscles in this sample problem gen¬
erate 438 N (98 lb), or approximately 79% of body weight, to
stabilize the great toe during roll off [30]. The joint reaction
force determined for this phase of gait is 424 N (95 lb), or
approximately 76% body weight.

Forces on the Great Toe during Gait

Published estimates of the joint reaction force at the metatar¬
sophalangeal joint of the great toe during normal locomotion
vary widely and range from approximately 30% to almost
100% of body weight [12,16,28,32]. These variations origi¬
nate, in part, from differences in the models used to calculate
the forces. However, all of the studies also report large
interindividual variability. A major contributor to the calcula¬
tion is the magnitude of the ground reaction force on the toe,
which depends on walking form and walking speed. Older
individuals appear to have smaller ground reaction forces
because they walk more slowly [32]. In running, which pro¬
duces much larger ground reaction forces, the external
moment to dorsiflex the metatarsophalangeal joint reaches
reported peaks ranging from 60 to 100 Nm [27].

Clinical Relevance

LARGE LOADS ON THE METATARSAL BONES: Large
joint loads at the first metatarsophalangeal joint are impli¬
cated in the degenerative changes commonly found at the
first metatarsophalangeal joint. Loads on the metatarsal
bones also are suspected to contribute to stress fractures
found in the metatarsal bones. Stress fractures of the
metatarsal bones are common in soldiers who participate in
long marches and in runners who rapidly increase their
training regimen. Walking and running apply large ground
reaction forces on the metatarsal bones and generate bend¬
ing moments within the bones (Fig. 46.2). Studies show that
the plantar fascia and extrinsic muscles help to reduce the
strain , or deformation , of the bones [8,24]. Clinicians can
help to prevent such injuries by educating patients to
modulate training to avoid prolonged walking or running
in the presence of severe muscle fatigue. Consideration
may also be given to the benefits of supportive footwear.

Figure 46.2: The loads on the metatarsal heads during gait
produce bending moments in the metatarsal bones that may
contribute to stress fractures.

LOADS ON THE PLANTAR SURFACE OF
THE FOOT DURING WEIGHT BEARING

This chapter demonstrates that the structures of the foot sus¬
tain large loads during weight-bearing activities, especially
locomotor activities. These loads are directly related to the
contact forces between the foot and the ground. While walk¬
ing, the foot collides with the ground at every step, and each
collision is even more vigorous in running. The foot possesses
many special structures to sustain such repeated collisions,
including the fat pad on the plantar surface of the heel, the
plantar aponeurosis, the plantar plates of the metatarsopha¬
langeal and interphalangeal joints, as well as the special bony
architecture of the foot. The magnitude and location of the
loads applied to the plantar surface of the foot contribute to
many complaints of foot pain and dysfunction, from sore feet
and blisters to diabetic ulcers. An understanding of factors
that contribute to the loading characteristics of the foot helps
clinicians identify ways to minimize the detrimental effects of
these loads.

Loading on the foot is typically described by pressure
that, like stress, equals the force/area, where the measured
force is perpendicular to the measuring device. In studies of
the foot, pressure is a close approximation of the vertical
stress on the foot. Investigation of the loading pattern of the
foot reveals, as expected, that the largest vertical loads and
pressures are applied to the heel at ground contact [7]. Peak

870

Part IV I KINESIOLOGY OF THE LOWER EXTREMITY

jogging has stimulated an entire industry that has led to numer¬
ous innovations in the design and construction of footwear to
enhance the foots ability to withstand these high-impact loads.

Figure 46.3: Although most of the plantar surface of the foot
sustains substantial pressures during the stance phase of gait, the
largest pressures are found at the heel, metatarsal heads, and
the great toe. (Redrawn with permission from Sammarco GJ,
Hockenbury RT: Biomechanics of the foot and ankle. In: Nordin
M, Frankel VH, eds. Basic Biomechanics of the Musculoskeletal
System. Philadelphia: Lippincott Williams & Wilkins, 2001.)

pressures on the heel of approximately 13 to 14 MPa are
reported during standing (Fig. 46.3). The fat pad of the heel
is specially equipped to absorb such high stresses.

Although almost the entire plantar surface of the foot is
exposed to pressure in most individuals, the next highest peaks
occur at the metatarsal heads, with the greatest pressures on
the second metatarsal (approximately 5.0 MPa). Such high
pressures likely contribute to the high incidence of stress frac¬
tures of the second metatarsal bone in runners and military
recruits [8,24]. The hallux sustains the largest pressures of the
five toes (approximately 2.0 MPa). The foot also withstands
large shear stresses during gait, as high as 8.5 MPa [11].
Several factors influence the pressures and shear stresses
applied to the foot, including its shape, arch height, and
supporting muscles. Individuals with pes cavus, an excessively
high medial longitudinal arch, report an increased prevalence
of foot pain compared to individuals with normal arch height
[5]. Not surprisingly, these individuals also demonstrate
increased plantar pressures on the rearfoot and an increased
duration of high pressures on the rearfoot and forefoot. These
changes in stress are consistent with a decrease in contact area
on the foot because of the elevated arch (stress = force/area).
Walking speed also appears to affect plantar pressures.
Increased walking speed reportedly increases pressures under
the heel, forefoot, and toes in healthy elders [4].

Footwear affects plantar pressures as well. Some shoes
increase the contact area between the foot and ground, thereby
decreasing stress. The structure of the shoe and the interface
between the foot and the shoe, including the sock, can alter the
plantar pressures and shear forces on the foot. The popularity of

Clinical Relevance

SKIN ULCERS IN THE INSENSITIVE FOOT: Individuals
who lack sensation to the foot are at high risk for skin
breakdown on the plantar surface of the foot. The data on
the pressures applied to the foot during normal walking
indicate that even the normal foot has discrete areas of high
stress. A high-arched foot may have increased stress under
the metatarsal heads, while afoot with excessive pronation
may sustain increased stress on the medial aspect of the
forefoot. Areas of high stress typically produce pain, and
under normal circumstances, the individual takes action to
reduce the pain, perhaps by wearing more comfortable
shoes or decreasing the load on the feet by resting.

However ; an individual who lacks sensation may be
unaware of the areas of high or excessive stress and there¬
fore do nothing to reduce the stress.

Prolonged stress can cause vascular changes within the
stressed tissue and lead to tissue damage that, in some
cases, may prove catastrophic. Some individuals with
impaired sensation also exhibit a compromised vascular sys¬
tem. For example, an individual with diabetes mellitus may
develop a peripheral neuropathy with resulting muscle
weakness, sensory loss, and vascular changes in the feet
and a common clinical scenario unfolds. As the patient con¬
tinues to ambulate, the gait pattern gradually changes
because of the weakness, and areas of high stress on the
plantar surface of the foot during gait develop [1]. With the
gradual diminution of sensation, the individual is unaware
of the areas of high stress on the bottom of the foot, and
the continued stresses lead to skin breakdown. The patient's
impaired peripheral vascular system inhibits healing, and
the ulcer fails to heal. Infection may set in, and in the worst
case, the patient faces amputation. By understanding the
normal pattern of loading on the foot and the factors that
produce abnormally high stresses, clinicians can help pre¬
vent the catastrophic skin lesions by teaching the patient to
identify areas of high stress and by recommending shoe
modifications to alter the stress patterns under the foot.

SUMMARY

This chapter uses examples of two-dimensional analysis
to demonstrate the magnitude of the loads sustained by the
ankle and metatarsophalangeal joint of the great toe. Although
these examples are oversimplifications of the three-dimen¬
sional nature of the forces on these joints as well as the
anatomical structures that exert loads at the joints, they pro¬
vide the clinician with a perspective to appreciate the normal

Chapter 46 I ANALYSIS OF THE FORCES ON THE ANKLE AND FOOT DURING ACTIVITY

871

wear-and-tear that the foot withstands with every step.
A review of the literature reveals that all of the structures of
the foot sustain large loads that are even larger during activ¬
ities that produce large ground reaction forces, such as
running and jumping. The chapter also demonstrates that
the high loads applied to the foot may contribute to patho¬
logical processes within the foot that produce pain and dis¬
ability. Examples in which the loads on the foot contribute
directly to pathology include stress fractures of the
metatarsals and skin ulcers in an insensitive foot that sustains
excessive plantar pressures. This chapter concludes the pres¬
entation of the mechanics and pathomechanics of the bones,
joints, and muscles of the lower extremity. The remaining
two chapters of this text apply the current understanding of
the structure and function of the musculoskeletal system to
the examination of their contributions to two functions that
are central to the function of human beings, upright posture
and locomotion.

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11. Hosein R, Lord M: A study of in-shoe plantar shear in normals.
Clin Biomech (Bristol, Avon) 2000; 15: 46-53.

12. Jacob HAC: Forces acting in the forefoot during normal gait-an
estimate. Clin Biomech 2001; 16: 783-792.

13. Kerrigan DC, Riley PO, Rogan S, Burke DT: Compensatory
advantages of toe walking. Arch Phys Med Rehabil 2000; 81:
38-44.

14. Lloyd J, Elsayed S, Hariharan K, et al.: Revisiting the concept
of talar shift in ankle fractures. Foot Ankle Int 2006; 27:
793-796.

15. Maganaris CN, Baltzopoulos V, Sargeant AJ: In vivo measure¬
ment-based estimations of the human Achilles tendon moment
arm. Eur J Appl Physiol 2000; 83: 363-369.

16. McBride ID, Wyss UP, Cooke TDV, et al.: First metatarsopha¬
langeal joint reaction forces during high-heeled gait. Foot Ankle
1991; 11: 282-288.

17. Michelson JD, Checcone M, Kuhn T, Varner K: Intra-articular
load distribution in the human ankle joint during motion. Foot
Ankle Int 2001; 22: 226-233.

18. Procter P, Paul JP: Ankle joint biomechanics. J Biomech 1982;
15: 627-634.

19. Reeck J: Support of the talus: a biomechanical investigation of
the contributions of the talonavicular and talocalcaneal joints,
and the superomedial calcaneonavicular ligament. Foot Ankle
Int 1998; 19: 674-682.

20. Robon MJ, Perell KL, Fang M, Guererro E: The relationship
between ankle plantarflexor muscle moments and knee com¬
pressive forces in subjects with and without pain. Clin Biomech
2000; 15: 522-527.

21. Rugg SG, Gregor RJ, Mandelbaum BR, Chiu L: In vivo moment
arm calculations at the ankle using magnetic resonance imaging
(MRI). J Biomech 1990; 23: 495-501.

22. Scott SH, Winter DA: Internal forces at chronic running injury
sites. Med Sci Sports Exerc 1990; 22: 357-369.

23. Scott SH, Winter DA: Talocrural and talocalcaneal joint kine¬
matics and kinetics during the stance phase of walking. J
Biomech 1991; 24: 743-752.

24. Sharkey NA, Ferris L, Smith TS, Matthews DK: Strain and load¬
ing of the second metatarsal during heel-lift. J Bone Joint Surg
[Am] 1995; 77: 1050-1057.

25. Simonsen EB, Dyhre-Poulsen P, Voigt M, et al.: Bone-on-bone
forces during loaded and unloaded walking. Acta Anat [Basel]
1995; 152: 133-142.

26. Stauffer RN, Chao EYS, Brewster RC: Force and motion analy¬
sis of the normal, diseased, and prosthetic ankle joint. Clin
Orthop 1977; 127: 189-196.

27. Stefanyshyn DJ, Nigg BM: Mechanical energy contribution of
the metatarsophalangeal joint to running and sprinting. J
Biomech 1997; 30: 1081-1085.

28. Stokes IA, Hutton WC, Stott JR: Forces acting on the
metatarsals during normal walking. J Anat 1979; 129: 579-590.

29. Wang CL, Cheng CK, Chen CW, et al.: Contact areas and pressure
distributions in the subtalar joint. J Biomech 1995; 28: 269-279.

30. Winter D: Biomechanics and Motor Control of Human
Movement. New York: John Wiley & Sons, 1990.

31. Wren TAL, Yerby SA, Beaupre GS, Carter DR: Mechanical
properties of the human achilles tendon. Clin Biomech 2001;
16: 245-251.

32. Wyss UP: Joint reaction forces at the first MTP joint in a normal
elderly population. J Biomech 1990; 23: 977-984.

PART

Posture and Gait

Chapter 47: Characteristics of Normal Posture and Common Postural Abnormalities
Chapter 48: Characteristics of Normal Gait and Factors Influencing It

873

PART V

T he first part of this textbook presents the basic principles needed to understand the mechanics and pathome-
chanics of the musculoskeletal system and presents the mechanical properties of the individual components
of the musculoskeletal system. Most of the text then examines the structural and functional properties of the
individual joint complexes in the body. This final portion of the textbook applies this knowledge to the analysis of two
intrinsically human functions, erect standing and bipedal locomotion. The goals of this final segment are to:

■ discuss the biomechanical demands of these two functions

■ demonstrate how a basic understanding of the structure and function of the components of the musculoskeletal
system leads to the ability to analyze functions that involve many different joint complexes

Patients seek help from rehabilitation experts typically for complaints of pain or difficulty in performing a task rather
than with complaints of impairments in specific anatomical structures. Clinicians must be able to observe the activity
in question, analyze the biomechanical demands of the activity, and determine what, if any, impairments contribute to
the pathomechanics producing the complaints. Examination and evaluation of posture and gait require an understand¬
ing of the basic biomechanical principles introduced in the first two chapters of this book and use knowledge of mus¬
cle and joint function to explain how an individual produces these characteristic human behaviors. Clinicians who can
evaluate posture and gait and can identify impairments that contribute to an abnormal movement pattern will be able
to apply these same skills to evaluate and treat any abnormal movement, including activities as diverse as lifting box¬
car hitches, performing a grand plie, typing at a computer, or operating a cash register at the local supermarket.

Chapter 47 describes the current understanding of "correct" posture and discusses the mechanisms to control the
posture. Chapter 48 presents the characteristics of normal locomotion and discusses the factors that influence it.

874

CHAPTER

Characteristics of Normal
Posture and Common
Postural Abnormalities

CHAPTER CONTENTS

NORMAL POSTURE.876

Postural Sway .876

Segmental Alignment in Normal Posture.876

Muscular Control of Normal Posture.884

POSTURAL MALALIGNMENTS.887

Muscle Imbalances Reported in Postural Malalignments.887

SUMMARY .889

P osture is the relative position of the parts of the body, usually associated with a static position. Clinicians

evaluate posture with the underlying assumptions that abnormal posture contributes to patients' complaints
and that many impairments within the neuromusculoskeletal system are reflected in an individual's posture.
Thus clinical interpretation of an individual's posture requires blending a description of an individual's posture with an
understanding of the person's physical condition and complaints.

Posture in erect standing is the focus of much clinical attention, but postures in sitting and during activities, such as
lifting or assembly line work, also may contribute to musculoskeletal complaints. This chapter focuses on standing
posture, but the issues considered to understand erect standing posture are applicable to any other posture as well. It
is important to recognize that even seemingly static postures such as erect standing exhibit small, random movements,
and typically, humans move in and out of several postures. As a result, assessment of a single posture may be insuffi¬
cient to understand the link between posture and a patient's complaints.

Analysis of posture is a well-established clinical tradition and forms a basic part of the physical examination for many
different health disciplines. Despite the frequency with which such evaluations are carried out, there remains a surpris¬
ing lack of unanimity in the description of "normal" posture. Although faulty posture has been associated with such
diverse complaints as headaches, respiratory and digestive problems, and back pain throughout the centuries, the
direct consequences of faulty posture are not well documented. The purposes of this chapter are to describe the cur¬
rent understanding of normal posture and to describe some common postural faults. Specifically, the objectives of this
chapter are to

■ Describe the alignment of the body in erect standing posture and its variability
■ Discuss the current understanding of the muscles needed to control erect standing posture
■ Describe common postural faults

■ Briefly discuss the purported consequences of postural faults

875

876

Part V I POSTURE AND GAIT

NORMAL POSTURE

Posture is evaluated by examining its stability and also by
describing the relative alignment of adjacent limb segments.

Postural Sway

Normal erect standing posture is often compared to the
movement of an inverted pendulum in which the base is
fixed and the pendulum is free to oscillate over the fixed
base (Fig. 47.1). Although erect standing appears static to
the casual observer, it is characterized by small oscillations
in which the body sways anteriorly, posteriorly, and side to
side; and the bodys center of mass, approximately locat¬
ed just anterior to the body of the first sacral vertebra,
inscribes a small, irregular circle within the base of support
[8,42]. This normal postural sway in erect standing also is
described by the movement of the center of pressure,
which is related to, but distinct from, the location of the
body’s center of mass [29,50]. The center of pressure

Figure 47.1: Standing posture often is modeled as an inverted
pendulum in which the body sways over the fixed feet.

locates the center of the distributed pressures under both
feet. In contrast, a vertical line through the center of mass
locates the center of mass within the entire base of sup¬
port. The normal sway of the body during quiet standing
moves the center of mass and the center of pressure of the
body anteriorly and posteriorly up to 7 mm [8,42,50,67].
Side-to-side excursions of the centers of mass and pressure
are only slightly less than those in the anterior-posterior
direction [8].

Clinical Relevance

ASSESSING STABILITY IN QUIET STANDING: Stability
in quiet standing is assessed in different populations to bet¬
ter understand why some individuals are at increased risk
for falling. Changes in the magnitude or frequency of pos¬
tural sway determined by the oscillations of either center of
pressure or center of mass are reported in healthy elders
and in individuals with impairments such as hemiparesis,
sensory deficits, flat and high-arched feet and vestibular
dysfunctions [8,42,50,63].

Segmental Alignment in Normal
Posture

SAGITTAL PLANE ALIGNMENT OF THE BODY
IN NORMAL POSTURE

Although both ideal posture and normal posture have
been described in the clinical literature, the criteria for the
ideal posture remain hypothetical [31,58]. Ideal posture is
variously described as the posture that requires the least
amount of muscular support, the posture that minimizes the
stresses on the joints, or the posture that minimizes the loads
in the supporting ligaments and muscles [1,31]. In the
absence of a clear understanding of the meaning of the
“ideal” posture, careful measurements of the positions
assumed by individuals without known musculoskeletal
impairments or complaints provide a perspective on the typi¬
cal, if not ideal, alignment of limb segments.

Table 47.1 presents the relative orientation of landmarks in
the sagittal plane with respect to the ankle joint from two stud¬
ies examining the posture of individuals without any known
musculoskeletal impairment or complaint [8,48]. Fig. 47.2
presents the relative location of the landmarks with respect to
a line through the center of mass, which lies approximately
4 to 6 cm anterior to the ankle joint [8,48]. The two studies
report similar relative alignments, and both also agree some¬
what with the “ideal posture” described by Kendall et al. [31].
The relatively large standard deviations at the superior
landmarks reported by Danis et al. are consistent with
the normal postural sway that occurs in quiet standing.

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

877

TABLE 47.1: Alignment in the Sagittal Plane of Body Landmarks with Respect to the Ankle
during Erect Standing

Opila et al. [48] a

Danis [S] b

Description of Landmark

Location 0 (cm)

Description of Landmark

Location 0 (cm)

Ankle

Lateral malleolus

Calculated joint center

Knee

Lateral epicondyle of Femur

5.1

Calculated joint center

4.24 ± 2.14

Hip

Greater trochanter

5.4

Calculated joint center

5.42 ± 2.86

Shoulder

Acromioclavicular joint

3.0

Acromion process

1.89 ± 3.01

Head/neck

Just inferior to the external
auditory meatus

5.4

Approximately the atlanto-
occipital joint

4.84 ± 4.03

a Based on 19 unimpaired males and females aged 21 to 43 years. Originally reported with respect to the body's center of gravity.
fa Based on 26 unimpaired males and females aged 22 to 88 years. Originally referenced to the ankle joint.
c Positive numbers indicate that the landmark is anterior to the ankle joint.

Figure 47.2: In erect standing, the body is aligned approximately
so that a line through the body's center of mass passes very close
to the ear, slightly anterior to the acromion process of the
scapula, close to the greater trochanter, slightly anterior to the
knee joint, and anterior to the ankle joint.

Trunk and Pelvic Alignment

The data presented in Table 47.1 describe the sagittal plane ori¬
entation of many body parts in erect standing but provide little
information regarding the normal alignment of the spine and
pelvis. The adult spine is characterized by a kyphosis in the
thoracic and sacral regions in which the curves are convex
posteriorly and a lordosis in the cervical and lumbar regions
in which the curves are concave posteriorly. At birth, the
spine is entirely kyphotic, and consequently, the thoracic and
sacral curves are primary curves. Development of head
control by approximately 4 months of age induces the devel¬
opment of a cervical lordosis, and a child’s progression to
upright standing and bipedal ambulation lead to the forma¬
tion of the lumbar lordosis. Hence these curves are known as
secondary curves and do not develop in the absence of
acquisition of the respective skill.

The most common means of characterizing the curva¬
tures of the spine use a radiographic method to assess the
total curve of a region. The Cobb angle describes the angle
formed by the surfaces of the superior and inferior verte¬
brae of a spinal region (Fig. 47.3). Mean Cobb angles of
20 to 70° are reported for the lumbar region and 20 to 50°
for the thoracic region [19,27,66,70]. These data demon¬
strate wide disparities and are influenced by the measure¬
ment procedures used in each investigation, but also reflect
the wide spectrum of spinal curvatures found in a popula¬
tion with no known pathology. Despite the differences
reported in the literature, some consistent findings are
found. The studies that examine both the thoracic and lum¬
bar curves consistently report a larger lumbar lordosis than
thoracic kyphosis [2,19,28,53,64]. Both thoracic and lumbar
curves appear to increase during growth and development
[39]. There is general agreement that the peak or apex of
the thoracic curve occurs at approximately the midthoracic
region, most often at T7, and the apex of the lumbar curve
typically is located at either L3 or L4 [2,17,19,64].

Although the Cobb method is the most frequently used
method of quantifying spinal curves, it requires radiographic
assessment and is not part of a routine physical examination.

878

Part V I POSTURE AND GAIT

Figure 47.3: Cobb angles in the thoracic and lumbar spines are
determined radiographically by determining the angles formed
between the superior surface of the most superior vertebra of
the region and the inferior surface of the most inferior vertebra
of the region.

Methods to evaluate the spinal curves from surface assessment
include the use of inclinometers to define the angulation, and
flexible rulers and computer-assisted surface digitizers to trace
the shape of the spinal curvature [18,46,62,77] (Fig. 47.4). The
surface curvature methods yield different measurements from
radiographic methods and may differ among themselves,
depending upon the mathematical analyses used to describe
the curves [46,59]. No current surface curvature methods
offer normative data defining the range of curvature values

Figure 47.4: Surface methods to assess spinal curves. A. Clinicians use
inclinometers to measure the curvature of spinal regions from sur¬
face palpations. B. Flexible rulers are used to trace the curvature in a
spinal region, and the tracing can be quantified mathematically.

found in a healthy population. Based on current knowledge,
clinicians lack well-accepted criteria for normal curvatures of
the spine in the sagittal plane using surface methods
and continue to rely on qualitative assessments of the
spinal curves [40].

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

879

Clinical Relevance

MONITORING CHANGES IN FORWARD-HEAD
POSTURE: Forward-head posture is associated with a wide
range of patient complaints including headaches; vertigo,
temporomandibular joint pain , and neck and shoulder pain.
A typical physical examination of a patient with any of
these complaints includes assessment of postural alignment
(Fig. 475). Although objective procedures to quantify head
position exist [18,24], the clinician often resorts to visual
observation of head postureassessing head alignment as
normal or noting a "mild/' "moderate/' or "severe" forward
head position. In the presence of an abnormal forward-head
posturethe clinician typically initiates an intervention to
improve or normalize the posture. However ; without opera¬
tional definitions of the postural deviations , it is difficult to
identify changes in posture objectively and to associate any
changes in the patient's complaints with changes in posture.
Third-party payers are challenging the value of interventions
to alter posture. Well-controlled outcome studies to measure
the effectiveness of postural interventions are needed , and
these studies demand more precise and more objective
measures of postural alignment.

Orientation of the pelvis is a common postural evaluation
performed in conjunction with the assessment of spinal
curves. Pelvic alignment is determined from the orientation
of the sacrum or by the orientation of pelvic landmarks. Most

Figure 47.6: Sacral alignments determined from radiographs
typically measure the angle between the superior surface of the
sacrum and the horizontal (0) or an angle between the posterior
surface of the sacrum and the vertical (a).

Figure 47.5: Forward-head alignment observed in the clinic
is often assessed qualitatively as mild, moderate, or severe.

measurements based on sacral alignment derive from radi¬
ographic assessment and report the angle made between a
vertical or horizontal reference line and either the superior or
posterior surface of the sacrum [27,66] (Fig. 47.6). The angle
between the horizontal and the superior surface of the first
sacral vertebra is known as the sacral slope. Pelvic inci¬
dence is another radiologic parameter that relates the sacral
slope to the location of the femoral heads.

Orientation of the pelvis from surface landmarks is reported
as the angle formed between the horizontal and a line con¬
necting the posterior superior iliac spine with the anterior
superior iliac spine [5,16,77] (Fig. 47.7). Typical measurements
of sacral and pelvic orientation are reported in Table 47.2.
Measurements based on the orientation of the sacrum are
larger than those based on the pelvis, and the two measure¬
ment procedures show only slight-to-moderate correlations
with each other [20].

Clinical literature suggests interdependence among the
spinal curves and pelvic alignment [31]. An increased lordosis
purportedly accompanies an increased thoracic kyphosis.
Similarly, an anterior pelvic tilt reportedly accompanies an
increased lumbar lordosis, while a decreased lumbar lordosis

880

Part V I POSTURE AND GAIT

Figure 47.7: Pelvic alignment from surface landmarks is defined
by the angle between a line drawn through the anterior superior
iliac spine (ASIS) and the posterior superior iliac spine (PSIS) and
the horizontal (0).

is reportedly associated with a posterior pelvic tilt. There is
limited evidence to support these purported relationships,
and the existing relationships may be more complex than
those reflected by the popular beliefs. The assessment
procedures as well as the populations studied appear to affect
the strength of the associations reported. A study of 100
adults over the age of 40 years reports a correlation between
the thoracic kyphosis measured between T5 and T12 and the
total lumbar lordosis but finds no association between the
kyphosis in the upper thorax and the lumbar lordosis [16]. A
study of 88 adolescents reports no relationship between the
thoracic kyphosis from T3 to T12 and the total lumbar lordo¬
sis [53]. However, the same study does find correlations

between the thoracic kyphosis and the lordosis between L5
and SI. Although additional research is required, these data
suggest some association between the thoracic and lumbar
curves, but their interdependence may be a function of age
and the specific morphology of an individuals spine.

Studies investigating the relationship of pelvic alignment
and lumbar lordosis also yield conflicting results. Studies that
use radiographic measures consistently demonstrate an associ¬
ation between pelvic tilt as measured by sacral alignment and
lumbar lordosis measured by the Cobb method [11,16,53].
These studies demonstrate the expected positive associations
between an anterior tilt of the sacrum and an increased lordo¬
sis and between posterior tilting and a flattening of the lordosis
(Fig. 47.8). Both sacral slope and pelvic incidence have strong
positive correlations with the lumbar lordosis, and pelvic inci¬
dence appears to be increased in individuals with spondylolis¬
thesis at L5-S1 [4,53]. Yet studies using surface methods to
assess pelvic and spinal alignment in static posture fail to
demonstrate any significant correlation between pelvic align¬
ment using pelvic landmarks and the amount of lumbar lordo¬
sis using inclinometers or flexible rulers [55,62,63]. In con¬
trast, studies using surface methods to assess the association
between pelvic tilt and lumbar position during active move¬
ment demonstrate that posterior pelvic rotations do appear to
decrease the lumbar lordosis [8,30]. Controversy continues
regarding the effect of an active anterior pelvic tilt and the lor¬
dosis, with studies showing an increased lordosis with an ante¬
rior tilt [7,30] and others showing no change [8].

The studies reported here present confusing results for cli¬
nicians. On the one hand, radiographic data support the gener¬
ally accepted clinical impression that pelvic alignment and
spinal curves are related, but assessments of those relationships
using the evaluation procedures typically applied in the clinic
reveal weak or absent relationships. What do these conflicts
mean to the clinician? Existing evidence appears sufficient to
justify the continued belief that pelvic and spinal alignments
are interdependent. However, current clinical assessment tools
may be influenced enough by soft tissue overlying the skeleton
that they do not reflect true bony alignment. The larger ques¬
tion that clinicians and researchers must answer is whether
knowing the alignment of the pelvis and the spine, regardless
of measurement technique, affects treatment outcomes.

TABLE 47.2: Measurements of Pelvic Orientation Reported in the Literature

Sacral Orientation

Pelvic Incidence

ASIS-PSIS Angle

Voutsinas and MacEwen [66]

56.5 ± 9.3 a

During et al. [13]

40.4 ± 8.8 6

Jackson and McManus [27]

50.4 ± 7.7 C

Boulay et al. [4]

53.0 ± 9 od

Levine and Whittle [35]

11.3 ± 4.3

Crowell et al. [5]

12.4 ± 4.5

a Based on the angle made by the superior surface of the sacrum and the horizontal.
b Based on the angle made by the posterior surface of the sacrum and the vertical.
c Based on the angle made by the superior surface of the sacrum and the horizontal.

d Based on the angle between a line perpendicular to the superior surface of the sacrum and a line from the superior surface of the sacrum to the center
of the hip joint.

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

881

Figure 47.8: An anterior pelvic tilt, which enlarges the angle (cf>)
formed by the horizontal and a line through the anterior superior
iliac spine, is believed to lead to an increased lumbar lordosis
(A) and a posterior pelvic tilt in which the angle (4>) decreases
and produces a decreased lumbar lordosis (B). Data supporting
these beliefs conflict.

Clinical Relevance

IS POSTURE REEDUCATION A USEFUL
INTERVENTION STRATEGY FOR A PATIENT WITH
LOW BACK PAIN?: A patient with low back pain provides
a good model to examine the role posture plays in some
treatment strategies. The patient reports pain with lumbar
extension and in quiet standing and decreased pain with
forward bending and sitting. Radiographs demonstrate a
spondylolisthesis at L4-L5. Spondylolisthesis is an anterior
displacement of one vertebra on the vertebra below , and
decreasing the lumbar curve would decrease the forces that
tend to increase the displacement. Although the evidence
regarding the effect of pelvic alignment on lumbar curvature
is conflicting; the clinician chooses to proceed with a

program to teach the patient to stand maintaining a poste¬
rior pelvic tilt to flatten the lumbar curve. The clinician
teaches the patient abdominal strengthening exercises and
posterior pelvic tilts. The patient learns to stand while con¬
tracting the abdominal muscles and the gluteus maximus,
rotating the pelvis posteriorly. The patient reports pain relief.

This case provides an example of the commonly reported
anecdotal evidence supporting the use of postural education
to treat patients' complaints. Anecdotal evidence by itself
however ; is insufficient to determine the effectiveness of the
intervention , since many factors besides pelvic alignment
may contribute to the reduction in symptoms; including the
placebo effect. Without well-controlled biomechanical studies
to determine the mechanical effects of pelvic alignment on
low back posture and without similarly well-controlled effec¬
tiveness studies , the role of postural interventions in rehabili¬
tation remains a firmly held belief.

FRONTAL AND TRANSVERSE PLANE ALIGNMENT
IN NORMAL ERECT POSTURE

In the frontal and transverse planes, normal posture suggests
a general right-left symmetry, with the head and vertebral
column aligned vertically, hips and shoulders at an even
height, the knees exhibiting symmetrical genu valgum within
normal limits, and symmetrical placement of the upper and
lower extremities in the transverse plane (Fig. 47.9).Gangnet
and colleagues using three-dimensional radiography report
very slight (<5°) deviations of the vertebrae to the left with
similarly slight rotations to the right in a sample of 34 asymp¬
tomatic adults during upright standing [17].

Scoliosis describes a postural deformity of the vertebral
column that is most apparent in the frontal plane but includes
both frontal and transverse plane deviations. The curve is
named according to its location in the spine and the side of its
frontal plane convexity. For example, a right thoracic curve
indicates that the curve is located in the thoracic region of the
spine and its convexity is on the right side.

Scolioses can be either structural or functional. A func¬
tional scoliosis results from soft tissue imbalances, but a
structural scoliosis includes bony changes as well as soft tis¬
sue asymmetries. As noted in Chapter 29, idiopathic scolio¬
sis is the most common form of scoliosis. It is a structural sco¬
liosis that is found most frequently in adolescent girls. The
curve usually involves at least two spinal regions, and the
curves typically are compensated, so that adjacent regions
have opposite convexities (Fig. 47.10). A structural scoliosis in
the thoracic region is accompanied by a rib hump on the same
side as the convexity as a result of the coupled movements of
the thoracic spine and their effects on the joints of the
ribs. (Chapter 29 reviews the mechanics producing a
>0^ rib hump.)

A popular theory in rehabilitation suggests that hand dom¬
inance induces muscle imbalances that lead to functional

882

Part V I POSTURE AND GAIT

Figure 47.9: Normal alignment of the head and trunk in the
frontal plane is characterized by a vertically aligned head and
vertebral column, with shoulder, pelvis, hips, and knees at the
same height, and the knees and feet exhibiting valgus and sub¬
talar neutral positions within normal limits.

A i

Figure 47.10: A. An individual exhibits a right thoracic left lumbar
idiopathic scoliosis. B. When flexed forward, the individual exhibits
a rib hump on the right, the side of the thoracic convexity.

scolioses and asymmetry in shoulder and hip alignment [31].
Few objective studies exist that test this hypothesis, but a study
of 15 females aged 19 to 21 years reports no statistically sig¬
nificant differences in frontal plane alignment of the scapula
between the dominant and nondominant sides, although 11
of 15 subjects demonstrated a lower right shoulder [57].
Horizontal distances between the medial border of the
scapula and the vertebral column range from 5 to 9 cm
[7,52,57]. Although asymmetry in hip height, or pelvic obliquity,
also is allegedly associated with hand dominance, there is no
known direct evidence to support or refute the contention [31].

The relative alignment of the hip, knee, and foot in the
frontal and transverse planes during erect standing is discussed
in some detail in the respective chapters dealing with each joint

(Chapters 38, 41, and 44, respectively). Figure 47.11 provides a
brief review of the characteristic alignments. Because the lower
extremities participate in a closed chain during erect standing,
lower extremity malalignments may indicate local deformities
but also may reflect compensations for more remote malalign¬
ments. Findings from a postural assessment lead a clinician to
hypothesize underlying impairments. Direct assessment of
joints can identify the impairments that contribute to or explain
the postural malalignments. A single contracture at either the
hip, knee, or ankle may produce the same posture as the indi¬
vidual compensates for the functional limb length discrepancy
produced by the contracture (Fig. 47.12). An under¬
standing of the mechanisms contributing to faulty pos¬
ture requires careful assessment of each joint.

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

883

Figure 47.11: In normal alignment, the femoral condyles are
aligned in the frontal plane so that the hip is in neutral rotation
and the feet exhibit out-toeing of approximately 15-25°.

A. Frontal view. B. Superior view.

Clinical Relevance

RELATING POSTURAL FINDINGS TO IMPAIRMENTS
OF THE NEUROMUSCULOSKELETAL SYSTEM: A CASE
REPORT: A 45-year-old male with rheumatoid arthritis
was evaluated in the clinic with hip, knee , and foot pain
bilaterally. An evaluation of his standing posture revealed
a pelvic obliquity , right side higher than left a slightly
plantarflexed right ankle , and increased out-toeing on the
left (Fig. 47.13). Many possible impairments could explain
these findings, and the clinician's initial hypotheses
included a structural leg length discrepancy and a plan-
tarflexion contracture. A thorough examination of all of
the joints of the lower extremities was required before an
explanation for the posture emerged. The patient demon¬
strated bilateral hip flexion contractures. In addition ,
range of motion assessments revealed that the patient
had a complex contracture of the left hip, holding it
flexed , laterally rotated , and abducted. The patient stood

Figure 47.12: Flexion contractures of the hip or knee functionally
shorten the lower extremity, and a common compensation is
plantarflexion to lengthen the limb so that the individual can
stand with the pelvis level. A plantarflexion contracture produces
a functionally lengthened lower extremity so that an individual
with a plantarflexion contracture may stand with a flexed hip
and/or knee to restore symmetry and stand with a level pelvis.
The resulting postures look approximately the same although the
precipitating factors differ.

with an anterior pelvic tilt and increased lordosis ,
consistent with the hip flexion contractures , but the lateral
rotation and abduction contractures on the left effectively
shortened the left lower extremity while turning the toes
outward. The patient stood with the left hip in obligatory
abduction secondary to the abduction contracture , while
the right hip was adducted , and consequently , the pelvis
was higher on the right. Correction of standing posture
required reduction of the contractures of both the left and
right hip. Although conservative treatment failed to reduce
the contractures on the left , a total hip replacement on
the left restored normal joint alignment , and standing
posture was immediately improved.

884

Part V I POSTURE AND GAIT

Figure 47.13: A patient with an abduction contracture of the left
hip stands with the left hip abducted. To maintain an upright
posture with the feet close together, the individual adducts the
right hip, producing a pelvic obliquity in the frontal plane. The
left hip is abducted and the right hip is adducted. The right
ankle plantarflexes to equalize limb length.

Muscular Control of Normal Posture

Examples throughout this textbook demonstrate that ground
reaction forces and body segment weights apply external
moments to the joints, which are balanced by internal
moments supplied by the surrounding muscles and noncon-
tractile connective tissue. The alignment of the body’s center
of mass relative to joint axes in quiet standing defines the
external moments applied to the joints during erect standing.
These external moments then are balanced by either active or
passive support to maintain the upright posture against the
ever-present gravitational forces tending to press the body into
the ground. Examination of the external moments applied to
the joints of the lower extremities, trunk, and head by the
ground reaction forces helps explain the forces needed to sup¬
port these joints (Fig. 47.14). Using the data from the studies
presented in Table 47.1, the sagittal plane external moments
on many joints of the body are presented in Table 47.3.
Biomechanical analysis of these moments and electromyo¬
graphic (EMG) studies combine to help explain the mecha¬
nisms used to maintain upright posture.

Figure 47.14: In quiet standing, the ground reaction force applies
a dorsiflexion moment at the ankle, extension moments at the
knee and hip, and flexion moments on the spine.

Although the external moments described in Table 47.3
are the predominant moments applied during quiet standing,
it is important to recall that standing posture is dynamic and
that even so-called quiet standing is characterized by oscilla¬
tions of the body over the fixed feet. Panzer et al. report that
during quiet standing, the EMG activity of muscle groups is
less than 10% of each group’s activity during a maximum vol¬
untary contraction (MVC) [50]. These investigators also note
that many of these muscle groups exhibit sudden, brief activ¬
ity levels of 30-45% of their MVC and suggest that these sud¬
den bursts may reflect a muscle group’s response to the sway
of the body’s center of mass.

Because the body’s center of mass generates a dorsiflexion
moment on the ankle during quiet standing, the plantar flexor

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

885

TABLE 47.3: External Moments Applied to the Joints Based on the Center of Mass Line

Opila et al. [40] a External Moment

Danis [6] b External Moment

Ankle

Dorsiflexion

Dorsiflexion 0

Knee

Extension

Hip

Extension

Back

Flexion

Head/neck

Flexion

Approximately zero d

a Based on 19 unimpaired males and females aged 21 to 43 years. Originally reported with respect to the body's center of gravity.
fa Based on 26 unimpaired males and females aged 22 to 88 years. Referenced to the ankle joint.
c Moment is reported directly in the study but is derived from the available data.

d Although the moment arm is 0.03 cm, the standard deviation is almost 4 cm, suggesting that some individuals sustain a flexion moment, and others sustain
an extension moment.

muscles generate a plantarflexion moment to maintain static
equilibrium. EMG data demonstrate activity of both the
soleus and the gastrocnemius during quiet standing
[1,50]. Brief, intermittent, and slight EMG activity
is also found in the dorsiflexor muscles, apparently in
response to postural sway [1,50]. Research suggests that
plantar flexion fatigue in young healthy adults produces an
anterior shift of the center of pressure in quiet standing as
well as an increase in postural sway [67].

In contrast to the ankle, the knee exhibits minimal muscle
activity during quiet standing [1,50]. In erect posture, the
ground reaction force applies an extension moment to the
knee allowing it to maintain extension using its passive con¬
straints, including the collateral and anterior cruciate liga¬
ments. Reports of slight electrical activity in the quadriceps
muscles (4-7% of MVC) and hamstrings (1% of MVC) are
consistent with the use of passive supports to sustain the
extended knee during quiet standing [50]. However, like the
muscle activity at the ankle, larger brief bursts of activity in
the quadriceps and hamstrings muscles may reflect the mus¬
cles’ response to sway.

Few studies examine activity of the hip musculature dur¬
ing erect posture. The ground reaction force produces an
extension moment at the hip, and EMG data reveal activity of
the iliacus in quiet standing, exerting a stabilizing flexion
moment [1]. Understanding the role of muscles and liga¬
ments in generating the internal moments needed to balance
the external moments exerted by body weight and ground
reaction forces allows the clinician to intervene to provide
postural stability in the absence of muscular support.

Clinical Relevance

MAINTAINING ERECT POSTURE IN THE PRESENCE
OF MUSCLE WEAKNESS: A PATIENT WITH
PARAPLEGIA: A patient with a spinal cord injury resulting
in loss of muscle function from the level of L2 is beginning
rehabilitation. Functional goals include standing for stimu¬
lation of bone growth and limited ambulation. Weakness
secondary to the spinal cord injury begins at the hip flexors

and extends throughout the rest of the lower extremities.

To teach the individual safe and efficient standing; the clini¬
cian uses an understanding of the effects of external
moments on the joints of the lower extremities and a recog¬
nition of the passive structures that are available to support
the joints.

The individual lacks muscular support at the hip, knee,
and ankle; but the astute clinician knows that the hip pos¬
sesses strong anterior ligaments, the iliofemoral pub¬
ofemoral and ischiofemoral ligaments. By maintaining the
hip in hyperextension, the individual can "hang on" these
anterior ligaments, even in the absence of the hip flexors.
Similarly, the knee normally maintains extension in erect
standing without muscular support , since the body's center
of mass falls anterior to the knee joint and exerts an exten¬
sion moment on the knee. As long as the knee remains
extended, no additional muscular support is needed. Thus
the individual can stand in hip and knee hyperextension
using passive supports at these joints.

Stable erect posture requires that the body's center of
mass remain over the base of support. To maintain hip and
knee hyperextension while keeping the body's center of
mass over the base of support, the individual's ankles
assume a dorsiflexed position, and the ground reaction
force applies an external dorsiflexion moment (Fig. 47.15).
With no muscle support at the ankle, the individual with
weakness from the hips distally requires external support
from an orthosis to exert a plantarflexion moment at the
ankle, balancing the external dorsiflexion moment. Thus the
individual can stand with minimal external support to stabi¬
lize the lower extremity by using the external moments gen¬
erated by the ground reaction force to apply external
moments at the knee and hip that can be balanced by pas¬
sive joint structures.

For the individual described in this case to stand with
minimal external support, he or she must be able to assume
a position of hip and knee hyperextension. Flexion contrac¬
tures at the hips or knees or plantarflexion contractures at
the ankle produce disastrous results, preventing the individ¬
ual from positioning the joints to use passive supports
(Fig. 47.16).

886

Part V I POSTURE AND GAIT

Figure 47.15: Standing posture of an individual with weakness at
the hip, knees, and ankles. By hyperextending the hip joints, the
individual uses the passive restraint of the anterior ligaments of
the hip joint to support the hip. Hyperextension of the knee
increases the extension moment at the knee that is supported by
passive structures of the knee. To maintain hyperextension of the
hip and knees while keeping the center of mass over the base of
support, the ankles dorsiflex, producing a dorsiflexion moment
that is withstood by an externally applied plantarflexion moment
using an orthotic device.

The weight of the trunk exerts an external flexion moment
on the back, requiring an extension moment to maintain erect
posture. EMG data show low-level activity of the erector
spinae and multifidus with intermittent bursts of increased
activity [1,50,69]. The cervical region also sustains an external
flexion moment because the heads center of mass is anterior
to the joints of the cervical spine. Active contraction of cervi¬
cal extensors maintains upright posture of the head and neck,
but as in the trunk, EMG data reveal that only slight activity
is required to hold the head erect. Although few studies
examine activity in the cervical muscles during quiet standing,
data show activity in the semispinalis muscles with no activity
in the splenius muscles [61].

The role of the abdominal muscles during quiet standing
continues to be debated. EMG studies of the abdominal mus¬
cles identify activity, particularly in the internal oblique mus¬
cle, with some activity in the external oblique muscle during

Figure 47.16: Effect of sagittal plane contractures on standing
posture and the external moments applied to the hip, knees, and
ankles. A. Flexion contractures at either the hip or knee cause an
individual to stand in a flexed position at both the hips and
knees, generating external flexion moments at both joints.
Consequently, the individual is unable to use the passive supports
at the hip and knee joints. B. Plantarflexion contractures at the
ankles prevent an individual from moving the center of mass
over the base of support while still maintaining hip and knee
hyperextension. To relocate the center of mass over the base of
support, the patient flexes the hip joints, thus requiring muscular
support to support the hip joints.

quiet standing [1,14,51,56]. Yet studies that investigate the
association between abdominal muscle strength as measured
by leg-lowering maneuvers and postural alignment of the
pelvis report either no association [68] or weak associations in
females and no association in males [76]. The leg-lowering
exercise often used to assess abdominal strength recruits the
rectus abdominis more than the oblique abdominal muscles
in most individuals and, consequently, may not reflect the
ability of the oblique abdominal muscles to participate in pos¬
tural support [51]. Contraction of the muscles of the abdom¬
inal wall appears to stabilize the pelvis and prevent excessive
anterior pelvic tilt during prone hip hyperextension [47].
Chapter 34 discusses the role of the abdominal muscles in sta¬
bilizing the spine. The data presented here suggest that the
oblique abdominal muscles are important in erect posture,

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

887

although their role may be to function with the transversus
abdominis muscle to stabilize the spine rather than to position
the pelvis.

The role played by muscles to maintain shoulder position
during quiet standing also lacks definitive conclusions. Inman
et al. demonstrate active contraction of the levator scapulae
along with the upper trapezius and upper portion of the ser-
ratus anterior muscles in quiet standing, suggesting that these
muscles are providing upward support for the shoulder girdle
and upper extremity [26]. However, Johnson et al. note that
only the levator scapulae and the rhomboid major and minor
muscles can directly suspend the scapula [30]. EMG studies
show that in the presence of voluntary relaxation of the upper
trapezius in quiet standing, there is an increase in EMG activ¬
ity of the two rhomboid muscles but a decrease in activity in
the levator scapulae [49]. These data support the notion that
the rhomboid muscles can and do support the upright posi¬
tion of the shoulder girdle, at least under certain circum¬
stances. Whether the levator scapulae contributes additional
support remains debatable.

POSTURAL MALALIGNMENTS

Health care providers evaluate posture on the premise that
postural malalignments contribute to altered joint and muscle
mechanics, producing impairments that lead to pain [24,31].
Complaints attributed to postural deviations of the head and
spine include circulatory, respiratory, digestive, and excretory
dysfunctions; headaches; backaches; depression; and a gener¬
alized increased susceptibility to disease [6,24,44,45]. Pain in
the back and lower extremities also is attributed to abnormal
alignment in the hips, knees, and feet [12,33,34,37,72].

Despite the presumption of associations between postural
abnormalities and patients’ complaints, studies examining
these associations vary in their findings. Correlations between
the incidence of reported head, neck, and shoulder pain are

reported in people with forward head, rounded shoulders,
and increased thoracic kyphoses [24]. Studies investigating
the association between low back postural deviations and low
back pain draw variable conclusions, with some reporting lit¬
tle or no difference in posture between those with and with¬
out low back pain [9,13,75], and others finding differences
between the two groups [27,28,53]. Malalignments of the
patellofemoral joint are associated with a variety of pain syn¬
dromes at the knee [25,41,55]. Considerably more research is
required to determine the role that postural abnormalities
play in musculoskeletal complaints and to determine the
effectiveness of treatments directed toward improving pos¬
ture to reduce pain.

Typical postural deviations are listed and defined in Tables
47.4 and 47.5 ( Fig.47.17 ). These postural abnormalities are
presumed to produce excessive or abnormally located stresses
(force/area) on joint surfaces or to contribute to altered
muscle mechanics by putting some muscles on slack while
stretching others [31]. Although evidence supports these
effects in some cases, evidence is lacking for others
[11,34,37]. Determining the role posture plays in the patho-
mechanics of musculoskeletal disorders requires continued
research in basic anatomy and biomechanics, as well as well-
controlled outcome studies examining the effectiveness of
treatments directed toward posture reeducation.

Muscle Imbalances Reported in Postural
Malalignments

A commonly held clinical perception is that postural
malalignments produce adaptive changes in the muscles
surrounding the malaligned joints. Specifically, it is believed
that muscles on one side of the joint are held in a lengthened
position and the antagonistic muscles are maintained in a
shortened position. Clinicians also suggest that these length
changes produce joint impairments including weakness and

TABLE 47.4: Common Postural Abnormalities in the Sagittal Plane

Postural Deviation

Description

Forward head

The mastoid process lies anterior to the body of C7

Forward shoulders

The acromion process lies anterior to the body of C7,
or the scapula tilts anteriorly

Excessive/flattened thoracic kyphosis

The sagittal plane curve of the thorax is excessive or inadequate

Excessive/flattened lumbar lordosis

The sagittal plane curve of the lumbar spine is excessive or inadequate

Anterior/posterior pelvic tilt

The angle made by a line through the ASIS and PSIS and the horizontal
increases/decreases from an angle of approximately 10-15°

Forward/backward translation of the pelvis

Determined by the location of the greater trochanter with respect to
the vertical line through the center of mass, which in normal alignment
passes approximately through the trochanter

Genu recurvatum

Angle between the mechanical axes of the leg and thigh in the sagittal
plane is greater than 0°

888

Part V I POSTURE AND GAIT

TABLE 47.5: Common Postural Abnormalities in the Frontal and Transverse Planes

Postural Deviation

Description

Head tilt

The line through the center of the head deviates from the midsagittal plane

Asymmetrical shoulder height

Measured by the height of the acromions or the inferior angles of the scapulae

Scoliosis

Frontal plane deviation of the vertebral column as assessed by the spinous processes

Pelvic obliquity

Asymmetrical height of the pelvis as measured by the iliac crests

Asymmetrical hip height

Measured by the height of the greater trochanters or gluteal folds

Genu varum/valgus

Angle between the mechanical axes of the leg and thigh in the frontal plane

Foot pronation/supination

Indicated by several different measures including (1) the frontal plane alignment of the heel and leg,

(2) the height of the navicular relative to the medial malleolus and the head of the first metatarsal,
and (3) the subtalar neutral position

In-toeing/out-toeing

The angle between the long axis of the foot and the malleoli is less than/greater than approximately 20°

limited range of motion that contribute to a patients com¬
plaints. Although these hypotheses are logical and may still
prove true, few studies to date identify clear associations
between malalignments and joint impairments [11,43].
Borstad reports that individuals with tight pectoralis minor
muscles demonstrate a rounded shoulder posture as defined by
the distance between the sternal notch and the coracoid

Figure 47.17: Excessive curves. This individual displays a common
but abnormal posture, characterized by excessive sagittal plane
curves of the spine.

process accompanied by internal rotation of the scapula [3].
Although these findings link muscle changes with postural
malalignments, they still do not relate the postural deviations to
joint impairments or functional deficits. Additional research is
needed to determine what links exist among postural abnor¬
malities, muscle impairments, and patients’ complaints.

As noted in Chapter 4, studies in animals demonstrate
that prolonged length changes in muscles produce structural
changes in muscle, although those changes depend upon
many factors besides length. These additional mitigating fac¬
tors include age, fiber arrangement within the muscles, and
fiber type within the muscle [36,38]. In general, prolonged
stretch of a muscle induces protein synthesis and the pro¬
duction of additional sarcomeres [21,22,60,71,73]. The
lengthened muscle hypertrophies, and as a result, peak con¬
tractile force increases with prolonged stretch [36,38]. The
structural remodeling that accompanies prolonged lengthen¬
ing appears to maintain the muscle’s original length-
tension relationship so that, although the muscle has a larger
peak torque, it generates the peak torque at a different joint
position. The clinical literature describes stretch weakness
in which a muscle that has been held in a stretched position
long enough to remodel appears weak when tested in the tra¬
ditional test position [23,31]. For example, at the shoulder,
stretch weakness suggests that a posture characterized by
rounded shoulders applies a prolonged stretch to the middle
trapezius, which undergoes the structural adaptations that
lead to weakness when assessed in the traditional manual
muscle test position. Although the changes described here
are logical and plausible, they remain unproved.

Animal studies examining prolonged shortening reveal
that shortening produced by immobilization appears to accel¬
erate atrophy, and muscles demonstrate a loss of sarcomeres
[21,60,73]. Studies examining the effect of prolonged length
changes in muscle reveal that the relationship between
muscle length and muscle performance is complex, requiring
independent investigation of the relationship with each mus¬
cle. The complexity of the association helps explain the
absence of clearly defined associations.

Attempts to confirm the expected muscle impairments with
postural abnormalities have failed to yield clear relationships.

Chapter 47 I CHARACTERISTICS OF NORMAL POSTURE AND COMMON POSTURAL ABNORMALITIES

889

Individuals with idiopathic scoliosis exhibit atrophy of the
muscles of the posterior thorax, particularly on the concave
side, and a higher percentage of type I muscle fibers than nor¬
mal on the convex side of the deformity [15,74,78]. The mus¬
cles of the thorax on the concave side of the curve are likely
shortened, while those on the convex side are lengthened; yet
both muscle groups exhibit atrophy Although this atrophy
may precede the development of the scoliosis, the expected
adaptive changes with prolonged lengthening apparently are
lacking. Similarly, attempts to relate scapular alignment and
muscle performance fail to reveal consistent associations
[3,11]. However, the scapula moves in a complex, three-
dimensional way, and studies so far may not accurately reflect
the effects of scapular malalignment on muscle length. These
data demonstrate the need for careful anatomical, biome¬
chanical, and clinical studies to identify and explain any detri¬
mental effects of postural malalignment.

Because of the lack of definitive studies that link postural
malalignments with patient complaints, impairments, and dis¬
abilities, clinicians continue to argue the significance and utility
of postural examinations [54]. Boulay et al. use a statistical
model to suggest that biomechanical efficiency in posture
depends on several interdependent factors [4]. They suggest,
for example, that an individual with a given pelvic incidence has
a range of sacral slopes, lumbar lordoses, and thoracic kyphoses
that can be combined to achieve biomechanical economy. If,
however, the individuals lordosis falls outside the range of
acceptable values, then his or her posture may lead to impair¬
ments and dysfunction. Such an interdependent and dynamic
postural adaptation suggests that clinicians may need to con¬
sider clusters of postural alignments and perhaps identify
thresholds beyond which malalignments should be treated.

SUMMARY

This chapter describes the relative alignment of body seg¬
ments identified in healthy adults during quiet standing. In the
absence of a validated description of “ideal posture,” the
documented alignments provide clinicians with a view of the
variability of alignments found in individuals without muscu¬
loskeletal complaints. Although individuals demonstrate a
wide spectrum of alignments, the overall image of upright pos¬
ture shows a head well balanced over the pelvis, which in turn
is well balanced over the feet. Using these alignments, the
chapter also demonstrates the external moments applied to
the joints of the lower extremities and trunk during upright
standing. The external moments are balanced by internal
moments generated by muscle contractions and noncontrac-
tile connective tissue support. EMG data are consistent with
the mechanical data, demonstrating low levels of activity in the
plantar flexors, hip flexors, and erector spinae muscles of the
lumbar and cervical regions. Additional activity in the oblique
abdominal muscles is consistent with their role as stabilizers of
the spine. In addition, other muscle groups such as the dorsi-
flexor muscles, the quadriceps, and the hamstrings demonstrate

very brief bursts of activity that may be required to control the
small, but persistent sway of the body that occurs throughout
quiet stance.

Postural alignment is commonly assessed clinically, and
some abnormal postures are associated with musculoskeletal
abnormalities and clinical complaints. However, many of the
commonly held beliefs regarding the associations between
postural abnormalities and musculoskeletal impairments lack
objective evidence. Although these associations may well exist,
additional research is required to identify such relationships
and to demonstrate the effectiveness of treating postural devi¬
ations to reduce pain or other impairments.

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CHAPTER

Characteristics of Normal Gait
and Factors Influencing It

CHAPTER CONTENTS

THE GAIT CYCLE, THE BASIC UNIT OF GAIT.893

KINEMATICS OF LOCOMOTION .894

Temporal and Distance Parameters of a Stride .895

Angular Displacements of Joints.896

MUSCLE ACTIVITY DURING LOCOMOTION .900

KINETICS OF LOCOMOTION .902

Joint Moments and Reaction Forces.902

Energetics of Gait: Power, Work, and Mechanical Energy.908

FACTORS THAT INFLUENCE PARAMETERS OF GAIT .911

Gender.911

Walking Speed .911

Age.912

SUMMARY .913

H abitual bipedal locomotion is a uniquely human function and influences an individual's participation and

interaction in society. Impairments in gait are frequent complaints of persons seeking rehabilitation services
and are often the focus of an individual's goals of treatment. Rehabilitation experts require a firm under¬
standing of the basic mechanics of normal locomotion to determine the links between impairments of discrete seg¬
ments of the musculoskeletal system and the patient's abnormal movement patterns in gait.

Therapists and other rehabilitation experts are called upon daily to analyze a patient's movements and determine the
cause of the abnormal, often painful, motion. A thorough understanding of normal locomotion and the factors that
influence it, as well as an understanding of the functions of the components of the musculoskeletal system, provides
a framework for evaluation and treatment of locomotor dysfunctions. This chapter describes the general characteristics
of normal locomotion and introduces the clinician to the basic concepts central to all movement analysis.

Normal human locomotion consists of stereotypical movement patterns that are immediately recognizable. Yet most
individuals also are able to distinguish the gait of close friends and associates by the sound of their footsteps in the
hallway. The purpose of this chapter is to describe the common characteristics of normal human locomotion and their
variability and to provide insight into how impairments within the musculoskeletal system may be manifested in
altered gait patterns. The specific objectives of this chapter are to

■ Describe the basic components of the gait cycle
■ Present the temporal and distance characteristics of normal gait

892

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

893

■ Detail the angular displacement patterns of the joints of the lower extremity, the trunk, and the upper
extremities

■ Describe the patterns of muscle activity that characterize normal locomotion

■ Briefly discuss the methods for determining muscle and joint loads sustained during normal locomotion
and present the findings from representative literature

■ Briefly consider the energetics of normal locomotion and the implications of gait abnormalities on the
efficiency of gait

Gait has been studied for millennia, and the last 50 years have seen an explosion in the research examining the charac¬
teristics of gait and the factors that control it. The current chapter is, of necessity, an overview of the characteristics of
locomotion that are useful to a clinician and that demonstrate the effect of the integrity of the musculoskeletal system
on gait. Several textbooks dealing only with locomotion provide details regarding the movement and methods of its
assessment, and insight into the central nervous system's role in controlling and modifying the movement of gait
[31,134,147,184].

THE GAIT CYCLE, THE BASIC
UNIT OF GAIT

Gait is a cyclical movement that, once begun, possesses very
repeatable events that continue repetitively until the individ¬
ual begins to stop the motion. The steady-state movement of
normal locomotion is composed of a basic repeating cycle, the
gait cycle (Fig. 48.1). The cycle is traditionally defined as the
movement pattern beginning and ending with ground contact
of the same foot. For example, using the right foot as the ref¬
erence foot, the gait cycle begins when the right foot contacts
the ground (usually with the heel) and ends when it contacts
the ground again. Thus a gait cycle consists of the time the
reference foot is on the ground (stance) and the time it is off
the ground (swing). The movement of both limbs that occurs
during the gait cycle is known as the stride.

The stance phase of gait makes up approximately 60%
of the gait cycle, so that the remaining 40% consists of the

swing phase. The gait cycle with respect to the right limb is
slightly out of phase with the gait cycle of the left limb. At
contact on the right, the left limb is just ending its stance
phase. At approximately 10% of the gait cycle on the right, the
left limb leaves the ground and begins its swing phase, return¬
ing to the ground at approximately 50% of the gait cycle of the
right limb. Thus the gait cycle is characterized by two brief
periods, each lasting approximately 10% of the gait cycle, in
which both limbs are in contact with the ground. These are
periods of double limb support, and the remaining cycle
consists of single limb support.

The stance phase can be divided into smaller periods asso¬
ciated with specific functional demands and identified by dis¬
tinct events (Fig. 48.2) [136]. The period immediately follow¬
ing ground contact is known as contact response, or wei ght
acceptance, and ends when the whole foot flattens on the
ground. During contact response, the limb absorbs the shock
of impact and becomes fully loaded. The foot flat event that

Double support

Single support

Double support Single support

Figure 48.1: The gait cycle of a single lower extremity consists of a stance and swing period and lasts from ground contact of one foot
to the subsequent ground contact of the same foot. It includes two steps that are defined as the period from ground contact of one
foot to the ground contact of the opposite foot. A single gait cycle includes two periods of double limb support and two periods of
single limb support.

894

Part V I POSTURE AND GAIT

Figure 48.2: The stance phase is divided into smaller phases that are demarcated by specific events. GC, ground contact; FF, foot flat;
HO, heel off; CGC, contralateral ground contact, TO, toe off.

ends contact response occurs at approximately 15% of the
normal gait cycle. It is important to recognize that loading
response includes double limb support and continues into
single limb support. The period following loading response is
midstance, also known as trunk glide, since during this
period the trunk glides over the fixed foot, moving from
behind the stance foot to in front of it. Heel off ends trunk
glide at approximately 40% of the gait cycle and begins ter¬
minal stance, which ends at 50% of the gait cycle when con¬
tralateral ground contact occurs. The final stage of stance,
from 50 to 60% of the gait cycle, is preswing and is charac¬
terized by double limb support. It ends with toe off.

The swing phase also is divided into early, middle, and late
periods, although it lacks distinctive events to delineate these
phases (Fig. 48.3). Early swing continues from 60% to
approximately 75% of the gait cycle and is characterized by
the rapid withdrawal of the limb from the ground. Midswing
continues until approximately 85% of the gait cycle and con¬
sists of the period in which the swing limb passes the stance
limb. Late, or terminal, swing finds the swing limb reach¬
ing toward the ground, preparing for contact.

Although normal gait is often assumed to be symmetrical,
substantial evidence exists to refute that assumption
[15,68,106,151]. Although the differences are small among
ambulators without pathology, the right and left limb move¬
ments are not mirror images of one another. Differences exist

Figure 48.3: The swing phase is divided into early swing, when
the limb is pulled away from the ground; midswing, as the swing
limb passes the stance limb; and late swing, when the swing limb
extends toward the ground.

in timing and movement patterns, in muscle activity, and in
the loads applied to each limb [55,150]. Asymmetry appears
greatest at slower walking speeds [57]. When evaluating the
gait patterns of individuals with asymmetrical impairments,
clinicians must remember that small asymmetries in gait are
normal, particularly when walking slowly

Consideration of the basic functional tasks of the swing
and stance phase of gait provides a framework for character¬
izing the movements in each phase of gait. While the over¬
riding goal of locomotion is forward progression, the stance
and swing phases contribute to that goal in different ways.
The stance phase has three tasks in locomotion: providing
adequate support to avoid a fall, absorbing the shock of
impact between the limb and the ground, and providing ade¬
quate forward and backward force for forward progress
[35,183]. The basic tasks of the swing phase are safe limb
clearance, appropriate limb placement for the next contact,
and transfer of momentum. By keeping these tasks in mind,
the clinician can understand the importance of discrete
movements of limb segments or the specific sequencing of
muscle activity and can begin to appreciate the significance of
specific joint impairments.

KINEMATICS OF LOCOMOTION

As noted in Chapter 1, kinematics describes a movement in
terms of displacement, velocity, and acceleration. The vast
majority of kinematic analyses of gait examines displacement
characteristics, and although velocity and acceleration data
are available and may provide useful information, this chap¬
ter reviews the more commonly cited displacement data.
Presented first is a description of the movement characteris¬
tics of the stride as a whole followed by descriptions of dis¬
crete movement patterns of individual joints.

Many factors affect the kinematic characteristics of gait,
including walking speed, age, height, weight or body mass
index, strength and flexibility, pain, and aerobic conditioning.
Walking speed and age have large and important effects on
gait and are discussed later in this chapter. Unless noted oth¬
erwise, the data reported come from trials in which the sub¬
jects walk at their self-selected, comfortable, or free, speed.

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

895

Figure 48.4: Several distance measures help describe a typical
gait cycle.

Temporal and Distance Parameters
of a Stride

A stride consists of the movement of both limbs during a gait
cycle and contains two steps. A step is operationally defined as
the movement of a single limb from ground contact of
one limb to ground contact of the opposite limb (Fig. 48.4).
The literature demonstrates that there is considerable differ¬
ence in step and stride characteristics among subjects and even
among trials of the same subject [53,133,171]. Despite this nor¬
mal variability, these parameters are capable of distinguishing
between individuals with and without impairments [83,176].

DISTANCE CHARACTERISTICS OF THE STRIDE

The typical distance parameters of gait are defined in
Table 48.1. A representative range of values also is presented
from the literature [24,65,82,101,104,120,122,131,133].

Stride and step lengths depend directly upon standing height,
so measures of absolute step or stride length, although fre¬
quently reported, are difficult to interpret. These measures can
be normalized by standing height or lower extremity length to
compare values from different individuals [30,81]. Estimates of
normalized stride length vary from approximately 60 to 110%
of standing height [24,30]. Judge et al. report a mean step
length of 74 ± 4% of leg length in young healthy adults [81].
Step width and foot angle are less frequently reported but pro¬
vide an indication of the size of the base of support.

TEMPORAL CHARACTERISTICS OF THE STRIDE

The temporal characteristics of the stride are defined in
Table 48.2 [43,49,59]. Included in this list is walking speed, or
gait velocity, although this is typically computed over several
strides. The normal gait cycle at free speed lasts approxi¬
mately 1 second, and walking speed is between 3 and 4 miles
per hour (4.8-6.4 km/h) or approximately 1.3 m/sec. Walking
speed is a function of both cadence (steps/minute) and step
length. An increase in either cadence or step length con¬
tributes to increased walking speed [7,62,101,119,159,176].

Walking speed affects swing and stance time differently.
Increased walking speed decreases the overall duration of
the gait cycle, but the decrease in cycle duration results in a
greater decrease in stance time than in swing time [7,119]. As
stance time decreases with less change in swing time, double
limb support time decreases, and single limb support time
increases. The difference between running and walking is
the absence of a double limb support phase in running. The
ratio between swing and stance time increases toward 1 with
increasing walking speed.

Many gait disorders lead to altered time and distance
parameters, typically decreased speed and stride length and,
in the case of unilateral disorders, altered swing and stance
times with abnormal swing-stance ratios. Such measures are
relatively easy to obtain in the clinic and serve as useful out¬
come measures, sensitive to change. On the other hand, many
different disorders produce similar temporal and distance
characteristics. For example, a patient with unilateral hip pain
and a patient with hemiparesis secondary to a stroke both
walk with decreased velocity, and both demonstrate
decreased single limb support time on the affected side and
increased double limb support time [119]. These parameters

TABLE 48.1: Distance Parameters of Stride in Young Healthy Adults

Parameter

Definition

Range of Values Reported in the Literature

Stride length

The distance between ground contact of one foot and
the subsequent ground contact of the same foot

1.33 ± 0.09 to 1.63 ± 0.11 m
[65,82,101,104,120,122,131]

Step length

The distance between ground contact of one foot and
the subsequent ground contact of the opposite foot

0.70 ± 0.01 to 0.81 ± 0.05 m
[65,120,159]

Step width (also known
as base of support) 3

The perpendicular distance between similar points on both
feet measured during two consecutive steps [25,104]

0.61 ± 0.22 to 9.0 ± 3.5 cm
[104,120,122,159,169]

Foot angle

Angle between the long axis of the foot and the line
of forward progression

5.1 ± 5.7 to 6.8 ± 5.6° [104]

a Step width is defined variably in the literature. Some measures incorporate the angle of the foot on the ground.

896

Part V I POSTURE AND GAIT

TABLE 48.2: Temporal Parameters of Stride in Young, Healthy Adults

Parameter

Definition

Values From the Literature

Stride time

Time in seconds from ground contact of one foot
to ground contact of the same foot

1.00 ± 0.23 to 1.12 ± 0.07
[104,120,122,131]

Speed (also known as velocity)

Distance/time, usually reported in m/sec

0.82-1.60 ± 0.16 [49,65,81,82,101,
104,119,122,131,159]

Cadence

Steps per minute

100-131 [30,43,49,81,82,104,119,
122,159]

Stance time

Time in seconds that the reference foot is on the
ground during a gait cycle

0.63 ± 0.07 to 0.67 ± 0.04
[104,120,122]

Swing time

Time in seconds that the reference foot is off the
ground during a gait cycle

0.39 ± 0.02 to 0.40 ± 0.04
[104,120,122]

Swing/stance ratio

Ratio between the swing time and the stance time

0.63-0.64 [82,122]

Double support time

Time in seconds during the gait cycle that two feet are
in contact with the ground

0.11 ± 0.03 to 0.141 ± 0.03
[104,119,120]

Single support time

Time in seconds during the gait cycle that one foot is in
contact with the ground

Not reported

distinguish between normal gait and abnormal walking pat¬
terns but are unlikely to identify the differences in gait pat¬
terns between the two patients, even though such differences
often are easily detected by an observer. Thus temporal and
distance parameters may be helpful in tracking a patients
progress but are insufficient to characterize a gait pattern
fully and to identify the mechanisms driving the movement
pattern. Patterns of joint excursions, however, can help the
clinician to identify the differences in gait patterns between
individuals with similar temporal and distance characteristics.

Clinical Relevance

EFFECTS OF STANDING HEIGHT ON DISTANCE AND
TEMPORAL CHARACTERISTICS OF GAIT: Two friends
agreed to participate in a 2-day Breast Cancer Walk. The
walk consisted of a 26.2-mile walk the first day and a 13.1-
mile walk the following day. The friends trained for the walk
by walking together several times per week, distances rang¬
ing from 4 to 20 miles. One friend was 5 feet 8 inches tall
and the other 5 feet 2 inches tall. Both were active, healthy
individuals of the same age. Neither had any known muscu¬
loskeletal problems or gait deviations.

At the beginning of the official walk, all participants
received pedometers. The friends completed the first day
without difficulty, walking together the entire distance. At the
end of the day they checked their pedometers and one
reported 65,000 steps and the other 48,000 steps. Was one
pedometer broken?! The friends walked the second day
together again and finished without incident. At the end of
the 2-day walk the pedometers read 99,500 and 63,000
steps! Not surprisingly, the shorter friend was wearing the
pedometer that read 99,500. That individual had taken over
50% more steps than the taller friend. Since they walked
together the entire time, the shorter friend must have used a
higher cadence (steps/minute) to stay with the taller friend.

Angular Displacements of Joints

The growth of photography in the mid- to late 19th century
allowed the systematic observation of discrete movements of
each joint during the complex activity of normal locomotion
[5]. Over the last 50 years improved photographic techniques
and the development of the computer have led to ever more
precise monitoring of the three-dimensional motion of individ¬
ual segments. The sagittal plane motions of the joints of the
lower extremity are the most thoroughly studied and best
understood, at least in part because sagittal plane motions are
the largest and easiest to measure. In contrast, frontal and
transverse plane motions of the joints of the lower extremities
and the three-dimensional motions of the upper extremities
and trunk are less frequently studied. Joint displacement data
reveal intra- and intersubject variability in all planes, although
the variability is greater in the frontal and transverse planes
than in the sagittal plane and across subjects than between
cycles of a single individual [16,42,63,82]. The smaller excur¬
sions in the frontal and transverse planes are particularly sensi¬
tive to differences in measurement procedure, which accounts
for some of the increased variability of these motions [75,145].
Despite the variability in magnitudes of the movements, the
patterns and sequencing of joint movements in gait are remark¬
ably consistent across trials and across subjects [14,34,35,119].

SAGITTAL PLANE MOTIONS

The classic studies by Murray remain the foundation for
understanding sagittal plane motion of the lower extremity
[119,120,122,123] (Fig. 48.5). More-recent studies confirm
the overall patterns of motion for the hip, knee, and ankle,
although there is variation in the reported maximal joint posi¬
tions. Because studies demonstrate both intra- and intersub¬
ject variability, the reader is cautioned that the pattern
of motion is the focus of the following discussion
rather than the specific magnitudes [45,82,184].
Values of peak excursion are mentioned to provide an image
of the motion rather than to define an absolute norm.

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

897

Figure 48.5: Sagittal plane excursions of the ankle, knee, and
hips. Plots indicate mean and 2 standard deviations. (Reprinted
with permission from Murray MP: Gait as a total pattern of
movement. Am J Phys Med 1967; 46: 290-333.)

The hip exhibits a single cycle of motion. Beginning at
ground contact, the hip is in maximum flexion (approximately
25°) and gradually extends, reaching maximum hip hyperex¬
tension (approximately 10°) at close to 50% of the gait cycle,
when contralateral ground contact occurs [88,99,119,184].
The magnitude of apparent hip hyperextension excursion
depends on the point of reference. As noted in Chapter 38, a
normal hip exhibits little or no hyperextension range of
motion. Consequently, the hyperextension reported at the hip
during locomotion is the result of pelvic motions in the trans¬
verse and sagittal planes. In most studies, the reported
hip hyperextension reflects the orientation of the thigh with
the trunk or with the room-fixed reference frame as seen in
Fig. 48.6. After reaching maximum extension, the hip begins

Figure 48.6: In most locomotion studies the hip excursion is
described as the angle between the length of the thigh and a
room-fixed coordinate system.

flexing again, reaching maximum flexion late in swing, at
80-85% of the gait cycle. The cycle repeats at ground contact.

The knee exhibits a slightly more complex movement
pattern, landing in extension, albeit usually a few degrees
short of maximum extension, at ground contact. The knee
flexes 10 to 20° immediately after contact, reaching maximum
flexion at about 15% of the gait cycle when the subject
achieves foot flat. At foot flat the knee begins to extend and
reaches maximum extension at about 40% of the gait cycle as
the heel rises from the ground. Flexion of the knee begins
again and reaches a maximum of approximately 70° in
midswing (approximately 75% of the gait cycle). Knee exten¬
sion resumes, and the knee reaches maximum knee extension
just before ground contact [23,88,100,119,149].

Ankle motion also exhibits several reversals in direction.
Ground contact occurs with the ankle close to neutral in
either slight plantarflexion or slight dorsiflexion [88,99,119].
Following contact, the ankle plantarflexes an additional 5 or
10°, reaching a maximum at about 5% of the gait cycle. As the
body glides over the stance foot, the ankle dorsiflexes, reach¬
ing a maximum just after the knee reaches full extension.
Ankle plantarflexion resumes, and the ankle reaches maxi¬
mum plantarflexion of approximately 20° just following toe
off. In swing, the ankle dorsiflexes slightly but may remain in
slight plantarflexion throughout swing.

Pelvic motions in the sagittal plane are small, with no con¬
sistent definition of neutral. However, studies suggest that the
pelvis anteriorly tilts whenever either hip is extending
[120,122,157,179]. The anterior pelvic tilt contributes to the

898

Part V I POSTURE AND GAIT

apparent hip hyperextension that occurs in late stance. Upper
extremity sagittal plane motion also shows a rhythmic oscilla¬
tion that is related to the movement of the lower extremities.
At free walking speed, flexion of the shoulder and elbow par¬
allel flexion of the opposite hip [119,123,175].

Clinical Relevance

ASSOCIATED MOVEMENTS IN AN INDIVIDUAL
FOLLOWING STROKE: Close examination of the sagittal
plane motions of the hip, knee*, and ankle reveal that only
for a very brief instant following toe off are these three joints
moving in the same direction with respect to the ground.

Just following toe off, all three joints are pulling the foot
away from the groundthe hip and knee are flexing, and
the ankle is dorsifiexing. At other points in the gait cycle the
joints move independently, so that one or two joints move
the foot toward the ground as the other(s) pull it away from
the ground. A common impairment found in patients follow¬
ing stroke is an inability to disassociate movements, and as
a result , a patient is compelled to move all three joints of
the lower extremity together in the same direction. For
example, to flex the knee, the patient may flex the knee and
hip and dorsiflex the ankle simultaneously in a flexion pat¬
tern , or synergy ; or extend the knee while simultaneously
extending the hip and plantarflexing the ankle in an exten¬
sion pattern ; or synergy Such obligatory movements
interfere with the normal timing and sequencing of joint
movements in gait. For instance, in late swing, as the
patient extends the knee toward the ground, the hip tends
to extend, and the ankle plantarflex, producing a foreshort¬
ened step and an abnormal foot position at ground contact.
A flexion pattern produces similar conflicts as the hip begins
to flex in terminal stance. At this time, the hip and knee
should be flexing while the ankle continues to plantarflex. A
flexion pattern stops the ankle plantarflexion and interferes
with the normal roll off of late stance.

FRONTAL PLANE MOTIONS

Frontal plane excursions are less well studied and more var¬
ied than sagittal plane movements (Fig. 48.7). Hip position in
the frontal plane is affected by the motion of the pelvis over
the femur and by the orientation of the femur as the subject
translates toward the opposite foot to keep the center of mass
over the base of support. The hip lies close to neutral abduc¬
tion at ground contact and then adducts during weight
acceptance as the pelvis drops on the contralateral side
[9,78,79,82] (Fig. 48.8). Adduction is amplified as the subject
shifts toward the stance side to keep the center of mass over
the foot. Adduction continues until late stance, when loading
begins on the opposite limb. At that instance, the pelvis drops
on the side in late stance, and the hip moves into abduction
(Fig. 48.9). Reported knee motion in the frontal plane is
slight, with estimates ranging from approximately 2 to 10° of
adduction, peaking in early swing [9,23,82,100].

B Gait cycle (%)

Percent of walking cycle

Figure 48.7: Frontal plane excursions of the hip (A), knee (B), and
foot (C) are much smaller than sagittal plane excursions but
show characteristic patterns of movement.

Frontal plane motion of the foot recorded during walking
reflects the inversion and eversion component of supination
and pronation of the foot. Although the position of the hind-
foot at ground contact is variable and the magnitude of the
reported excursions differs among reports, data consistently
demonstrate a motion pattern following ground contact char¬
acterized by eversion, consistent with pronation, continuing
until mid to late stance when the hindfoot begins inverting or
supinating [29,91,115,138,139,189]. Forefoot motion is similar
to hindfoot motion, although forefoot pronation during stance
begins after hindfoot pronation has begun [74,139,189].

TRANSVERSE PLANE MOTIONS

Transverse plane motions of the limbs and trunk also demon¬
strate more variability and smaller excursions than those seen
in the sagittal plane (Fig. 48.10). Transverse plane rotations of
the hip are a function of the transverse plane motion of the
pelvis as well as the transverse plane motion of the femur

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

899

Figure 48.8: At weight acceptance, the individual shifts laterally to
keep the center of mass close to the stance foot, and the pelvis
drops on the unsupported side. The stance hip is in adduction.

(Fig. 48.11). Pelvic rotation in the transverse plane accompanies
hip flexion, so that the pelvis rotates forward on the side of the
flexing hip, reaching maximum forward rotation at approximately
ground contact [51,82,89,119]. Forward rotation of the pelvis
contributes to lateral rotation of the hip. At the same time, the

Figure 48.9: During weight acceptance, the hip drops on the
unsupported side, which is abducted.

A Gait cycle (%)

Figure 48.10: Transverse plane motions of the hip and knee.
(Reprinted with permission from Kadaba MP, Ramakrishnan HK,
Wootten ME, et al.: Repeatability of kinematic, kinetic, and elec¬
tromyographic data in normal adult gait. J Orthop Res 1989; 7:
849-860.)

opposite hip is in maximum extension, and the relative backward
position of the pelvis on that side allows the hip to appear hyper-
extended. The transverse plane alignment of the pelvis on the
extended hip tends to medially rotate the extended hip.

Independent femoral movement provides its own contri¬
bution to hip position. At ground contact, the femur is aligned
close to neutral but rotates medially from contact to mid-
stance. Lateral femoral rotation then begins and continues
into mid swing when medial rotation resumes. Hip joint posi¬
tion is the sum of the pelvic contribution and the femoral con¬
tribution to joint position. Although there is disagreement
about the hip position at ground contact among the reported
data, there is good consistency regarding the direction of the
hip motion, medial rotation from ground contact to mid- or
late stance and then lateral rotation until late swing or ground
contact [78,79,82,129].

The knee, too, exhibits transverse plane motion with medi¬
al rotation following ground contact and gradual lateral rota¬
tion from midstance through most of swing, although there is
more disagreement about knee motion in swing
[9,23,82,100,129]. Transverse plane motion of the knee is
linked to the motion of the foot and to the sagittal plane
motion of the knee, particularly during stance, when the
lower extremity functions in a closed chain. As the foot
pronates, the tibia medially rotates and allows the knee to
flex. This coupled motion assists in shock absorption during

900

Part V I POSTURE AND GAIT

Figure 48.11: The pelvic position in the transverse plane and the
femoral rotation in the transverse plane both contribute to the
transverse plane hip joint position during the gait cycle. At
ground contact the femur is medially rotating, but the forward
alignment of the pelvis contributes to lateral rotation of the hip.
At heel off the opposite is true.

loading response [137]. Later in stance, the foot supinates as
the tibia rotates laterally, and the knee extends while the body
rolls forward onto the opposite limb.

MOTIONS OF THE TRUNK

Studies of the head and trunk reveal that these segments
undergo systematic translation and rotation in three dimen¬
sions and exhibit both intrasubject and intersubject variability
[97,174]. The trunk exhibits slight flexion and extension during
the gait cycle, is more erect or extended during single limb sup¬
port, and is more flexed during double limb support [32,97].
Frontal plane motion of the trunk is consistent with the need
to keep the center of mass over the stance foot. So the trunk
leans slightly to the stance limb at each step [97,119,157,174].
In the transverse plane, the rotation of the trunk is opposite the
rotation of the pelvis, with the trunk rotating forward on the
side in which the shoulder is flexing [97,119,157].

Clinical Relevance

THE TRUNK S CONTRIBUTION TO SMOOTH GAIT:

The gait pattern of a toddler learning to walk is character¬
ized by large lateral leans with little forward rotation of the

trunk and shoulders [11]. As the child matures , the pattern
becomes smoother and more stable , and trunk rotation
moves out of phase with the pelvis. The coupling motion of
the trunk and pelvis contributes to the efficiency and stability
of gait. Patients who lack the ability to rotate the trunk sep¬
arately from the pelvis , such as patients with Parkinson's
syndrome or patients with low back pain , may lose gait effi¬
ciency and expend more energy to walk.

MUSCLE ACTIVITY DURING
LOCOMOTION

Studies that examine the electrical activity of muscles during
locomotion have played a central part in defining the role of
muscles in producing and controlling locomotion. Data from
Winter and Yack [188] demonstrate the normalized elec¬
tromyographic (EMG) data for 16 muscles recorded in up to
19 subjects (Fig. 48.12). These data reveal important principles
regarding muscle activity during gait. First, the duration of
large bursts of activity for most muscles is quite brief, and most
of these bursts occur at the transitions between swing and
stance or between stance and swing. These data demonstrate
the considerable variability in muscle activity across individuals.
Studies also demonstrate variability within a single individual,
although there is less than across individuals [22,76,82,188].

Despite the variability of muscle activity, certain consistent
functions for specific muscle groups emerge from the EMG
data [76,82,92,161,180]. In order to understand the role muscles
play during gait it is important to recall that each lower extrem¬
ity functions in both an open and closed kinetic chain, open
through the swing phase and closed through the stance phase.
Consequently, a muscle contraction can affect not only the joint
crossed by that muscle, but also joints throughout the chain.

The gluteus maximus and hamstrings are active prior to and
following ground contact, exerting a deceleration force on the
hip and knee at the end of swing. Their activity also helps to
initiate hip extension during early stance. By controlling the
femur the gluteus maximus also helps to accelerate the knee
toward extension during early single-limb support [10].

The gluteus medius contracts just before ground contact
and continues its activity through most of stance, until load¬
ing begins on the opposite side. The activity of the hip abduc¬
tors provides essential frontal plane stability to the pelvis
throughout stance and adds to hip and knee extension sup¬
port in mid to late stance [2,10]. The hip flexors contract in
late stance and continue their activity into early swing to slow
hip extension and initiate hip flexion [10,61]. The iliopsoas
also contributes to the flexion velocity of the knee in mid-
stance [58].

Muscle activity at the knee is characterized by co-contrac¬
tion of the hamstrings and quadriceps for approximately the
first 25% of the gait cycle, during loading response and early
midstance. During this period, the knee is flexing and then
extending, and the quadriceps activity is essential in controlling

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

901

500-

400-

300-

200 -

100 -

0 -

Lateral hamstrings

N = 17
CV = 59%
MEAN = 52.6

I-1-1-1-1-1

0 20 40 60 80 100

0 20 40

"1-1

80 100

A % of Stride

% of Stride

300

200 -

100

300-

20 40 60

% of Stride

400

300-

200 -

100 -

N = 11
CV = 61 %
MEAN = 113.0

Lateral

N = 10

gastro¬

CV = 57%

cnemius S'

V MEAN = 79.2

Soleus

N = 18

<\ CV = 31%

\ MEAN = 113

80 100

0 20 40 60 80 100

% of Stride

Figure 48.12: Electrical activity of lower extremity muscles during
gait. (Reprinted with permission from Winter DA, Yack HJ: EMG
profiles during normal human walking: stride-to-stride and inter¬
subject variability. Electroencephalogr Clin Neurophysiol 1987;

67: 402-411.)

this movement. Some individuals exhibit activity of either the
quadriceps, especially the rectus femoris, or hamstrings at
the transition from stance to swing, but this activity is both
variable and smaller in magnitude than the activity at the

beginning of stance [8,125]. Most of swing proceeds with no
muscle activity at the knee joint.

The ankle also exhibits co-contraction of the dorsiflexor
and plantarflexor muscles. Dorsiflexors of the ankle exhibit
slight activity throughout swing to hold the foot away from
the ground. The activity continues at ground contact and
through the loading response, controlling the descent of the
foot onto the ground. The plantarflexor muscles gradually
increase their activity from ground contact through most of
stance, with the greatest burst of activity from heel off to toe
off as the body rolls over the plantarflexing foot. Through
the mechanics of the closed chain, the plantarflexors also
help control the hip and knee joints [10,162]. The plan¬
tarflexor muscles provide the majority of support to the
lower extremity during the latter portion of the stance phase
[2]. With the iliopsoas, the gastrocnemius contributes to the
flexion velocity of the knee [58].

Review of the muscle activity of these large muscle
groups demonstrates that much of the activity is character¬
ized by an eccentric contraction followed by a concentric
contraction. For example, the gluteus maximus contracts
eccentrically as the hip flexes late in swing and then con¬
tracts concentrically as the hip begins to extend. The same
pattern is found in the gluteus medius, hip flexors, quadri¬
ceps, and dorsiflexors. The plantarflexors also exhibit
lengthening and then shortening, although at least some of
the change in length is a passive stretch and shortening in
the tendo calcaneus (Achilles tendon), so the actual change
in muscle fiber length may be small [52]. The hamstrings
also begin their activity with an eccentric contraction in late
swing, but their subsequent length is more difficult to dis¬
cern, since at loading response the hip is extending while the
knee is flexing. The overall length change in the hamstrings
during loading response may be negligible. The lengthening
contractions that begin many muscles’ activity in gait decel¬
erate each joint, and then the subsequent concentric con¬
tractions begin the joints forward movement.

This pattern of eccentric then concentric contraction is
known as the stretch-shortening cycle and is used by most
muscles during gait as an efficient means of generating
muscle force and storing energy (see Chapter 4 for more
details). Some of the energy stored by the stretched muscle
is released during the muscle s lengthening to help propel
a limb segment without requiring additional muscle
contraction [126]. Thus normal gait utilizes muscle contrac¬
tions in a very efficient way to generate force and produce
movement.

It is worth noting that at most joints, the motion occurring
during the concentric contraction continues after the con¬
traction ceases. For example, the hip continues to extend long
after the peak activity of the gluteus maximus and hamstrings,
and the hip flexes after cessation of hip flexor activity.
Similarly, the knee continues to extend without significant
quadriceps activity, and the ankle continues to dorsiflex after
the burst of dorsiflexor activity early in stance. Thus the
chief functions of the muscles of the lower extremity during

902

Part V I POSTURE AND GAIT

locomotion are to slow one motion and to provide an initial
burst, or pull, in the opposite direction. How motion contin¬
ues in the absence of active muscle contraction is related to
the kinetics of the movement.

Clinical Relevance

MUSCLE WEAKNESS AND CHANGES IN GAIT: Muscles
play complex roles during gait including controlling and pro¬
pelling the individual jointfs) they cross as well as supporting and
propelling joints throughout the lower limbs. Consequently
weakness of even a single muscle can produce significant
changes in movement patterns throughout the lower extremities
[95,162]. For example, in the presence of significant quadriceps
weakness the individual will avoid knee flexion while bearing
weight on the affected limb. In order to avoid knee flexion, how¬
ever, the individual must alter other joint movements as well. The
subject may avoid the ankle dorsiflexion position as a means of
ensuring the knee remains extended or avoid the use of the

plantaiflexor muscles because they contribute to acceler¬
ation of the knee. Similarly, weakness in hip muscula¬
ture may alter hip, knee, or ankle joint movements.

KINETICS OF LOCOMOTION

Kinetics examines the forces, moments, and power generated
during a movement and, in the case of locomotion, includes
the moments generated by the muscles, the forces applied
across joints, and the mechanical power and energy generated.
A discussion of the kinetics of gait allows consideration of the
efficiency of gait.

Joint Moments and Reaction Forces

As indicated in the preceding sections, gait consists of com¬
plex cyclical movements occurring in a coordinated sequence
that is controlled by muscle activity. In addition, gait entails
the repetitive impact loading of both lower extremities in
each gait cycle. Thus it is easy to recognize that normal loco¬
motion produces large forces between the foot and the
ground, requires significant muscle forces, and generates
large joint reaction forces. Many impairments in gait are
related to an individuals inability to generate sufficient mus¬
cular support or to sustain the large reaction forces of gait.

DYNAMIC EQUILIBRIUM

Researchers and clinicians have long been interested in the
forces sustained by the muscles and joints during normal and
abnormal locomotion [17,110,168]. Chapter 1 of this text
describes the principles used to determine the loads in mus¬
cles and on joints during activity. Newtons first law defines
the conditions of static equilibrium (2F = 0, 2M = 0), stat¬
ing that an object remains at rest (or in uniform motion)

unless acted upon by an unbalanced external force.
Throughout this text, two-dimensional examples of static
equilibrium problems are provided to analyze the forces in
the muscles and on joints during static tasks or in tasks where
acceleration is negligible. However, during gait, limb seg¬
ments undergo large linear accelerations, and joints exhibit
large angular accelerations. As a result, the assumption used
in static equilibrium analysis, that acceleration is negligible, is
not valid when applied to gait.

Newtons second law of motion, 2F = ma, states that the
unbalanced force on a body is directly proportional to the
acceleration of that body. The specific relationships between
the accelerations and the forces and moments can be deter¬
mined by applying the principles of dynamic equilibrium.
The conditions of dynamic equilibrium are very similar to the
conditions of static equilibrium. To determine the forces on
an accelerating body in a two-dimensional analysis, the fol¬
lowing conditions must be satisfied:

2F X = ma x , 2F y = may,

2M = I X a (Equation 48.1)

In three-dimensional analysis, the conditions for dynamic
equilibrium are

2F X = ma x , 2F y = may,

2F Z = ma z (Equation 48.2)

and

2M X = I X a x , 2M Y = I X a Y ,

2M Z = I X a z (Equation 48.3)

where F. is the force in the ith direction, a. is the linear accel-
eration in the ith direction, M. is the moment about the ith
axis, a. is the angular acceleration in the ith direction, and I is
the moment of inertia. The moment of inertia indicates a
body’s resistance to angular acceleration and depends on the
body’s mass and distribution of mass. The larger the mass and
the farther the mass is from the body’s center of mass, the larger
is the body’s moment of inertia. Elite gymnasts tend to possess
short and compact bodies (smaller moments of inertia) that
allow high angular accelerations producing rapid rotations
about horizontal bars and in tumbling routines. The accelera¬
tion quantities in each of the equations of dynamic equilibrium,
ma. and I X a., are known as inertial forces and are intu-
itively explained by the awareness that it takes more force to
push a car to start or stop its rolling, that is, to accelerate or
decelerate it, than it takes to keep the car rolling.

Solutions to the conditions of dynamic equilibrium, also
known as equations of motion, require knowledge of several
parameters, including mass and moment of inertia. Mass
is usually determined from tables derived from cadaver
measurements, as demonstrated in examples throughout this
textbook [40]. Similarly, these tables provide means to calcu¬
late moments of inertia of a limb or limb segment from easily
obtained anthropometric measurements, although methods
also exist to compute the moment of inertia of some segments

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

903

directly [25,155]. Regardless of the method chosen, the prop¬
erties of mass and moment of inertia can be estimated and
entered into the equations of motion to allow solutions.

Theoretically, the equations of motion in dynamic equilib¬
rium can be used to calculate a body’s acceleration from all of
the forces on the body. This approach is useful to determine
the response of an airplane or rocket to an applied force.
However, in the case of human movement, where forces can¬
not be measured directly, the equations of motion are used
more often to determine the forces on the body when the
accelerations are known. This approach, known as inverse
dynamics, allows estimation of the forces on the human body
and requires direct determination of the acceleration.
Application of inverse dynamics in static equilibrium is
straightforward because the accelerations are, by definition,
zero, and the examples of two-dimensional analysis through¬
out this book demonstrate the use of inverse dynamics.

Chapter 1 reminds the reader that acceleration is the
change of velocity over time, and velocity is the change in

displacement over time. Therefore, if a body’s displacement is
known over time, then velocity and acceleration can be deter¬
mined. Precise calculations of velocity and accelerations of the
body or of any limb segment requires careful measurement of
the displacement, which can be accomplished by a number of
techniques including high-speed cinematography, videogra-
phy, or electromagnetic tracking devices [99,119,124,140].
Appropriate signal processing of the displacement data and
mathematical calculations yield satisfactory estimations of
velocity and accelerations of the body of interest. A thorough
discussion of the methods and challenges in these techniques
is beyond the scope of this book; suffice it to say that the nec¬
essary acceleration values are available, so that the equations
of motion can finally be solved for the applied forces.

Examining the Forces Box 48.1 provides an example of the
equations of motion for the leg-foot segment during the
swing phase of gait. Using anthropometric data from
Dempster [40], the mass (m) and moment of inertia (I) are
entered directly into the calculations. Videographic data are

904

Part V I POSTURE AND GAIT

Figure 48.13: Free body diagram of the leg-foot segment during
stance includes the forces: weight of the leg-foot (W), joint reac¬
tion force (J), muscle force (M), ground reaction force (GRF), iner¬
tial forces -ma and -la, where m = mass, a = linear acceleration,

I = moment of inertia, and a = angular acceleration.

collected at a rate of 60 Hz (hertz, or cycles per second) and
manipulated so that the linear and angular accelerations of
the leg-foot segment are determined for every 1/60 of a sec¬
ond and entered into the equations. The equations of motion
are solved repeatedly for the muscle force (F) at each incre¬
ment of time. A similar procedure is applied to the stance
phase of gait, but the external forces on the foot also include
the ground reaction forces (Fig. 48.13). The direction and
magnitude of these forces must be known to solve the equa¬
tions of motion during stance and can be measured directly
by force plates. The characteristics of the ground reaction
force during gait are discussed in the following section.

The example presented in Examining the Forces Box 48.1
assumes that only one muscle group is active. However, the
EMG data described earlier in this chapter provide convincing
evidence that there is co-contraction of the hamstrings and
quadriceps during late swing and early stance and sometimes at

the transition from late stance to early swing as well. Ligaments
also apply significant loads to the knee joint during gait
[67,160]. Thus there is more than one structure applying force
at the knee joint, producing a dynamically indeterminate sys¬
tem. As noted in Chapter 1 and elsewhere in this book, sophis¬
ticated mathematical solutions for indeterminate systems exist,
and they are applied frequently in locomotion research to
approximate the muscle and joint reaction forces [27,158].

Using inverse dynamics, many studies report the joint
reaction forces in the body during the gait cycle [3,17,33,
44,67,94,158,165]. Peak joint reaction forces at the hip, knee,
and ankle reported in the literature are presented in Table
48.3. These data reveal wide variation in the forces reported
at each joint. Several factors influence these calculations,
including the estimates of the body segment parameters of
mass and moment of inertia, the accuracy of the displacement
data and the procedures to determine accelerations, the use
of two- or three-dimensional analysis, as well as the analytical
approach used to complete the calculations [1,4,36,39,96,191].
Values reported here are intended to demonstrate that
regardless of the precise magnitude, all of the joints of the
lower extremity sustain large and repetitive loads during loco¬
motion. Running and jumping produce even larger muscle
loads and joint reaction forces [21,112,172].

To avoid the problem of indeterminacy, researchers often
solve only the moment equations, calculating the external
moments applied to the limb by external forces such as
weight and ground reaction forces and inferring the internal
moments applied by the muscles and soft tissue [85].
Authors report either the internal [181] or external moment
[88,98], and the reader is urged to read the literature carefully
to identify which moment is reported. The limitation of this
approach is that it prevents calculations of the forces in spe¬
cific muscles and at the joints, but joint moments provide
insight into the primary roles of muscle groups during gait
and support the roles already suggested by EMG.

Typical internal moments generated at the hip, knee, and
ankle in the sagittal plane during normal locomotion are
reported in Fig. 48.14. The internal moment at the hip joint
at ground contact and contact response is an extension
moment, consistent with the EMG activity of the gluteus
maximus and hamstrings [54,82]. The moment changes direc¬
tion in midstance at about the time the hip extensors cease
their activity and the flexors become active. The moment at
the hip in swing is minimal until late swing when the hip
extensors resume activity.

The knee demonstrates a small and brief flexor moment at
ground contact, consistent with hamstring activity, but then a

TABLE 48.3: Reported

Peak Joint Reaction Forces during

Normal Gait in Units of

Body Weight

Anderson
et al. [3]

Komistek [94]

Duda
et al. [44]

Seireg and

Arvikar [158]

Hardt [67]

Simonsen
et al. [165]

Hip 4

2.0-2.5

5.25

Knee 2.7

1.7-2.3

n.r. a

2.75

4.5

Ankle 6

1.25

n.r.

3.5

a Not reported.

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

905

Figure 48.14: Internal moments at the hip, knee, and ankle in
the sagittal plane. (Reprinted with permission from Kadaba MP,
Ramakrishnan HK, Wootten ME, et al.: Repeatability of kinematic,
kinetic, and EMG data in normal adult gait. J Orthop Res 1989;

7: 849-860.)

larger and more prolonged extensor moment that is consistent
with quadriceps activity. In midstance, the knee exhibits a small
flexor moment that is attributable to activity of the gastrocne¬
mius. A small extension moment helps control knee flexion at
the end of stance and in early swing, just as the flexion moment
at the end of swing slows the rapid knee extension.

A small dorsiflexion moment at ground contact and con¬
tact response reflects the dorsiflexor activity controlling the
descent of the foot onto the ground. It is followed by a steadily
increasing and prolonged plantarflexion moment controlling
advancement of the tibia through the rest of stance. Although
there has been disagreement about whether the plantarflex-
ors actually propel the body forward [135], recent studies

provide convincing evidence that these muscles contribute
some of the propulsion moving the body forward [126,
142,144,153]. A very small dorsiflexion moment following toe
off pulls the foot and toes away from the ground.

Moments in the transverse and frontal planes also are
reported and appear to be important in the mechanics and
pathomechanics of locomotion [46,73,107]. However, less
consensus exists regarding the magnitude and even the pat¬
tern of these moments. Moments in the frontal and trans¬
verse planes are smaller than those in the sagittal plane, and
smaller moments are more sensitive to measurement errors,
including the location of the joint axes and the kinematics of
the movements [20,72].

Clinical Relevance

KNEE ADDUCTION MOMENT: The external frontal
plane moment at the knee , known as the adduction
moment has been implicated in the development and pro¬
gression of knee osteoarthritis (OA) as well as the pain and
disability associated with knee OA (Fig. 48.15) [73].

(continued )

Figure 48.15: Adduction moment on the knee in gait. The
ground reaction force (GRF) applies an external adduction
moment (M AD ) on the knee during the stance phase of gait.

906

Part V I POSTURE AND GAIT

(Continued)

Treatments to reduce the adduction moment and reduce
pain include lateral shoe wedges; knee braces that apply a
valgus stress to the knee ; and surgical osteotomy to realign
the knee joint [6].

Winter describes a support moment for the stance
phase of gait that is the sum of the internal sagittal plane
moments in which all of the moments that tend to push the
body away from the ground or support the body are posi¬
tive (Fig. 48.16) [70,181]. The net support moment during
stance is positive, indicating the overall role of the muscles
to support the body and to prevent collapse during weight
bearing. Data suggest that although the net support
moment is consistent across walking trials, individuals with¬
out pathology demonstrate variability in the individual joint
moments, indicating that individuals with normal locomo¬
tor systems may exhibit flexibility in the ways they provide
support [182].

Figure 48.16: The support moment is the sum of the moments at
the hip (M^, knee (M K ), and ankle (M A ) needed to support the
body weight during stance.

Clinical Relevance

A PATIENT WITH QUADRICEPS WEAKNESS: A patient
with quadriceps weakness lacks the ability to support the
knee actively during the stance phase of gait. To generate
adequate support during the stance phase of gait , this
patient may increase activity of the hip extensor muscles
and of the soleus to increase the hip and ankle contribu¬
tions to the net support moment (Fig. 48.17).

GROUND REACTION FORCES

With every stride, each foot applies a load to the ground and
the ground pushes back, applying a ground reaction force
to each foot. The magnitude and direction of this ground
reaction force changes throughout the stance phase of each
foot and is directly related to the acceleration of the body’s
center of mass. The center of mass of the body rises and falls
as the individual moves from double support when the

Figure 48.17: An individual may increase the activity in the
soleus and the gluteus maximus to support the knee in exten¬
sion by preventing forward movement of the tibia or the
femur, respectively.

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

907

Figure 48.18: Ground reaction forces during gait. (Reprinted with
permission from Meglan D, Todd F: Kinetics of human locomo¬
tion. In: Rose J, Gamble JG, eds. Human Walking. Philadelphia:
Williams & Wilkins, 1994; 23-44.)

center of mass is low to single support when the center of
mass is high [77,119,154]. Similarly, the center of mass
moves from side to side as the individual passes from stance
on the right to stance on the left [119]. The ground reaction
force is measured directly by force plates imbedded in the
walking surface.

The ground reaction force typically is described by a verti¬
cal force as well as anterior-posterior and medial-lateral
shear forces. The vertical ground reaction force under one

foot is characterized by a double-humped curve (Fig. 48.18).
The two peaks are greater than 100% of body weight and
occur when the body accelerates upward. The valley between
the peaks is less than 100% of body weight and occurs during
single limb support. Examining the Forces Box 48.2 uses
dynamic equilibrium to demonstrate how acceleration of the
center of mass of the body alters the ground reaction force.
The vertical ground reaction force also is characterized by a
brief but high peak just following ground contact, which
reflects the impact of loading [164].

Clinical Relevance

GROUND REACTION FORCES AND JOINT PAIN: Vertical
ground reaction forces contribute significantly to joint reaction
forces; and large joint reaction forces contribute to pain in
patients with joint pathology such as arthritis. Patients with
arthritis walk more slowly [84], and their vertical ground reac¬
tion forces demonstrate smaller peaks and valleys as the result
of smaller vertical accelerations [163,166]. A reduction in walk¬
ing speed, producing a reduction in accelerations, may be an
effective way to reduce joint loads and, consequently, joint
pain. These changes may represent appropriate adaptations to
protect a painful joint and to maintain overall function.

The posterior and anterior shear components of the
ground reaction force also demonstrate a consistent pattern
in normal locomotion. The ground exerts a posterior force on

908

Part V I POSTURE AND GAIT

the foot during the initial portion of stance, decelerating the
foot; consequently this period is known as the deceleration
phase. In midstance, the ground applies an anterior shear
force on the foot, contributing to the forward propulsion of
the body. The second half of the stance phase is known as the
acceleration phase of the gait cycle. Walking on ice demon¬
strates the importance of these posterior and anterior shear
forces. Because there is little friction between the foot and
the ice, the posterior and anterior shear forces between the
ground and the foot are small when walking on ice, and
forward progress is impaired. In the absence of any posterior
and anterior shear forces, forward progress is impossible.

The medial and lateral shear forces during gait are smaller
and more variable than the vertical forces or posterior-
anterior shear forces. They reflect forces associated with the
shift of the body from side to side between the supporting
feet. Although plots of the ground reaction forces demon¬
strate rather stereotypical shapes, it is important to recognize
that like kinematic variables, these forces exhibit normal
intra- and intersubject variability [55,68].

The ground reaction force vector is the sum of the indi¬
vidual components of the ground reaction force. Whether
described as a single force vector or as three individual com¬
ponents, the ground reaction force generates external
moments on the joints of the body in all three planes
(Fig. 48.19). Realistic computation of joint moments and

Figure 48.19: The ground reaction force vector (GRF) is the sum
of the vertical, anterior-posterior, and medial-lateral ground
reaction forces. The force vector applies external moments to the
joints of the lower extremities about all three axes.

Figure 48.20: Progression of the center of pressure during
locomotion. (Reprinted with permission from Sammarco GJ,
Hockenbury RT: Biomechanics of the foot and ankle. In: Nordin
M, Frankel VH, eds. Basic Biomechanics of the Musculoskeletal
System. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2001;
222-255.)

forces during gait must include the three components of the
ground reaction force or the force vector.

The location of the ground reaction force with respect to
the foot indicates the path of the center of pressure through
the foot. In the normal foot, the center of pressure progresses
in a relatively straight line from the posterior aspect of the
plantar surface of the heel through the midfoot and onto the
forefoot where it deviates medially onto the plantar surface of
the great toe [64,66] (Fig. 48.20). Inability to roll over a
painful toe or the interrupted forward progress of the body’s
center of mass because the knee suddenly hyperextends, are
examples of gait deviations that produce changes in the pat¬
tern of the progression of the center of pressure.

Energetics of Gait: Power, Work,
and Mechanical Energy

Normal locomotion appears to be a remarkably efficient
movement. Individuals without impairments, walking at a
self-selected cadence, require less oxygen consumption than
when walking at lower or higher cadences [12,116].
Individuals with locomotor impairments expend more energy
during ambulation than individuals without impairments
[18,111,167]. The efficiency of locomotion depends on many
factors, including the mechanics of the muscular control of
gait described earlier in this chapter and the conservation of
mechanical energy that results from the synergistic move¬
ment of the limb segments.

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

909

JOINT POWER

Mechanical power is the product of force and linear
velocity or, in rotational motions such as the joint move¬
ments in locomotion, the product of joint moment and
angular velocity:

P = M • CO (Equation 48.4)

where P is power in watts, M is a joint moment, and CO is the
angular velocity of the limb segment. Power is a useful indi¬
cation of the muscles’ role in controlling motion; it is negative
when the body absorbs energy during eccentric muscle activ¬
ity and is positive when the body generates energy during
concentric muscle activity. Power also can be described as
work (W) per unit time (t) (i.e., W/t), where work is the prod¬
uct of force and displacement, or in angular terms, the prod¬
uct of moment (M) and angular displacement (0):

W = M • 0 (Equation 48.5)

Angular velocity, CO, is equal to angular displacement over
time (co = 0/t) and therefore:

P = M • 0/t (Equation 48.6)

and

P = W/t (Equation 48.7)

Thus concentric muscle activity generates power, or does
work, and eccentric activity absorbs power, and work is done
on the segment [187]. A pogo stick (Pogo™) provides a use¬
ful example of positive and negative power, work done on or
by the pogo stick (Fig. 48.21). In landing, the weight of the
child does work on the pogo stick, and energy is absorbed by
its spring, but in takeoff, the spring releases its energy and
performs work on the child, pushing the child and pogo stick
off the ground.

Analysis of joint powers provides increasing understanding
of the role of muscles in propelling and controlling movement
during locomotion [144,151]. The joint powers at the hip,
knee, and ankle during gait derived from two-dimensional
analysis are pictured in Fig. 48.22. These demonstrate that
positive power generation, when muscles are generating
power and doing positive work, occurs at the hip at loading
response as the hip extends and again at the end of stance as
it flexes. Both of these periods are characterized by concen¬
tric muscle contractions. In contrast, the knee has only a brief
period of power generation, producing only a small amount of
power. Like the hip, the ankle generates considerable positive
power at the end of stance when the plantarflexors contract
concentrically. These data suggest that the hip flexors and
extensors and the plantarflexors contribute important energy
to the lower extremity during normal locomotion. It is worth
noting that the power generated by the plantarflexors is
considerably larger than that of any other muscle groups. The
plantarflexor muscles, particularly the gastrocnemius muscles,
appear to play an essential role in forward propulsion [60,142].

Figure 48.21: Energy storage and release. A. Weight bearing on
the Pogo stick™ compresses its spring and work is done on the
stick. B. As weight is removed, the spring is released, and the
Pogo stick™ does work on the body, lifting it into the air.

910

Part V I POSTURE AND GAIT

15-.

0 20 40 60 80 100

Gait cycle (%)

0 20 40 60 80 100

Gait cycle (%)

Figure 48.22: Joint powers at the hip, knee, and ankle from two-
dimensional analysis. (Reprinted with permission from Meglan D,
Todd F: Kinetics of human locomotion. In: Rose J, Gamble JG, eds.
Human Walking. Philadelphia: Williams & Wilkins, 1994; 23-44.)

Clinical Relevance

JOINT POWERS IN INDIVIDUALS WITH GAIT
DYSFUNCTIONS: Joint powers during free-speed walking
are altered in elders and in individuals with weaker lower
extremity muscles [41114]. The decrease in plantarflexion
power and concomitant increase in hip flexor power genera¬
tion noted in elders and in individuals with weakness may
help to explain the decrease in velocity and step length
reported in these individuals, as well as their mechanisms of
compensation [41,113,114]. As an individual is unable to
generate power through plantarflexion for forward progres¬
sion, active hip flexion appears to provide the forward
propulsion needed to swing the limb forward. These patients

may benefit from exercise to improve plantarflexion force
production.

The use of joint kinetics in conjunction with EMG is also
useful in evaluating the complex gait deviations in individu¬
als with central nervous system disorders such as cerebral
palsy. These analyses provide more insight into the mechan¬
ics of the gait abnormalities than can be provided solely by
clinical observation and lead to more informed treatment
decisions [132,148].

MECHANICAL ENERGY

The cyclic movement inherent in locomotion and the ability
of the muscles to store energy contribute to the inherent effi¬
ciency of normal gait. The cyclic nature of gait has led to its
description as an inverted pendulum in which the body
swings repeatedly over the stance limb [128]. The mechani¬
cal energy of a pendulum changes form from potential to
kinetic energy, thereby maintaining its swing with little
additional energy input. The image of gait as the movement
of an inverted pendulum has spurred investigators to study
the mechanical energy of gait as a means of assessing its effi¬
ciency Potential (PE) and kinetic (KE) energy are related to
the distance of a body’s center of mass from the earth and to
the body’s linear and angular velocity, as indicated by the fol¬
lowing relationships:

PE = mgh (Equation 48.8)

where m is the mass of the body, g is the acceleration due to
gravity, and h is the distance from the body’s center of mass to
the earth; and

KE = ^ my2 + \ Ico 2 (Equation 48.9)

where m is the body’s mass, v is its linear velocity, I is its
moment of inertia, and CO is its angular velocity. In an ideal sys¬
tem, conservation of energy dictates a complete transforma¬
tion between potential and kinetic energy, so that an ideal
roller coaster continues in motion indefinitely (Fig. 48.23).
When the cars are at their peak height, potential energy is
maximized and kinetic energy is minimized. At its lowest
point, the roller coaster’s potential energy is minimum and its
kinetic energy is maximum. Since the work done on a body
equals the change in total energy, an ideal system requires no
work to continue moving, since the change in the body’s total
energy is zero. Studies of the mechanical energy of the limb
segments during gait suggest that an exchange of kinetic and
potential energy can account for most of the energy change in
the distal leg at the beginning and end of swing [146,186]. This
energy exchange improves when walking normally at free
speed and is greater at steady-state walking than at the initia¬
tion of gait [109,117,177]. Assessment of the energy transfer
during locomotion demonstrates the efficiency of gait and sug¬
gests that minimizing the expenditure of mechanical energy
may actually be a dominant characteristic of normal gait [141].

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

911

Figure 48.23: In an ideal roller coaster, potential and kinetic energy
are transformed from one form to the other with no loss of ener¬
gy. Potential energy (PE = mgh) is maximum when the roller
coaster is farthest from the ground, at the same time the kinetic
energy (KE = 1/2 mv 2 ) is at its minimum. As the roller coaster
descends the track it gains speed, increasing its kinetic energy
while it is losing potential energy as it moves closer to the ground.

The ability of the muscles to absorb and generate energy
contributes to the overall efficiency of gait and explains how
many of the movements can proceed without muscle con¬
traction [156]. Energy flows between adjacent limb segments
during locomotion in much the same way that energy flows
between the vaulter and the pole during a pole vault or
among children playing “crack the whip.” Like the pole used
by the vaulter, much of the energy released by the muscles in
gait is elastic energy stored within the passive elastic compo¬
nents of the muscle [156]. (See Chapter 4 for details about
the passive elements of muscle.) Examination of the energy
flow between limb segments reveals that the energy generated
by the plantarflexors at push off is transferred passively to the
leg and thigh, facilitating the initiation of swing. Similarly, the
hamstrings absorb energy at the end of swing, and that energy
is transferred to the trunk at ground contact, assisting in
the trunks forward progression. The transfer of energy from
segment to segment depends on the normal sequencing of
the angular changes described earlier in this chapter.

Thus normal, efficient gait consists of complex motions of
several limb segments whose movement and control are inter-
dependent. Alterations at a single joint are likely to pro-
KM C\ duce changes in movement patterns throughout the
body and diminish the efficiency of the movement.

Clinical Relevance

ENERGY TRANSFER AMONG LIMB SEGMENTS
IN ABNORMAL GAIT: Energy transfer among limb seg¬
ments depends on the power generated and absorbed at
joints and requires precise coordination among the moving
segments. Since power is a function of the velocity of a limb
segment a limb segment that has a low angular velocity
also has low power generation or absorption and , conse¬
quently , has less ability to transfer energy from one segment

to another. A patient with arthritis producing a stiff knee is
unable to transfer energy from the plantarflexors to the
thigh; a patient with Parkinson's disease, which is character¬
ized by generalized rigidity , has difficulty transferring energy
through the lower extremity and into the trunk because the
joints lack the freedom of movement to allow the sequential
movement patterns of the joints of the lower extremity.

A study of patients with multiple sclerosis demonstrates an
inverse relationship between the metabolic cost of walking
and the patients' ability to rapidly flex and extend the knee.
This finding is consistent with a diminished capacity to
transfer energy through the knee joint [130]. Thus treat¬
ments directed toward reducing joint stiffness or rigidity may
lead to improved gait efficiency in these individuals.

FACTORS THAT INFLUENCE
PARAMETERS OF GAIT

Several factors influence gait performance and must be con¬
sidered by clinicians evaluating and treating a person with a
locomotor dysfunction. Factors considered here are gender,
speed, and age.

Gender

Although most observers would report differences between the
gait patterns of males and females, few studies provide direct
comparisons. Women walk with higher cadences than men and
shorter strides [15,88,119]. Yet when the distance characteris¬
tics of the gait cycle are normalized by height, females demon¬
strate a similar or slightly larger stride length [47,88].

A study directly comparing 99 males and females of simi¬
lar ages reports statistically different joint kinematics,
although these differences are on the order of 2-4°, and the
clinical significance of these differences is negligible [88]. The
same study also reports that females exhibit a statistically
greater extension moment at the knee at initial contact and a
greater flexion moment in preswing with increases in power
absorption or generation at the hip, knee, and ankle. A simi¬
lar study found no gender differences in flexion or adduction
moments at the knee during stance [90]. These studies sug¬
gest that while slight differences exist in some kinetic vari¬
ables of gait, none of these differences are enough to explain
the higher incidence of knee osteoarthritis in women.

Walking Speed

Gait speed affects several parameters of gait performance. As
noted in the discussion of the temporal and distance character¬
istics of gait, cadence, step length, and stride length increase
with increased walking speed and decrease with decreased
speed [7,119]. Increased speed appears to increase the variabil¬
ity of some temporal and spatial gait parameters such as step
width [159]. Angular excursions generally increase with walking

912

Part V I POSTURE AND GAIT

speed, but these changes vary with the joint and the direction of
motion [34,170,173]. Increases in joint excursions at the proxi¬
mal joints are related to the increase in stride length associated
with increased speeds [34].

Increased walking speeds also lead to increased ground
reaction forces [7,28] and changes in the pattern of muscle
activity. The relationship between walking speed and muscle
activity is somewhat complex and depends on the muscle
[71,178]. In general, peak muscular activity increases with
walking speed [173,178]. The duration of muscle activity may
increase at both very fast and very slow walking speeds
[118,173]. In general, muscle activity during free-speed walk¬
ing is more reproducible than that at speeds slower or faster
than free speed [26,93]. With increased peak muscle activity,
it is not surprising that joint moments and joint reaction forces
also increase with walking speed [13,103,191]. Similarly
increased mechanical work and power at all of the lower
extremity joints accompany increased walking speed [80,103].
However, the relative contribution of the hip flexors and exten¬
sors to propulsion increases with walking speed [142,143].

Clinical Relevance

WALKING SPEED IN INDIVIDUALS WITH GAIT
IMPAIRMENTS: Many abnormal gait patterns found in
individuals with impairments are characterized by decreased
walking velocities. Patients with dysfunctions associated with
low back pain , strokehemiparesis, and anterior cruciate lig¬
ament tears all frequently exhibit altered gait patterns that
include decreased step length , smaller joint excursions , and
decreased walking speed. Because decreased walking speed
is associated with decreased step length and joint excursion ,
are the gait deviations exhibited by these patients merely
the consequence of their walking speed? If a goal of treat¬
ment is to improve the gait pattern , the clinician must
attempt to discern what characteristics of the gait pattern
are attributable to the gait speed alone , and what character¬
istics are the result of the patient's impairments.

Age

Age appears to affect gait rather dramatically, as witnessed by
the development of gait in the toddler and the apparent dete¬
rioration of gait in older adults. While the gradual acquisition
of stable bipedal ambulation is a normal part of human devel¬
opment, it is unclear whether the alterations commonly seen
in gait in the elderly are the normal consequence of aging or
reflect functional deficits resulting from impairments associ¬
ated with neuromusculoskeletal disorders commonly found in
elders [37,47,56,102,190].

Table 48.4 lists commonly reported changes in gait with
aging. The ages of the elders studied range from approxi¬
mately 60 years to over 100 years, and studies vary in the
magnitude of changes reported. Despite the overwhelm¬
ing data demonstrating changes in gait with increasing
age, the nature of the relationship between age and loco¬
motor function remains unclear. One of the most consis¬
tent findings with age is a decrease in free-walking speed
[50,69,86,102,105,121], but many of the other changes
reported with aging also are consistent with the changes
reported earlier in this chapter for walking speed alone
[48]. Specifically, decreased walking speed produces
reductions in step length, joint excursions, and ground
reaction forces [50,69,87,185]. Consequently, many of the
changes that occur with aging appear to be secondary
changes associated with walking speed. However, even
when controlling for speed, elders demonstrate a signifi¬
cant increase in variability in gait characteristics as well as
an increased energy cost of walking [19,108,133].

The decrease in walking velocity reported with age
appears to depend on an individuals level of fitness and other
factors besides age itself. Coexisting joint impairments;
strength of the quadriceps, plantarflexors, and hip flexors; hip
and knee passive ranges of motion; and maximal oxygen
uptake all help explain the diminished walking velocity seen
with age [22,37,47,56,81]. Treatment of gait dysfunctions in
elders requires consideration of the contributions made to the
dysfunction by discrete impairments in the neuromuscu¬
loskeletal and cardiorespiratory systems.

TABLE 48.4: Commonly Reported Changes in Gait in Older Adults

Change with Increased Age

Speed

Decreased [50,69,86,102,105,121]

Cadence

Increased [50,80]

Step/stride length

Decreased [48,50,69,80,81,121,185]

Double support time

Increased [48,80,185]

Joint angular

Decreased [80,87,121]

excursions

Unchanged [50]

Muscle activity

Increased [50]

Joint powers

Decreased generation in hip extension and plantarflexion and increased generation in hip flexion [80,81,87,185]

Gait variability

Increased [19,133]

Energy cost of gait

Increased [108]

Chapter 48 I CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

913

Clinical Relevance

EVALUATION AND TREATMENT OF GAIT
DYSFUNCTION IN ELDERS: Data describing the gait of
elderly individuals reveal that many of the changes thought
to be characteristic of aging can be explained by a reduc¬
tion in walking speed. Consequently , a clinician must alter
the standards of "normal" used to judge the adequacy of
gait The gait patterns of elders walking at reduced speeds
are not comparable to the patterns of subjects walking at
faster speeds, regardless of age. Similarly treatment may be
most successful when directed toward those factors that
contribute to diminished speed, including strength of the
quadriceps, plantarflexors, and hip flexors and extensors.

SUMMARY

This chapter reviews the kinematic and kinetic variables of
normal gait. The kinematic variables presented in this chap¬
ter include the more global parameters of time and distance
as well as the discrete displacement patterns of joints.
Although all of these variables are subject to intra- and inter¬
subject variability, representative values from the literature
are presented to provide the reader with a frame of reference
for normal locomotion.

Joint excursions are largest in the sagittal plane and exhib¬
it stereotypical patterns and sequences. In normal locomo¬
tion, the hip, knee, and ankle rarely move together toward or
away from the ground. Activity of the major muscle groups of
the lower extremity is reviewed. Their activity is typically
brief, characterized by initial eccentric activity followed by
concentric activity. In most cases, joint movement continues
after muscle activity has ceased.

The kinetic variables described in this chapter include
ground and joint reaction forces, muscle forces, and joint
moments, as well as joint power and mechanical energy. The
principle of dynamic equilibrium is used to explain the deri¬
vation of muscle and joint reaction forces, joint moments, and
joint power. Like the kinematic variables, the kinetic variables
exhibit intra- and intersubject variability that reflects the nor¬
mal variability of individuals and populations, but kinetic
parameters also are quite sensitive to differences in measure¬
ment procedures. The kinetic variables reveal that locomo¬
tion generates large muscle and joint forces. Kinetic analysis
also demonstrates the remarkable efficiency of normal loco¬
motion in which energy is stored and released, reducing the
amount of work the muscles must perform to achieve the
movement. Impairments in the neuromusculoskeletal system
decrease the efficiency of gait.

Finally this chapter discusses factors that influence walk¬
ing patterns, including gender, walking speed, and age. The
discussion reveals a complex interdependence between walk¬
ing speed and age effects on gait, and the clinician is cau¬
tioned to keep these factors in mind when judging the walk¬
ing performance of an individual.

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Index

Note: Page numbers followed by f, t, or b indicate figures, tables, or boxed text, respectively

Abdominal belts, 616
Abdominal wall muscles, 596-597,
596b-598b, 596t, 598f
exercises for, 598b, 614-616, 615f
in standing, 886-887
Abduction contractures, of hip,

883, 884f

Abduction splint, for median nerve injury,
366, 366f

Abductor digiti minimi, 357, 357f, 856,
856b, 856f
actions of, 357-358
attachments of, 357b
innervation of, 357b
weakness of, 359

Abductor hallucis, 855-856, 856b, 856f
Abductor pollicis brevis, 352, 352f
actions of, 352-353
attachments of, 352b
innervation of, 352b
moment arms of, 319, 319f
in normal pinch, 373
palpation of, 352b
weakness of, 353, 353f
Abductor pollicis longus, 317, 317f

actions of, 318-4320, 318f, 319f, 320b-321b
attachments of, 318b
innervation of, 318b
moment arms of, 319, 319f
palpation of, 318b
tightness of, 320
weakness of, 320
Acceleration, 18, 18f, 18t

cervical spine injuries due to,

516-518, 517f
in gait cycle, 908
instantaneous, 18

Accessory lateral collateral ligament,
209-210, 210f
Accessory motion, 107
Accommodating resistance, in
dynamometry, 794
Acetabulum, 688-689, 688f

alignment of, 694-698, 694f-698f
femoral head and, 690
Achilles tendon, 844-845, 844f
forces on, 867
rupture of, 845, 845b
Aconeus, 233f, 236-237, 238f
Acromioclavicular joint, 131-133
dislocation of, 131-132
ligaments of, 130-131, 131f
motions of, 132-133, 132f

in arm-trunk elevation, 142-143
loss of, 145-146
osteoarthritis of, 133, 145
structure of, 130-133, 131f, 132f
Acromion, 122f, 123
ACTH, connective tissue and, 94
Actin, 47, 47f
Active insufficiency

of finger muscles, 323-324, 323f, 342
muscle weakness and, 55, 221
Addition, vector, 6, 6b
Adduction moment, in knee, 797, 905, 950f
Adductor brevis
action of, 719
attachments of, 718b
functional contributions of, 720
innervation of, 718b
structure of, 717-718, 718f
tightness of, 720
weakness of, 720

Adductor hallucis, 858-859, 858b, 858f
Adductor longus, 706f
attachments of, 718b
functional contributions of, 720
innervation of, 718b
structure of, 717-718, 718f
tightness of, 720
weakness of, 720
Adductor magnus, 706f, 71 If
action of, 719
attachments of, 719b
functional contributions of, 720
innervation of, 719b
structure of, 717-718, 718f
tightness of, 720
weakness of, 720
Adductor pollicis, 352f, 354
actions of, 355-356, 355f
attachments of, 355b
innervation of, 355b
tightness of, 356
weakness of, 356, 356f
Adhesive capsulitis, 136
Adrenocorticotropic hormone (ACTH),
connective tissue and, 94
Aerobic exercise, low back injury and, 614
Age-related changes. See also Elderly
in bone, 42

in cartilage, 76, 79, 535b
in healing, 97

in ligaments, 92-94, 92f, 93f, 97
in locomotion, 912-913, 912t
in muscle strength, 64
at knee, 786

in tendons, 92-94, 92f, 93f, 97
in Youngs modulus, 42
Aggregate modulus, 73
Alar ligaments, 478-479, 478f, 479b
Alignment. See also specific structures
abnormal. See Malalignment
valgus

of ankle, 818, 831
definition of, 205
of elbow, 201, 205-206, 206f,
208-209, 209f
of knee, 750-752, 751f, 756,

756f, 757f

varus

of ankle, 818
definition of, 205-206
of elbow, 205-206, 206f
of knee, 750-752, 751f, 756, 756f
Alimentary canal, 424, 425f
Allografts. See Grafts
Ambulation. See Gait; Locomotion;
Walking

Amphiarthrodial symphysis, 637
Amyotrophic lateral sclerosis, 667b
Anal canal, 669
Anal sphincter
external, 660
internal, 669
Anal triangle, 658, 658f
Anatomical force couple, 156, 156f, 162,
165, 165f, 167, 508, 509f
Anatomical snuffbox, 256, 261f, 317, 317f,
322b, 322f

Android pelvis, 635b, 635f
Angle of application, 50, 50f
moment arm and, 57
Angle of Louis, 126
Anisotropy

of bone, 39-40, 40f
of cartilage, 76
Ankle. See also Foot

avulsion fracture of, 810b, 855b
bones of, 808-810, 808f,

809b-810b, 809f

collateral ligaments of, 817-818, 817f,
818t, 821

eversion of, 821, 82If
forces on, 865-867, 866b, 867f
in standing, 884-885, 885t
joint power at, 909, 910f
manual therapy techniques for, 819b
motions of, 818-819, 819b, 819f
in locomotion, 897-900, 897f, 898f
pronation, 815, 815t

919

920

INDEX

Ankle. See also Foot ( Cont.)
muscles of, 838-839

deep, of posterior compartment,
849-853, 850f
dorsiflexor, 838-843
lateral, 853-855
in locomotion, 901
relative strength of, 860-861, 860t,

86 It

in standing, 884-885, 885t
superficial, of posterior

compartment, 844-849
osteoarthritis of, 817b, 867b
range of motion of, 819, 819f, 845-846,
846f

sprains of
high, 816b

inversion, 821b, 855b
stability of, 818, 818f
structure of, 817-818, 817f, 818t
Ankylosing spondylitis, chest expansion
in, 533b

Annular ligament, 207, 209,

210-211, 211f

Anorectal continence, 668t, 669-670,
670b, 672

Anorectal flexure, 658, 658f
Antalgic gait, 732

Anterior cruciate ligament, 752-754
closed-chain exercises for, 799
forces on, 798-799
function of, 752-754, 753f, 755
injuries of, 755

diagnosis of, 753, 754, 754f
estrogen and, 94
osteoarthritis and, 79, 80
patellar tendon grafts for, 99
treatment of, 98, 99
peak strain in, 89t
structure of, 752, 753f
Anterior drawer test, 753, 754f
Anterior inferior iliac spine, 693
Anterior interosseous nerve, impingement
of, 315b-316b

Anterior scalene, 505-506, 505b, 505f
Anterior superior iliac spine, 628b-629b
Anterior tibialis, 839, 840f
actions of, 839-841
attachments of, 840b
innervation of, 840b
palpation of, 840b
tightness of, 841
weakness of, 841, 841f
Anthropoid pelvis, 635b, 635f
Anulus fibrosus, 573, 574f
tissue failure and, 610
Anus, 663f

Ape thumb deformity, 316, 316f, 366, 366f
Apertures, facial, 392
Apical ligament, 479f, 480
Aponeuroses, palmar, 340-342, 341f

Arch index, 830
Arcuate popliteal ligament, 755
Arcuate pubic ligament, 648
Arcus tendineus, 656f, 657
Arm-trunk motion, 140-146, 141f-144f
Arthritis. See Osteoarthritis; Rheumatoid
arthritis

Arthrokinematics, 107, 580
Arthroplasty, total joint. See Total joint
arthroplasty

Articular cartilage, 10, 69. See also
Cartilage
Articular cavity, 10

Articular disc, of temporomandibular
joint, 443, 443b, 445b, 445f
mandibular condyle alignment with,
462-463, 463f
Articularis genu, 770
Artificial joints. See Total joint
arthroplasty

Arytenoid cartilages, 415, 415f, 416f
Asphyxia, swallowing disorders and,

423, 436

Aspiration pneumonia, 423, 436
Asymmetrical tonic neck reflex, 236, 236f
Athletes

cervical spine impact loading in,
515-516, 516b

muscle length (stretch) in, 56
osteoarthritis in, 80
shoulder impingement syndrome in,
139, 161

throwing, valgus stress during,

209, 209f

upper extremity weight bearing in,

183, 184f

Atlantoaxial joints, 477, 477f. See also
Cervical spine

range of motion of, 484-485, 485f,

485t, 486f

Atlantoaxial membrane, 479-480
Atlanto-occipital joints, 477. See also
Cervical spine

joint reaction forces on, 511, 512b
range of motion of, 474b, 483-484,
483t, 484t

Atlanto-occipital membrane, 479-480
Atlas, 474, 474f
Atrophy, muscle
disuse, 63
in scoliosis, 889
Auriculares, 394-395, 395b
Autografts. See Grafts
Avascular necrosis
of femoral head, 692
of hip, 734
of lunate, 261, 262f
of scaphoid, 261
Avulsion fracture, 90
of ankle, 810b, 855b
of tibial tuberosity, 795-796

Axes of motion, 105, 106f, 109-110, 109t
forearm muscles and, 295, 296f
Axioclavicular muscles, 150-167, 152f
Axiohumeral muscles, 179-184
Axioscapular muscles, 150-167, 152f
Axis

craniovertebral, 475, 475f
of rotation, 17, 17f, 109-110

Back pain

disc fluid content and, 574b
extensor muscle exercises for,
595b-596b
facet joints and, 572b
gluteus maximus tightness and, 714
inextensible hamstrings and, 579b
injury causing

prevention of, 612
rehabilitation after, 612-617,
614f-616f

intervertebral disc and, 577b-578b
postural training for, 881
psoas major contraction in, 708
walking and, 603b

Ball-and-socket joints, 109-110, 109t
Barringtons area, 665
Bell’s palsy, 400b, 400f
Bending, 32-33
intervertebral disc and, 575,
575b-576b, 576f, 577f
lumbosacral region in, 680
Bent-knee sit-ups, 613
Biaxial (condyloid) joints, 109-110, 109t,
273, 284

Biceps brachii, 220-223, 220f-224f
actions of, 221-223, 230-232, 231t
attachments of, 220b
electromyography of, 230-232, 231t
functional contribution of, 228-232,
229f, 230f
innervation of, 220b
length of, 228-230, 229f, 230f
moment arm of, 228-230, 229f
structure of, 220, 220f

comparative analysis of, 228-230
tightness of, 223, 223f, 224f
weakness of, 223, 232
Biceps femoris, 706f, 711f, 723, 775-778,
775f. See also Hamstrings
Biceps tendon, dislocation of, 126
Bicipital groove, depth of, 126
Biconcave joint surface, 111
Biconvex joint surface, 111
Biglycan, 87
Biomechanics
definition of, 3
overview of, 3-4
Bipennate muscles, 49f, 50
Bite force, 469, 469b
Bladder, 667

INDEX

921

Bladder neck, 667
Blinks
reflex, 396
spontaneous, 396
Body planes, 7, 7f
Body weight. See Weight
Bone, 36-43. See also specific bones
age-related changes in, 42
anisotropic, 39-40, 40f
biomechanics of, 38-42
cancellous, 37-38
compact, 37-38
cortical, 37-38
elastic constants for, 40-41
fracture toughness of, 41
healing of, 42-43
loading rate and, 42
orthotropic, 40
spongy, 37-38
strain rate for, 41-42
strength of, 41
structure of, 37-38, 37f, 38f
function and, 38-39
geometry of, 39
trabecular, 37-38
transversely orthotropic, 40
woven, 37

Youngs modulus of, 40
Bone density

loading forces and, 42
loss of, 42
Bone remodeling

in fracture healing, 43
in osteoporosis, 42
Border facet, patellar, 744, 745f
Boutonniere deformity, 348b, 348f
Bowel function, 668t, 669-670, 670b
Bowstringing, 342, 342f, 839
Boyle s law, 535

Brachialis, 220f, 224-225, 224b, 225
actions of, 224, 224b, 230-232, 231t
attachments of, 224b
electromyography of, 230-232, 23It
functional contribution of, 228-232,
229f-231f
innervation of, 224b
length of, 228-230, 229f, 230f
moment arm of, 228-230, 229f, 230f
tightness of, 225
weakness of, 224, 232
Brachioradialis, 220f, 226, 297f
actions of, 226, 230-232, 231f, 231t
electromyography of, 230-232, 23It
functional contribution of, 228-230,
229f-231f

length of, 228-230, 229f, 230f
moment arm of, 228-230, 229f, 230f
weakness of, 232

Bracing, patellar, for lateral tracking, 802
Braxton Hicks contractions, 671
Breaking point, 27, 29

Breast cancer, radical mastectomy for,
pectoralis major in, 180
Breathing

control of, speech and, 420b
mechanics of respiration and, 535, 535f
muscle activity of, 552-553, 553b, 553f
paradoxical, 548b-549b, 549f
British units, 4, 4t
Brittle material, 29, 30
Bruxing, 457b-458b
Buccinator, 399, 399f, 406, 406b, 463
Bulbospongiosus, 658, 659f, 660t
Bursitis, pes anserinus, 783
Burst fracture, of thoracic spine, 560b

Cadence, 895, 896t
height and, 896

Calcaneocuboid joint, 824, 824f
Calcaneus, 811-812, 811f
fractures of, 812b
Calcium, in muscle contraction, 47
Callus, fracture, 43
Cancellous bone, 37-38
Cancer, breast, radical mastectomy for,
pectoralis major in, 180

Cane

hip loading and, 729-732, 730b-731b
wrist loading and, 332, 333f
Caninus, 403-404
Capitate, 259-260, 259f, 260f, 262
Capitulum, 199, 200f, 201, 201f, 203
Capsular ligaments, 481
Capsuloligamentous complex,

136-138, 137f
Cardinal planes, 7, 7f
Carpal bones, 259-260, 259f, 260f,
262-263, 263f
motions of, 273-275
Carpal ligaments, 342-343, 343b
Carpal tunnel syndrome, 270

finger flexor muscle forces and, 385
management of, 343b, 385b
Carpometacarpal joint, 278-283
extension of, 318-320, 321
of fingers, 281-283, 282f, 283f
hypothenar muscles and, 359
of thumb, 278-281, 280f, 280t, 281f
Carrying angle, 201, 205-206, 206f
of interphalangeal joints, 286-287
Cartilage, 38

age-related changes in, 76, 79, 535b
aggregate modulus of, 73
articular, 69
biomechanics of, 69-81
biphasic model of, 72
classification of, 69
composition of, 70, 71f

mechanical properties and, 75-76
confined compression test for, 73,

73f, 74

costal, movement of, 534-535,
534f, 535b
elastic, 69
of elbow, 201, 202
fibrocartilage, 69
fluid flow in, 71
functions of, 69
of hip, 690

indentation test for, 74, 74f
of knee

articular, 745-746
meniscal, 746-748
in osteoarthritis, 79, 80
laryngeal, 413-416, 413f-416f
loading effects on, 71, 76-78,
76f-78f

osteoarthritis and, 79-80
material properties of, 72-751, 75
mechanical failure of, 76-78,
76f-78f

mechanical properties of, 72
collagen and, 75, 75f
composition and, 75-76
glycosaminoglycan and, 75, 75f
water content and, 75
modeling in, 72
in osteoarthritis, 72. See also
Osteoarthritis
permeability of, 72-74
shear stress in, 76-78, 76f-78f
shear tests for, 75
single edge notch test for,

78, 78f

solid matrix of, 71f, 72
assessment of, 75
split lines in, 71
stiffness of, 75
structure of, 70-72, 71f
tear test for, 78
tensile tests for, 76, 76f
tidemark in, 71
trouser tear test for, 78, 78f
Youngs modulus of, 73
zones of, 71-72, 71f
Cartilage-specific collagen, 37
Casts. See Immobilization
Cavus foot, 841
Center of gravity, 13, 14f
Center of mass, 876, 884-885
of head, arms, and trunk, 708
Center of pressure, 876
Center of rotation, 17, 17f
instant, 107-108, 108f, 109f
in ankle, 818-819
in elbow, 213
in shoulder, 140
of lever, 12, 13f
Cervical discs, 480-481
degeneration of, 513, 513f
Cervical headaches, 494b-495b
Cervical lordosis, 507

922

INDEX

Cervical muscles, 492-509

contractions of, types of, 508, 508t
extensors, 493, 506t
in deep plane, 493-495
in semispinalis plane, 495-497
in splenius and levator scapulae
plane, 497-500
in superficial plane, 500-502
flexors, 502, 506t

longus capitis/longus colli, 503-504,
504b, 504t

rectus capitis lateralis/rectus capitis
anterior, 504, 505b
scalene, 505-506, 505b, 505f, 506b
sternocleidomastoid, 502-503, 502b,
503b, 503f

functions of, 506-507, 506t
interactions of, activation patterns and,
507-508, 508f, 508t
posture and, 508-509, 509f
in standing, 886
Cervical spine

acceleration injuries of, 516-518, 517f
bones of, 473-477, 474f-476f
dynamic loading of, 515-518, 516f, 517f
forces on, 511-518
impact loading of, 515-516, 516f
joints of, 477-482
ligaments of, 481-482, 482f
loads on, 514-518
motions of, 507, 507f
muscles of. See Cervical muscles
range of motion of, 482-489, 482t, 483t
craniovertebral, 483-486
head posture and, 486, 486b
lower, 486-489
segmental, 483-489
total, 482-483

static loading of, 514-515, 514f
structure of, 473-477, 474f-476f
traction of, temporomandibular joint
and, 470b, 470f

Cervical vertebral column, 475-477,

475f, 476f
Chairs, rising from

chair height and, 775, 775f
extension moment in, 775, 775f
flexion moment in, 778, 778f
Charpy impact test, 30-31, 30f
Chest expansion, 533, 533f

ankylosing spondylitis and, 533b
Chewing. See Mastication
Childbirth, 668t, 672
Children

hip adductor spasticity in, 720
hip dysplasia in, 694-698, 720-721.

See also Infants
Chin-ups, 232, 232f
Chondrocytes, 70
Chondroitin sulfate, 87
proteoglycans and, 74f

Chopart s joint, 823-825
Chronic synovitis, 105
Chuck pinch, 373

Circular fibers, of sphincter urethrae,

658, 660t

Clavicle

displacement of, 128-129, 128f
fractures of, 129, 129f
function of, 121

motions of, 128, 128f, 130, 130f
in arm-trunk elevation, 142-143,

142f, 143f

crank-like, 143, 143f
palpation of, 130

sternoclavicular joint and, 127-130,
128f-130f

structure of, 121, 121f, 128, 128f
Claw hand deformity, 302, 315, 315f,

364f, 365, 365f

Claw toe deformity, 826b, 842, 842b
Closed chain, 113-114
definition of, 794
in knee exercises, 794-796, 795f,

800, 802f

for ACL-deficient knees, 799, 799f
in knee motion, 743, 794-796, 795f
rib cage and, 531
Closing reflex, in defecation, 670
Cobb angle, 877, 878f
Coccygectomy, 640b
Coccygeus, 656f, 657t, 658
Coccygodynia, 639b
Coccyx, 626, 627f

sacrococcygeal junction and, 639b-640b
Collagen, 37
in bone, 37

in cartilage, 37, 70, 75, 75f
composition of, 85
in connective tissue, 85-87
crimp pattern of, 86f, 87, 88
disorders of, 86-87
production of, 85
temperature effects on, 90-91
types of, 86
Collateral ligaments

of ankle, 817-818, 817f, 818t, 821
of elbow, 207-210, 208f, 210f, 215-216
of fingers, 284-285, 284f, 287
function of, 112
of knee, 112, 750-752
assessment of, 752, 752f
function of, 112, 750-752, 751f, 755
injuries of, 90, 94, 98, 752, 755
stress test for, 752, 752f
structure of, 105, 112, 112f, 750, 750f
of thumb, 283, 283f
of wrist, 266, 271-272, 271f, 272t
Colles’ fracture, 257
Compact bone, 37-38
Complete failure region, in stress-strain
curve, 88, 88f

Component motion, 107
Component resolution, of vector,

5-6, 5f

Compound joint, 443
Compression, 23, 24f
Compression fractures

of lumbar vertebrae, 566b, 575, 575f
of thoracic vertebrae, 522b,

557b-558b

Compressor urethrae, 658, 660t
Concave-convex rule, 107, 140, 742
Concentric muscle contraction
cervical spine and, 508
definition of, 58

strength and, 58-59, 59f, 60, 60f
velocity of, 58-59, 59f
Condyloid joints, 109-110, 109t,

273, 284

Confined compression test, 73, 73f, 74
Congenital hip dislocation, 694-698,
720-721

Congruence angle, 757, 758f
Congruent joint surfaces, 110, lllf
Connective tissue. See also Ligaments;
Tendons and specific joints
age-related changes in, 92-94, 92f, 93f
in collagen, 85-87
composition of, 85-87, 85f
dense, 85
elastin in, 87

extracellular matrix of, 85-87
ground substance of, 87
healing of, 96-98
hormone effects on, 94
injuries of

early mobilization in, 97
treatment of, 98
load-deformation curves for, 87
loose, 85

mechanical properties of, 87-94
in muscle, 48, 48f
rate of force application and, 90
stress-strain curves for, 88-90, 88f
structure of, 85-87, 85f
temperature effects on, 90-92, 91f
types of, 85

Conoid ligament, 131, 131f
Constitutive equation, for stress
and strain, 40
Constrictor muscles
of mouth, 399, 399f
pharyngeal, 428-429, 429b, 429f
Contact response, in gait cycle,

893-894, 894f
Continence, anorectal, 668t,

669-670, 670b

Contraction, muscle. See Muscle
contraction
Contractures

abduction, of hip, 883, 884f
Dupuytrens, 341b, 34 If

INDEX

923

flexion

of elbow, 223, 223f, 224f
of hip, 694, 709-710, 710, 711f, 883,
883f, 884f

of knee, 750, 778, 780, 883, 883f, 884f
plantarflexion, 848
postural abnormalities and, 883,

883f, 884f
splints for, 32
Coordinate systems, 7, 7f
Coracoacromial ligament, 131f, 132
Coracobrachialis, 178-179, 178b, 178f
Coracoclavicular ligament, 130-131, 13 If
Coracohumeral ligament, 137-138, 137f
Coracoid process, 122f, 123
Corniculate cartilage, 416
Coronal plane, 7, 7f
Coronoid fossa, 200-201, 201f
Coronoid process, 202, 202f
Corrugator supercilii, 396
actions of, 397, 397f
attachments of, 397b
innervation of, 397b
weakness of, 397
Cortical bone, 37-38
Cortisol, connective tissue and, 94
Cosine, 5, 5f

Costal cartilages, movement of, 534-535,
534f, 535b

Costoclavicular ligament, 128f, 129
Costosternal junctions, pain at, 528b
Costotransverse joints, 526, 527, 528f
Costovertebral joints, 526
Coxa valga deformity, 695-696, 695f,
732-733

Coxa vara deformity, 696, 696f, 732-733
Craniomandibular joint. See

Temporomandibular joint
Craniovertebral joints, 477, 477f, 478f
ligaments of, 478-480, 478f, 479f
segmental motions of, 483-486,
483t-485t, 485f, 486f
Craniovertebral vertebrae, 474-475,

474f, 475f

Cranium, 439-443, 439f
Crazy bone, 200
Creep, 32

in ligament grafts, 98
Creep test, 73, 73f
Crepitus, temporomandibular joint
and, 448b

Cricoarytenoid muscles

lateral, 416, 417, 417b, 417f
posterior, 418, 418b, 418f
Cricoid cartilage, 413, 413f, 414f, 415f
Cricothyroid muscle, 418-419,

418b, 419f

Crimp pattern, 86f, 87, 88
Cross product, of vectors, 6-7, 7f
Cross-bridges, in muscle contraction,

47, 47f

Cruciate ligaments, of knee. See Anterior
cruciate ligament; Posterior
cruciate ligament
Crutch walking

elbow loading in, 247-249, 248b,

249f, 250f

shoulder loading in, 195
wrist loading in, 333
Cubital tunnel, 200
Cuboid, 813
Cuneiform bones, 813
Cuneiform cartilage, 416
Cystocele, 671-672

Darcy’s law, 73

De Quervain’s disease, 320b-321b
Deceleration phase, of gait cycle, 908
Decorin, 87

Deep transverse perineus, 658, 659f, 660t
Defecation, 668t, 669-670, 670t
Defecation reflexes, 670
Deformation
elastic, 25
inelastic, 27, 29f

load-deformation curves for, 27-30,

29f, 87
plastic, 27, 29f

Degrees of freedom, 105-106, 107f,
109-110

Delayed-onset muscle soreness, 59-60
Deltoid, 151, 167-170, 168f, 169f
actions of, 144-145, 144f, 167-168,

169, 170

anterior, 167-169, 169f

attachments of, 168b

force calculation for, 15-16, 15b

innervation of, 168b

manual muscle test for, 168, 169f

middle, 169f, 170f

moment arm of, 10b

palpation of, 168b

posterior, 169-170, 169f

rotator cuff and, 175-176

scapular motion and, 144-145,

144f, 168

in shoulder elevation, 175-176
structure of, 167, 168f, 169f
tightness of, 169, 170
weakness of, 169, 169f
Deltoid tuberosity, 125f, 126
Dematan sulfate, 87
Dens, fracture of, 478b
Density, 22

Depressor anguli oris, 404
actions of, 405, 405f
attachments of, 404b
innervation of, 404b
weakness of, 405, 405f
Depressor labii inferioris, 404, 404b
Depressor septi, 399, 399b

Derived units of measurement, 4
Detrusor, 667

Detrusor hyperreflexia, 672
Detrusor-sphincter dyssynergia, 672
Developmental hip displacement,
694-698, 720-721
Diaphragm, 550, 55If
actions of, 550-551, 551b-552b, 552f
attachments of, 551b
impairment of, 552
innervation of, 551b
pelvic, 655, 656, 657t
urogenital, 658, 660t, 662f
Diarthrodial joints, 69, 104-105, 109-110.

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